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Journal of Experimental Botany, Vol. 65, No. 10, pp. 2603–2615, 2014
doi:10.1093/jxb/ert426 Advance Access publication 18 December, 2013
Review paper
Selective protein degradation: a rheostat to modulate
cell-cycle phase transitions
Pascal Genschik1,2,*, Katia Marrocco2, Lien Bach2, Sandra Noir1 and Marie-Claire Criqui1
1 Institut de Biologie Moléculaire des Plantes, Centre National de la Recherche Scientifique, Unité Propre de Recherche 2357,
Conventionné avec l’Université de Strasbourg, 67084 Strasbourg, France
2 Laboratoire de Biochimie et Physiologie Moléculaire des Plantes, Institut de Biologie Intégrative des Plantes ‘Claude Grignon’, UMR
CNRS/INRA/SupAgro/UM2, Place Viala, 34060 Montpellier Cedex, France
* To whom correspondence should be addressed. E-mail: [email protected]
Received 10 September 2013; Revised 12 November 2013; Accepted 17 November 2013
Abstract
Plant growth control has become a major focus due to economic reasons and results from a balance of cell proliferation in meristems and cell elongation that occurs during differentiation. Research on plant cell proliferation over the
last two decades has revealed that the basic cell-cycle machinery is conserved between human and plants, although
specificities exist. While many regulatory circuits control each step of the cell cycle, the ubiquitin proteasome system
(UPS) appears in fungi and metazoans as a major player. In particular, the UPS promotes irreversible proteolysis of
a set of regulatory proteins absolutely required for cell-cycle phase transitions. Not unexpectedly, work over the last
decade has brought the UPS to the forefront of plant cell-cycle research. In this review, we will summarize our knowledge of the function of the UPS in the mitotic cycle and in endoreduplication, and also in meiosis in higher plants.
Keywords: APC/C, cell cycle, cullin, DNA replication, endoreduplication, meiosis, mitosis, SCF, ubiquitin.
Introduction
The typical eukaryotic cell cycle is divided into four phases,
the M phase (mitosis) in which sister chromatids are separated and distributed to the newly forming daughter cells,
the S phase in which the nuclear DNA becomes replicated,
and two gap phases, G1 and G2, that separate the M and S
phases. In particular, transition from G1 to S phase as well as
progression and exit from mitosis are key steps that need multiple levels of control, one of which is assumed by the ubiquitin proteasome system. In this pathway, ubiquitin ligases
(E3s) facilitate the transfer of ubiquitin moieties to a substrate protein, the preparative step for degradation via the 26S
proteasome (Ciechanover et al., 2000). Similar to metazoans,
based on specific commonly shared structural motifs, plant
genomes also encode hundreds of different E3s (Vierstra,
2009). Two families of E3s, the SCF [Skp1, Cdc53 (cullin),
and F-box] and the anaphase promoting complex/cyclosome
(APC/C), dominate DNA duplication and cell division in all
eukaryotes (Pesin and Orr-Weaver, 2008; Marrocco et al.,
2010; Heyman and De Veylder, 2012; Mocciaro and Rape,
2012). SCF belongs to the Cullin-RING family of ubiquitin
E3 ligases (CRLs), the most prevalent class of E3s (Petroski
and Deshaies, 2005). SCF is a multimeric E3 in which the
CULLIN1 (CUL1) protein serves as a molecular scaffold.
This scaffold brings together a catalytic module composed
of a RING-finger domain protein (RBX1), a ubiquitin-conjugating enzyme (E2), and a specific substrate-recognition
module composed of the adaptor SKP1 (so-called ASK1/2
in Arabidopsis), and one protein of the F-box family that
physically interacts with the target substrate(s) (Fig. 1B)
(Hua and Viestra, 2011). The APC/C is composed of a larger
Abbreviations: CDK, cyclin-dependent kinase; CKI, cyclin-dependent kinase inhibitor; MCC, mitotic checkpoint complex; QC, quiescent centre; SAC, spindle
assembly checkpoint; UPS, ubiquitin proteasome system.
© The Author 2013. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved.
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2604 | Genschik et al.
Fig. 1. Different classes of ubiquitin E3 ligases involved in cell-cycle regulation. (A) Monomeric RING ubiquitin E3 ligases can interact
directly with their substrates and the E2 ubiquitin-conjugating enzyme. (B–D) The SCF/CRL1, CRL3, and CRL4 complexes are
composed of the scaffold proteins CUL1, CUL3, and CUL4, respectively, and the RING-finger protein RBX1. These ubiquitin E3 ligases
have a similar modular structure where cullins play the role of scaffold proteins bringing together the catalytic module (RBX1 and the E2
ubiquitin-conjugating enzyme) and the module in charge of substrate specificity, including F-box proteins, BTB domain proteins, and
DCAFs as substrate recruiters of SCF/CRL1, CRL3, and CRL4, respectively. (E) APC/C is the largest multimeric ubiquitin E3 ligase,
requiring WD40 activator proteins (CDC20/FZ and CDH1/FZR/CCS52) and most likely also the DOC1/APC10 subunit to recruit its
substrates. The RING-H2 finger subunit APC11 (red) interacts with an E2 ubiquitin-conjugating enzyme to stimulate the ubiquitylation
reaction. (This figure is available in colour at JXB online.)
number of subunits (Primorac and Musacchio, 2013), two
of which (APC2 and APC11) are related to SCF and constitute the catalytic core of the complex (Fig. 1E). APC2 is
a distant member of the CULLIN protein family, whereas
APC11 is similar to the RING-H2 RBX1 protein. To be
active, the APC/C requires crucial co-activators, called CELL
DIVISION CYCLE (CDC20) and CDC20 HOMOLOG 1
(CDH1) in mammals, FIZZY and FIZZY-RELATED in
Drosophila, and CDC20 and CELL CYCLE SWITCH52
(CCS52) in plants. These co-activators are WD40 repeat
proteins that assume, with the APC10/DOC1 core subunit,
most of the APC/C’s selectivity. Indeed, throughout the cell
cycle they recruit various substrates through short destruction motifs, predominantly the D-box and the KEN-box. For
instance, in all eukaryotes, the D-box found in mitotic cyclins
and other regulatory proteins is a small degenerated but conserved motif of 9 aa (RxxLxxIxN). Deletion or point mutations in this motif inhibit proteolysis (Brandeis and Hunt,
1996; Genschik et al., 1998; Yamano et al., 1998). Beside SCF
and the APC/C, other classes of E3s have also been linked
to cell-cycle regulation, such as monomeric RINGs and two
other Cullin-RING ubiquitin ligases, CRL3 (formerly BTB
CRLs) and CRL4 (formerly DCAF/DWD CRLs) (Fig. 1A,
C and D) (Marrocco et al., 2010; Genschik et al., 2013). So
far, the molecular mechanisms indicating a role of E3 ubiquitin ligases in the regulation of the cell cycle are far better
described in yeast and animals than in plants. Thus, in the
following sections, we will briefly introduce these proteolytic
processes in more advanced systems and put them into perspective with plant research in this field.
Progression from G1 to S phase
When cells replicate their DNA, they are committed to
divide, and therefore the transition from G1 to S phase is considered a key regulatory step in cell-cycle regulation (Nurse,
2000). Moreover, various mechanisms incorporating endogenous information, such as nutrient status and hormonal
signals, with exogenous environmental conditions impact
on the G1-to-S-phase transition. In both fungi and metazoans, this step requires the degradation of cyclin-dependent
kinase inhibitors (CKIs) to release cyclin-dependent kinase
(CDK) activity, which in turn allows the phosphorylation
of regulatory proteins required to enter S phase. In budding
yeast, one CKI, SIC1, is eliminated after its phosphorylation by the G1 cyclin CDK activity and ubiquitylation via
SCFCDC4 (CDC4 being a WD40-type F-box protein) (Schwob
Selective protein degradation to modulate the cell cycle | 2605
et al., 1994; Feldman et al., 1997). A similar mechanism
exists in mammals, where the CKI protein p27KIP1 becomes
unstable when cells approach S phase and its degradation
requires phosphorylation by the cyclin E–CDK2 complex
in order to be recognized by SCFSKP2 (SKP2 being a LRRcontaining F-box protein; Fig. 2A) (reviewed by Starostina
and Kipreos, 2012). Furthermore, SCFSKP2 also targets other
cell-cycle regulatory proteins such as p21CIP1, another CKI,
the transcription factor E2F1, and the chromatin licensing
and DNA replication factor 1 (CDT1) among many others (Frescas and Pagano, 2008). Besides SCFSKP2, other E3
ligases are also required to fine-tune mammalian CKI protein levels during the cell cycle and development (Starostina
and Kipreos, 2012). For instance, the RING-finger protein
Fig. 2. Progression through G1 to S phase. The transition from G1 towards the S phase requires a decreased level of CKI proteins
(green curve) in order to release CDK activity (red curve), resulting in interconnected regulation. (A) In mammals, two ubiquitin E3 ligases,
at least, trigger the degradation of the p27 CKI protein: the RING-finger protein KPC targets non-phosphorylated p27 in the cytoplasm,
whereas in the nucleus SCFSKP2 targets not only p27 once phosphorylated by the CyclinE/CDK2 complex but also another CKI, p21.
The CRL4CDT2 E3 complex also targets p21 when it interacts with proliferating cell nuclear antigen (PCNA) in order to prevent DNA overreplication. Additionally, CRL4CDT2 is involved in the destruction of CDT1 to restrict its DNA replication activity. Fine-tuning of the S-phase
progression further involves the SCFSKP2 complex in the regulation of many other cell-cycle regulators including CDT1 and the E2F1
transcription factor, which activates S-phase-promoting genes. (B) In plants, different CKI proteins, members of the KRP family, are also
regulated by distinct ubiquitin E3 ligases and, as in mammals, their degradation might be conditioned by their phosphorylated status.
In sporophytic tissues, the degradation of KRP1 is regulated by two distinct ubiquitin E3 ligases, the RING protein RKP and SCFSKP2B,
while SCFSKP2A has been shown to mediate destruction of the E2Fc transcription factor, which negatively affects cell-cycle progression.
Recent studies of Arabidopsis gametogenesis have demonstrated the implications of ubiquitin E3 ligases [the RING-H2 group F 1a
(RHF1a) and RHF2a, and SCFFBL17] in post-meiotic cell-cycle regulation. Direct interaction and genetic evidence support a role of these
ubiquitin E3 ligases in the proteasome-mediated degradation of KRP6 and KRP7. (This figure is available in colour at JXB online.)
2606 | Genschik et al.
KPC1 (Kip1 ubiquitylation-promoting complex1) promotes
the degradation of p27KIP1 in the cytoplasm in a phosphorylation-independent manner during G1 phase (Nakayama
and Nakayama, 2004; Fig. 2A), while the CRL4CDT2 (CUL4DDB1-CDT2) ubiquitin E3 ligase is in charge of p21CIP1
turnover during S phase, and also after DNA damage (Abbas
et al., 2008; Kim et al., 2008a). Notably, a physical interaction of p21 with proliferating cell nuclear antigen, mediated
by the PIP box degron present in p21, is required for its efficient CRL4CDT2-dependent destruction. Another substrate
of mammalian CRL4CDT2 is the chromatin licensing factor
CDT1, which also contains a PIP box and thus interacts with
proliferating cell nuclear antigen, ensuring that its ubiquitylation occurs only on the active chromatin-bound pool of the
substrate (Havens and Walter, 2011). Together, these findings
illustrate that substrates sometimes need to be modified at the
post-translational level and/or associate with specific factors
to become ubiquitylated by multiple E3s that can act at different time points of the cell cycle and at different subcellular
locations.
In contrast to the complex, but well-documented situation occurring in mammalian cells, our understanding of
proteolytic events at the G1-to-S-phase transition in plants
is still limited. The Arabidopsis Cdk1 homologue CDKA;1,
which is required for both S-phase entry and mitosis, is
negatively regulated by CKIs (Verkest et al., 2005a). Two
classes of CKIs have been identified so far in Arabidopsis:
the INTERACTOR/INHIBITOR OF CDK, also called
KIP-RELATED PROTEINS (with seven members in
Arabidopsis, hereafter called KRP1–7) (Torres Acosta et al.,
2011), and the SIAMESE-RELATED (SMRs), named after
their founding member SIAMESE (Churchman et al., 2006;
Peres et al., 2007). Both classes of plant CKIs show very
restricted similarities with the mammalian Kip/Cip proteins.
At the functional level, constitutive overexpression in transgenic Arabidopsis plants of all KRPs tested so far can block
both M and S phases, leading not only to growth retardation, including a reduction in cell number and organ size, but
also to different developmental abnormalities, such as leaf
serration (Verkest et al., 2005a). Therefore, protein levels of
plant CKIs must be tightly regulated. Indeed, it was shown
that KRP2 proteasomal degradation is conditioned by its
CDK-dependent phosphorylation (Verkest et al., 2005b), a
situation reminiscent of the mammalian p27KIP1 SCFSKP2dependent degradation. However, at present, the identity of
the ubiquitin E3 ligases responsible for the ubiquitylation and
degradation of plant CKIs at the sporophytic stage remains
mysterious, although a picture is starting to emerge in gametogenesis (see below and Fig. 2B).
Two distantly related Arabidopsis LRR-containing F-box
proteins, called SKP2A and SKP2B, are proposed to be the
metazoan homologues of SKP2; however, their function in
the turnover of KRPs remains presently unclear. SKP2A is
involved in cell-cycle regulation as it binds to the transcription factor E2Fc and its partner DPB to mediate their degradation (del Pozo et al., 2006). The E2Fc/DPB dimer acts
as a negative regulator of cell division by counteracting the
activation of E2F-responsive genes. Despite a high similarity
in protein sequence between the two F-box proteins, SKP2B
targets different substrates and KRP1 is likely to be one of
them (Ren et al., 2008). Indeed, SKP2B overexpression in
Arabidopsis was shown to suppress the leaf serration phenotype conferred by KRP1 overexpression. Nevertheless, a
KRP1–GUS reporter protein did not accumulate to a higher
level in a skp2a skp2b double mutant (Ren et al., 2008), nor
did the endogenous KRP2 (Marrocco et al., 2010). This
might suggest the implication of other ubiquitin E3 ligases.
Based on protein similarities with the mammalian RING
protein KPC1, the Arabidopsis RING protein named RKP
(Related to KPC1) was identified and suspected to be also
responsible for KRP1 proteasomal degradation (Ren et al.,
2008). Nevertheless, no obvious developmental defects that
would be expected for strong KRP protein accumulation
could be observed in any of these mutant lines, not even in
the triple skp2a skp2b rkp mutant, suggesting the existence
of additional and/or even more critical ubiquitin E3 ligase(s)
that await discovery.
Possible candidates for such ubiquitin E3 ligases have
emerged recently from studies of Arabidopsis mutants deficient in cell division during gametogenesis. For instance,
RHF1a and RHF2a are two similar RING-finger proteins
that, when mutated in the rhf1a rhf2a double mutant, impair
normal cell divisions during pollen and embryo sac development (Liu et al., 2008). Interestingly, a reduction in KRP6
expression rescues in part the rhf1a rhf2a mutant phenotype,
providing genetic evidence for a role of RHF E3s in KRP
degradation during both male and female gametogenesis.
Another key player in cell-cycle regulation during male gametogenesis is the F-box protein FBL17, as its corresponding
loss-of-function mutants fail to undergo pollen mitosis II,
which generates the two sperm cells in mature pollen grain
(Kim et al., 2008b; Gusti et al., 2009). Genetic evidence
here also supports a function of FBL17 in KRP degradation during gametogenesis, as different KRP loss-of-function
mutations suppressed, at least partially, the pollen defect
phenotype (Gusti et al., 2009; Zhao et al., 2012). However,
whether FBL17 functions beyond gametogenesis is presently
unknown.
Progression through M phase
APC/C: an evolutionarily conserved ubiquitin E3
regulating mitosis
The key regulator for mitotic progression and exit is undoubtedly the APC/C (Thornton and Toczyski, 2006; Pesin and
Orr-Weaver, 2008; van Leuken et al., 2008). This ubiquitin
E3 ligase is composed of many different subunits and has
an approximate size of 1.5 MDa. The APC/C is activated
from early mitosis to late G1/early S phase. To be active
and in order to select its substrates, the APC/C requires crucial co-activators, the CDC20/FIZZY and CDH1/FIZZYRELATED proteins. Besides co-activators, the temporal
regulation of APC/C activity during the cell cycle is also
achieved by reversible phosphorylation and the action of
Selective protein degradation to modulate the cell cycle | 2607
inhibitory proteins. The best-characterized APC/C inhibitory
protein is the metazoan EARLY MITOTIC INHIBITOR 1
(EMI1). During the S and G2 phases, EMI1 restricts APC/C
activity to enable the accumulation of cyclins A and B, allowing replication and promoting the transition from G2 phase
to mitosis, respectively (reviewed by Peters, 2006; Fig. 3A).
At the molecular level, EMI1 binds to the APC/CCDH1 using
its D-box motif as a pseudosubstrate, thus blocking the
accessibility of substrates to the D-box receptor site on the
APC/C. Its zinc-binding region and RL tail domains also
inhibit APC/C E3 ligase activity, mainly by interfering with
ubiquitin chain elongation (Miller et al., 2006; Frye et al.,
2013; Wang and Kirschner, 2013). Once EMI1 is degraded
in prophase by the SCFβ-TrCP ubiquitin E3 ligase upon phosphorylation by Plk1 (Guardavaccaro et al., 2003; MargottinGoguet et al., 2003), the APC/C activity is released and can
thus orchestrate mitosis by inducing sequential ubiquitylation and finally proteolysis of different cell-cycle regulators
(Pesin and Orr-Weaver, 2008). Expression of CDC20 precedes
that of CDH1, and one of the first substrates of the APC/
CCDC20 in animals is cyclin A, which is degraded right after
nuclear envelope breakdown, during prometaphase (Pines,
2006; Fig. 3A). Soon after, at metaphase, polyubiquitylation
by the APC/CCDC20 and degradation of PDS1/SECURIN (a
protease inhibitor) is required for sister chromatid separation
(Peters, 2006). PDS1/SECURIN destruction leads to activation of the Separase protease, which cleaves the cohesin complex that physically attaches sister chromatids. Notably, this
degradation step is part of a surveillance mechanism called
the spindle assembly checkpoint (SAC), keeping the APC/
CCDC20 in check until all chromosomes are properly attached
to the mitotic spindle and bioriented at the metaphase
plane (Kim and Yu, 2011; Musacchio and Ciliberto, 2012).
This mechanism involves the sequestration and, even more
importantly, the degradation of CDC20 through the interaction with MAD2, BUB3, and BubR1, the mitotic checkpoint complex (MCC) proteins. Recent findings have revealed
that the APC/C subunit APC15 (MND2 in yeast) actually
targets CDC20 for proteolysis when bound to the MCC
(Foster and Morgan, 2012; Uzunova et al., 2012; Fig. 3A).
Moreover, P31COMET, a MAD2-interacting protein identified
only in higher eukaryotes, is operating in the same pathway
as APC15, although their exact relationship remains unclear
(Musacchio and Ciliberto, 2012). When the SAC is satisfied,
the APC/CCDC20 sets off polyubiquitylation and thus the degradation of not only PDS1/SECURIN but also B-type cyclins (Fig. 3A). Consequently, the inhibition of Cdk1 activity
induces different cellular processes such as disassembly of the
mitotic spindle, chromosome decondensation, cytokinesis,
and reformation of the nuclear envelope.
In the last decade, substantial advances have been made in
our understanding of plant APC/C composition and function, although we still know only a limited number of its
substrates. While it has been recognized that most APC/C
subunits are evolutionarily conserved in plants (Capron
et al., 2003a), the complex was only recently biochemically
isolated from Arabidopsis cell-suspension cultures (Van
Leene et al., 2010). Most subunits were identified with the
exception of APC9, APC13 (Bonsai), and CDC26, which,
however, were classified as non-essential subunits of APC/C
in yeast (Thornton and Toczyski, 2006). Likewise, the APC15
subunit was not identified by tandem affinity purification of
the Arabidopsis APC/C (Van Leene et al., 2010), but is also
conserved in plant genomes (Uzunova et al., 2012). Thus
we can speculate that, as in mammals, APC15 is dispensable
for APC/C activity directed against common APC/C targets
(Mansfeld et al., 2011; Uzunova et al., 2012).
As expected for such an important regulator of mitosis,
knockout of different Arabidopsis APC/C subunits blocks
female gametogenesis by arresting mitotic division after
meiosis (Capron et al., 2003b; Kwee and Sundaresan, 2003;
Pérez-Pérez et al., 2008; Wang et al., 2012, 2013). In these
mutants, it was shown that mitotic cyclin reporter proteins
carrying a D-box accumulated in the arrested embryo sacs,
although it remains to be demonstrated that the cause of the
arrest results from the non-degradation of one or more cellcycle proteins. Recently, it has been shown that the APC/C
has also a function during male gametophyte development
(Zheng et al., 2011). Interestingly, this work also highlighted
that, in addition to its role in protein degradation, the APC/C
seems also to play a role in transcriptional regulation of
the CYCB1;1 gene. At the sporophytic stage, the APC/C is
required for normal plant organ development, including
roots, stems, leaves, and flowers (Blilou et al., 2002; Serralbo
et al., 2006; Saze and Kakutani, 2007; Pérez-Pérez et al., 2008;
Marrocco et al., 2009; Eloy et al., 2011; Zheng et al., 2011). In
these organs, reduced APC/C activity alters both cell division
and expansion rates, although the molecular details of this
pleiotropic phenotype remain to be elucidated. Conversely,
overexpression of certain APC/C subunits promotes plant
growth, in part, by stimulating cell division (Rojas et al.,
2009; Eloy et al., 2011). How this is achieved is also unknown
at the mechanistic level.
As well as APC/C core subunits, its co-activators have also
been studied intensively in several plant species. Arabidopsis
has five CDC20-like genes (CDC20-1 to CDC20-5) (Kevei
et al., 2011) and three CCS52 genes (CCS52A1, CCS52A2,
and CCS52B) (Tarayre et al., 2004). Expression of CDC20-1,
CDC20-2, and also CCS52B peaks in M phase and organs
with dividing cells (Fülöp et al., 2005; Kevei et al., 2011),
suggesting mitotic functions. Indeed, knocking down both
CDC20-1 and CDC20-2 genes reduces root meristem size
and leads to smaller leaves with fewer cell numbers. While
these observations strengthen the implication of the involvement of these proteins in cell-cycle regulation, the identity of
the targets of the plant APC/CCDC20 is still mysterious. The
function of CCS52 genes has been linked to cell-cycle exit
and endoreduplication (see below). In particular, Arabidopsis
CCS52A2 was shown to regulate mitosis exit and meristem
maintenance in the root tip by controlling mitotic activity
in quiescent centre (QC) cells (Vanstraelen et al., 2009). In
order to identify potential targets of the APC/CCCS52A2 in the
QC, Heyman et al. (2013) co-purified CCS52A2-associated
proteins by tandem affinity purification. The identified candidates were further challenged for their ability to promote QC
cell proliferation upon ectopic expression. This strategy led
2608 | Genschik et al.
Fig. 3. APC/C activity from entry towards exit of the M phase. (A) APC/C is activated from early mitosis and remains active throughout
mitosis and the G1 phase until early S phase. In metazoans, two main inhibitors keep the APC/C complex in check during the cell cycle.
EMI1 operates in the S and G2 phases, whereas the spindle assembly checkpoint (SAC) proteins target APC/CCDC20 in mitosis. The
mitotic checkpoint complex (MCC), which contains the three SAC proteins (MAD2, BUBR1, and BUB3) together with CDC20, is regarded
as the SAC effector. During SAC arrest, CDC20 turnover depends on its association with the SAC proteins and requires the APC/C
subunit APC15. When proper alignment and attachment of the duplicated chromosomes to the mitotic spindle are achieved towards
the end of metaphase, MCC and APC/C are dissociated from each other leaving the single MCC subunit protein, CDC20, with APC/C
to polyubiquitylate SECURIN/PDS1 for 26S proteasomal degradation and thus successful chromosome segregation. APC/CCDC20 also
polyubiquitylates mitotic CYCLIN B for proteasomal destruction to lower CDK activity and allows mitotic exit. (B) In plants, UVI4 and OSD1/
GIG1 are the functional homologues of EMI1. Similar to the EMI1 gene, E2Fa and E2Fb transcription factors regulate UVI4 transcription at
the G1/S-phase transition in Arabidopsis. UVI4 mediates the inactivation of APC/CCCS52A during DNA replication and correspondingly allows
the CYCA2;3 cyclin to accumulate. This A-type cyclin activates the CDKB1;1 kinase activity that is required for entering mitosis. A lossof-function uvi4 mutation causes a premature endocycle onset. Controlled plant endoreduplication in the dividing phase of Arabidopsis
leaves is also orchestrated by the atypical E2Fe/DEL1 repressor that inhibits transcription of the CCS52A2 co-activator of the plant APC/C
complex. The APC/C inhibitor OSD1/GIG1 interacts and inhibits the APC/CCDC20 complex during mitosis. Intriguingly, a loss-of-function
gig1 mutation in cotyledon cells causes endomitosis. Therefore, UVI4 and OSD1/GIG1 operate sequentially during the mitotic cell cycle
and inhibit the endocycle and endomitosis, respectively. As in fungi and metazoans, orthologous SAC proteins have been characterized in
Arabidopsis, but their roles in this surveillance mechanism need to be clarified. (This figure is available in colour at JXB online.)
Selective protein degradation to modulate the cell cycle | 2609
to identification of the transcription factor ERF115, a ratelimiting factor of QC cell division. Notably, ERF115 represents the first plant APC/C substrate that does not belong to
the basic cell-cycle machinery.
As in metazoans, APC/C inhibitory proteins have recently
been identified and characterized in Arabidopsis (Fig. 3B).
One of these is ULTRAVIOLET-B-INSENSITIVE 4 (UVI4),
which co-purified with the APC/C by tandem affinity purification and was shown to interact physically with CCS52A1/
A2 (Van Leene et al., 2010; Heyman et al., 2011; Iwata et al.,
2011). Although poorly conserved at the protein sequence
level, UVI4 shares some structural features with the animal
APC/C inhibitor EMI1 such as the D-box, which might interact with CCS52A1, whereas a C-terminal MR tail together
with the GxEN motif might mediate the interaction with
the APC/C holocomplex. Similarly to the EMI1 gene, UVI4
transcription at the G1-to-S-phase transition is regulated by
E2F transcription factors (Heyman et al., 2011). UVI4 loss
of function reduces cell division activity in roots and leaves.
At the molecular level, good evidence has been provided that
UVI4 restricts APC/CCCS52A1 activity in dividing cells, thus
allowing the cyclin CYCA2;3 to accumulate at levels required
for mitotic entry (Heyman et al., 2011; Fig. 3B). Arabidopsis
UVI4 has a homologue called OMISSION OF SECOND
DIVISION 1 (OSD1)/ GIGAS CELL 1 (GIG1) (Hase et al.,
2006; d’Erfurth et al., 2009; Heyman et al., 2011; Iwata
et al., 2011). OSD1/GIG1 like UVI4 interacts with APC/C
activators and associates in vivo with the APC/C (Van Leene
et al., 2010; Iwata et al., 2011). OSD1/GIG1 seems to act
during mitosis, most likely to repress APC/CCDC20 activity,
which would result from the accumulation of CYCB (Iwata
et al., 2011, 2012). Strikingly, an OSD1/GIG1 loss-of-function mutation triggers endomitosis in certain cotyledon cell
types, a phenomenon even exacerbated when combined with
CDC20 overexpression (Iwata et al., 2011). The exact mechanism of how OSD1/GIG1 represses endomitosis is unknown;
however, UVI4 and OSD1/GIG1 may have temporally different activities during the cell cycle (UVI4 may function earlier
than OSD1/GIG1 in the mitotic cell cycle) (Fig. 3B).
Beside these conserved regulators, the APC/C also incorporates plant-specific components such as the SAMBA protein
identified as a negative regulator of the APC/C (Eloy et al.,
2012). However, the mode of action of SAMBA at the level
of the APC/C or its substrate remains unclear. Inactivation of
SAMBA leads to a higher accumulation of cyclin CYCA2;3
protein level during early development. This CYCA2;3 accumulation in samba mutant plants may explain the increased
cell proliferation reported only at early developmental stages.
Additionally, loss of function of SAMBA also leads to defective pollen mitosis I. Whether this defect in male gametogenesis relies on stabilized cyclins has not been addressed.
A possible role of the APC/CCDC20 in the plant SAC
While mitotic cyclins are probable substrates of the APC/
CCDC20, the targeting of plant securin-like proteins by
this ubiquitin E3 ligase is still under debate. Moreover,
these proteins are poorly conserved, which precludes their
identification. However, several observations support a role
for ubiquitin-dependent proteolysis in the plant SAC. Hence,
when synchronized tobacco BY2 cells are treated with the
proteasome inhibitor MG132, they arrest in metaphase
(Genschik et al., 1998). Furthermore, the anti-microtubule
drugs propyzamide and oryzalin inhibit the degradation
of D-box containing reporter proteins and lead to the stabilization of cyclin CYCB1 (Genschik et al., 1998; Criqui
et al., 2001), suggesting that the APC/C is inactivated when
the SAC is on. Moreover, the APC15 subunit that triggers
CDC20 proteolytic ubiquitylation when the SAC is activated
is conserved in plants (Uzunova et al., 2012). Finally, orthologues of the SAC proteins BUBR1, BUB3, and MAD2 have
recently been investigated in Arabidopsis and their function
seems conserved in plants (Caillaud et al., 2009). Importantly,
Arabidopsis CDC20-1 and CDC20-2 interact at least in yeast
with MAD2 and BUBR1/MAD3, supporting a conserved
function of CDC20 proteins in the formation of the MCC
and SAC (Kevei et al., 2011). Although overall these data
support a role of the plant APC/CCDC20 in SAC regulation,
the observation that knockout of Arabidopsis APC/C does
not block cells in metaphase (Capron et al., 2003b; Kwee and
Sundaresan, 2003; Pérez-Pérez et al., 2008; Wang et al., 2012)
points to the existence of other means of disassembly of the
MCC, as reported in yeast and animals cells (Rieder and
Maiato, 2004; Primorac and Musacchio, 2013).
Endoreduplication
The endocycle, also called endoreduplication or endoreplication, is an alternative cell cycle where cells duplicate
their DNA without cell division, allowing them to increase
their ploidy. Endoreduplication is widespread in plants,
and often mitotically dividing cells switch into the endocycle as they differentiate (Breuer et al., 2010; De Veylder
et al., 2011). Endoreduplication is, however, not restricted to
plant cells and, although less common, this phenomenon is
well described in some mammalian and insect cells (Edgar
and Orr-Weaver, 2001; Lee et al., 2009). In particular, in
Drosophila salivary gland cells, APC/CFZR activity is required
in endocycling cells to mediate Geminin protein oscillation as
well as other proteins (Narbonne-Reveau et al., 2008; Zielke
et al., 2008). Geminin proteins are known to regulate DNA
replication by inhibition of the DNA replication licensing
factor CDT1, but, so far, orthologues of Geminin have not
been identified in plants.
A role of the APC/C in regulation of the plant endocycle was first demonstrated in leguminous Medicago plants
where silencing of a CCS52 gene led to a reduction in DNA
ploidy levels in leaves (Cebolla et al., 1999). The activity of
CCS52A homologues is also required in developmental
processes subjected to extensive endoreduplication such as
tomato fruit development (Mathieu-Rivet et al., 2010) and
endosperm development of rice kernels (Su’udi et al., 2012).
In Arabidopsis, downregulation of APC/C activity also leads
to a significant reduction in endoreduplication in leaves
and root cells (Serralbo et al., 2006; Marrocco et al., 2009).
2610 | Genschik et al.
Consistent with a role of A-type CCS52 genes in regulation
of the endocycle in different plant species, both Arabidopsis
CCS52A1 and CCS52A2 are also required for endoreduplication in leaf (including trichomes) and root cells (Lammens
et al., 2008; Vanstraelen et al., 2009; Kasili et al., 2010). Given
that A-type CCS52 genes act as key factors for the switch from
the mitotic to the endocycle, their expression must be tightly
controlled. Hence, it was shown that the atypical E2F transcription regulators E2Fe/DEL1 repress CCS52A2 expression
during the dividing phase of Arabidopsis leaf development to
avoid premature entry into the endocycle (Hase et al., 2006;
Lammens et al., 2008). It was shown as well that expression
of CCS52A1 is repressed by the GT2-LIKE1 (GTL1) transcriptional regulator, in order to promote termination of
ploidy-dependent cell growth such as in trichomes (Breuer
et al., 2012). This raises the question of which are the critical
substrates of the APC/CCCS52A in the regulation of endoreduplication. An interesting observation was that ectopic expression of a D-box-deficient and thus non-degradable CYCA2;3
significantly restrains endocycles in various plant organs
(Imai et al., 2006), assigning this cyclin as a good candidate
substrate for the APC/C in this process. Moreover, in UVI4deficient plants, the presumed loss of APCCCS52A1 repression
leads to increased degradation of CYCA2;3 and could thus
explain the premature entry in endoreduplication (Heyman
et al., 2011), showing that UVI4 plays an important function
as an inhibitor of the endocycle onset.
While we have slightly unravelled the function of the APC/C
in plant endoreduplication, it is likely that other ubiquitin E3
ligases are also involved in this process. For instance, it was
suggested recently that, while APC/C function is key for cell
entry into endoreplication cycles by eliminating mitotic CDK
activity, CULLIN4 (probably as part of a CRL4 complex)
might be essential for progression through endoreplication
cycles by degrading still unknown CDK inhibitor(s) to generate oscillating levels of S-phase CDK activity (Roodbarkelari
et al., 2010). Another interesting ubiquitin E3 ligase is the
HECT-type KAKTUS, which represses endoreduplication in trichomes (Downes et al., 2003; El Refy et al., 2003).
Here again, the substrate(s) regulated by KAKTUS needs
to be identified. Finally, as plant CKIs operate in a dosagedependent manner (e.g. at a low level these CKIs block only
the mitotic cycle, whereas at high level both the endocycle and mitotic cycle are arrested; reviewed by De Veylder
et al., 2011), it is likely that ubiquitin E3 ligases controlling
CKI protein stability will also have an important impact on
endoreduplication.
Meiosis
While mitosis produces genetically identical cells to the
mother cell, meiosis is another type of cell division implicated in sexual reproduction to reduce the ploidy of the
original cell and permit exchanges of genetic material by
recombination. This process involves two rounds of chromosome segregation that follow a single round of chromosome
duplication leading to the production of haploid gametes.
The complexes of cyclins and CDKs are essential for progression through both mitotic and meiotic cell cycles. The transition from meiosis I to meiosis II requires a fine balance in
cyclin–CDK activity, which must be sufficiently low to exit
meiosis I, but must nonetheless be maintained at a level sufficiently high to suppress DNA replication and promote entry
into meiosis II. This fine-tuning of CDK activity has been
shown to rely on the APC/C ubiquitin E3 ligase. Hence, in
Xenopus oocytes, at the end of meiosis I, CyclinB is only partially degraded and the resulting low level of Cdc2/CyclinB
activity is essential for entry into meiosis II (Iwabuchi et al.,
2000). This partial degradation is controlled by inhibition of
the APC/C by specific inhibitors, known as Emi2 in mouse
and Xenopus (Madgwick et al., 2006; Ohe et al., 2007) and
Mes1 in Schizosaccharomyces pombe (Kimata et al., 2008). In
addition, budding yeast also encodes a meiosis-specific coactivator of the APC/C, called AMA1, which is more similar to Cdh1 than to Cdc20 (Cooper and Strich, 2011). The
APC/CAMA1 targets several substrates including B-type cyclins (CLB in yeast), but not the anaphase inhibitor Pds1, and
is required for meiosis I reductional division and spore formation. Recently, it has been shown that degradation by the
APC/CAMA1 of NDD1, a subunit of a mitotic transcriptional
activator complex inducing the expression of CLB1/2 and
other regulators, suppresses mitotic cell-cycle control during
prophase I (Okaz et al., 2012). Indeed, an extended duration
of prophase I, controlled by the recombination checkpoint,
is essential for homologous chromosome recombination
to occur.
In plants, recent progress has indicated that APC/C inhibition is also required to promote meiotic progression, and this
is achieved by OSD1/GIG1. Mutations of OSD1 were shown
initially to trigger failures to enter the second division during both male and female meioses producing diploid spores
or gametes (d’Erfurth et al., 2009). Moreover, the demonstration that OSD1 is structurally and functionally related to the
animal Emi2 or yeast MES1 proteins was published recently
(Cromer et al., 2012). OSD1/GIG1 contains three APC/C
interaction domains (a D-box, a KEN-box, and an MR tail)
and physically interacts with most Arabidopsis APC/C co-activators (Iwata et al., 2011; Cromer et al., 2012). Evidence that
OSD1 inhibits the APC/C is also supported by the fact that
its expression in mouse oocytes leads to a metaphase I arrest.
Interestingly, mutation of TARDYASYNCHRONOUS
MEIOSIS (TAM) encoding the A-type cyclin CYCA1;2 leads
to a similar phenotype as osd1 (i.e. blocking of the transition
from meiosis I to meiosis II; d’Erfurth et al., 2010). Combining
tam and osd1 mutations leads to a failure of the transition
from prophase to the first meiotic division (d’Erfurth et al.,
2010). As the level of CDK activity is important for regulation of these transitions, a moderate decrease in CDK activity
in osd1 and cycA1;2/tam single mutants may cause failure to
enter meiosis II without impairing the prophase-to-meiosis-I
transition. In contrast, the coincident depletion of OSD1 and
CYCA1;2/TAM may further decrease CDK activity, impairing entry into meiosis I. To date, CYCA1;2/TAM is the only
known cyclin affecting meiotic progression, but its exact function will need further investigation. As CDKA;1 associated
Selective protein degradation to modulate the cell cycle | 2611
with CYCA1;2/TAM is able to phosphorylate OSD1/GIG1,
at least in vitro, this meiotic cyclin may control OSD1/GIG1
activity and/or stability.
Conclusion and some perspectives
Although we are still far from understanding the mechanistic details of how ubiquitin E3 ligases control the plant cell
cycle, it is clear that, in the last decade, a number of important contributions from several laboratories have provided
a first picture of these regulations. In particular, new results
have highlighted the importance of the plant APC/C core
complex and its activators during different type of processes
such as gametogenesis, plant growth, and endoreduplication
onset. Conversely, less understood is the role of SCF-type
E3s in cell-cycle regulation and especially in the transition
from G1 to S phase. While the SCFFBL17 and the RINGfinger proteins RHF1a/RHF2a are key cell-cycle regulators
during gametogenesis, the situation is far less clear at the sporophytic stage where essential E3s still wait to be discovered.
It is also unclear whether other CRL families such as CRL3
and CRL4 contribute to the regulation of the plant cell cycle
as they do in metazoans.
Another important challenge for the future will be to identify substrates for all these plant E3s, and it is likely, based on
animal models, that each will have more than one substrate.
Identifying these substrates will be complicated further by the
fact that some need to be modified at the post-translational
level in order to bind their ubiquitin E3 ligases. This is of
peculiar importance for SCFs where phosphorylation of the
substrate is often a prerequisite for their recognition by these
E3s. At present, we know very little about post-translational
modifications of plant regulatory cell-cycle proteins.
After binding of the substrate by its dedicated E3 ubiquitin ligase, the ubiquitylation reaction can proceed, and it is
well established that the formation of polyubiquitin chains
through lysine 48 (K48) triggers degradation of the substrate
by the proteasome. Interestingly, some substrates are also
modified by atypical polyubiquitin chains and/or by monoubiquitylation. For instance, in mammals it was found that
the APC/C assembles polyubiquitin chains linked to lysine
11 (K11) (Jin et al., 2008) that nevertheless also lead to proteasomal degradation. Whether this is true for the degradation of mitotic substrates by the plant APC/C remains to
be demonstrated. In mammals, CRL3 E3 ligases have been
shown to play important functions during mitosis (reviewed
by Genschik et al., 2013). In particular, some of these E3s
mono-ubiquitylate cell-cycle kinases such as Aurora B and
Polo-like kinase 1 (PLK1) (Maerki et al., 2009; Beck et al.,
2013), controlling their subcellular localization but not their
stability. Whether such mechanisms exist in plants remains
presently also at the level of pure speculation.
Besides unravelling conserved cell-cycle functions of plant
E3s, another interesting research avenue is to elucidate how
these enzymes regulate and are regulated by endogenous
and exogenous plant signals. For instance, it was proposed
recently that an abiotic stress such as drought mediates mitotic
exit and earlier onset of endoreplication by modulating the
activity of the APC/C (Claeys et al., 2012). Drought, but
also other stresses, acts by decreasing the amount in bioactive gibberellins, resulting in an increase in the accumulation
of the nuclear-localized growth repressing DELLA proteins
(reviewed by Achard and Genschik, 2009). How DELLAs are
linked to APC/C activity will require further investigations.
A link between the APC/C and auxin has also been highlighted in several reports (Blilou et al., 2002; Lindsay et al.,
2011; Wang et al., 2012) but remains poorly understood at the
mechanistic levels. Auxin has also been shown to positively
affect the stability of the cell-cycle transcription factor E2Fb
that promotes cell division (Magyar et al., 2005). However,
the mechanism by which auxin avoids E2Fb proteolysis is still
not known. Moreover, a recent striking finding is that auxin
binds directly to the Arabidopsis F-box protein SKP2A to promote E2Fc/DPB degradation, directly connecting auxin with
cell-cycle control (Jurado et al., 2010). Auxin also induces the
degradation of SKP2A, thus adding another level of control.
Thus, the above-mentioned examples represent only a few
challenges among others that await our research community
in the near future. Nevertheless, our success in this area will
probably depend on our capacity to adapt methods for plants
from the animal ubiquitin toolbox and advanced proteomics.
References
Abbas T, Sivaprasad U, Terai K, Amador V, Pagano M, Dutta A.
2008. PCNA-dependent regulation of p21 ubiquitylation and
degradation via the CRL4Cdt2 ubiquitin ligase complex. Genes and
Development 22, 2496–2506.
Achard P, Genschik P. 2009. Releasing the brakes of plant growth:
how GAs shutdown DELLA proteins. Journal of Experimental Botany
60, 1085–1092.
Beck J, Maerki S, Posch M, Metzger T, Persaud A, Scheel H,
Hofmann K, Rotin D, Pedrioli P, Swedlow JR, Peter M, Sumara
I. 2013. Ubiquitylation-dependent localization of PLK1 in mitosis.
Nature Cell Biology 15, 430–439.
Blilou I, Frugier F, Folmer S, Serralbo O, Willemsen V, Wolkenfelt
H, Eloy NB, Ferreira PC, Weisbeek P, Scheres B. 2002. The
Arabidopsis HOBBIT gene encodes a CDC27 homolog that links
the plant cell cycle to progression of cell differentiation. Genes and
Development 16, 2566–2575.
Brandeis M, Hunt T. 1996. The proteolysis of mitotic cyclins in
mammalian cells persists from the end of mitosis until the onset of S
phase. EMBO Journal 15, 5280–5289.
Breuer C, Ishida T, Sugimoto K. 2010. Developmental control of
endocycles and cell growth in plants. Current Opinion in Plant Biology
13, 654–660.
Breuer C, Morohashi K, Kawamura A, Takahashi N, Ishida T,
Umeda M, Grotewold E, Sugimoto K. 2012. Transcriptional
repression of the APC/C activator CCS52A1 promotes active
termination of cell growth. EMBO Journal 31, 4488–4501.
Caillaud MC, Paganelli L, Lecomte P, Deslandes L, Quentin
M, Pecrix Y, Le Bris M, Marfaing N, Abad P, Favery B. 2009.
2612 | Genschik et al.
Spindle assembly checkpoint protein dynamics reveal conserved and
unsuspected roles in plant cell division. PLoS One 4, e6757.
Capron A, Okrész L, Genschik P. 2003a. First glance at the plant
APC/C, a highly conserved ubiquitin-protein ligase. Trends in Plant
Science 8, 83–89.
Capron A, Serralbo O, Fülöp K, et al. 2003b. The Arabidopsis
anaphase-promoting complex or cyclosome: molecular and genetic
characterization of the APC2 subunit. Plant Cell 15, 2370–2382.
Cebolla A, Vinardell JM, Kiss E, Oláh B, Roudier F, Kondorosi
A, Kondorosi E. 1999. The mitotic inhibitor CCS52 is required for
endoreduplication and ploidy-dependent cell enlargement in plants.
EMBO Journal 18, 4476–4484.
Churchman ML, Brown ML, Kato N, et al. 2006. SIAMESE, a
plant-specific cell cycle regulator, controls endoreplication onset in
Arabidopsis thaliana. Plant Cell 18, 3145–3157.
Ciechanover A, Orian A, Schwartz AL. 2000. Ubiquitin-mediated
proteolysis: biological regulation via destruction. Bioessays 22,
442–451.
Claeys H, Skirycz A, Maleux K, Inzé D. 2012. DELLA signaling
mediates stress-induced cell differentiation in Arabidopsis leaves
through modulation of anaphase-promoting complex/cyclosome
activity. Plant Physiology 159, 739–747.
KAKTUS gene encodes a HECT protein and controls the number of
endoreduplication cycles. Molecular Genetics and Genomics 270,
403–414.
Eloy NB, de Freitas Lima M, Van Damme D, Vanhaeren H,
Gonzalez N, De Milde L, Hemerly AS, Beemster GT, Inzé D,
Ferreira PC. 2011. The APC/C subunit 10 plays an essential role in cell
proliferation during leaf development. The Plant Journal 68, 351–63.
Eloy NB, Gonzalez N, Van Leene J, et al. 2012. SAMBA, a plantspecific anaphase-promoting complex/cyclosome regulator is involved
in early development and A-type cyclin stabilization. Proceedings of
the National Academy of Sciences, USA 109, 13853–13858.
Feldman RM, Correll CC, Kaplan KB, Deshaies RJ. 1997. A
complex of Cdc4p, Skp1p and Cdc53p/cullin catalyzes ubiquitination
of the phosphorylated CDK inhibitor Sic1p. Cell 91, 221–230.
Foster SA, Morgan DO. 2012. The APC/C subunit Mnd2/Apc15
promotes Cdc20 autoubiquitination and spindle assembly checkpoint
inactivation. Molecular Cell 47, 921–932.
Frescas D, Pagano M. 2008. Deregulated proteolysis by the F-box
proteins SKP2 and β-TrCP: tipping the scales of cancer. Nature
Reviews Cancer 8, 438–449.
Cooper KF, Strich R. 2011. Meiotic control of the APC/C: similarities
& differences from mitosis. Cell Division 6, 16.
Frye JJ, Brown NG, Petzold G, et al. 2013. Electron microscopy
structure of human APC/C(CDH1)-EMI1 reveals multimodal
mechanism of E3 ligase shutdown. Nature Structural and Molecular
Biology 20, 827–835.
Criqui MC, Weingartner M, Capron A, Parmentier Y, Shen
WH, Heberle-Bors E, Bögre L, Genschik P. 2001. Sub-cellular
localisation of GFP-tagged tobacco mitotic cyclins during the cell
cycle and after spindle checkpoint activation. The Plant Journal 28,
569–581.
Fülöp K, Tarayre S, Kelemen Z, Horváth G, Kevei Z, Nikovics K,
Bakó L, Brown S, Kondorosi A, Kondorosi E. 2005. Arabidopsis
anaphase-promoting complexes: multiple activators and wide
range of substrates might keep APC perpetually busy. Cell Cycle 4,
1084–1092.
Cromer L, Heyman J, Touati S, et al. 2012. OSD1 promotes
meiotic progression via APC/C inhibition and forms a regulatory
network with TDM and CYCA1;2/TAM. PLoS Genetics 8, e1002865.
Genschik P, Criqui MC, Parmentier Y, Derevier A, Fleck J.
1998. Cell cycle-dependent proteolysis in plants. Identification of the
destruction box pathway and metaphase arrest produced by the
proteasome inhibitor MG132. Plant Cell 10, 2063–2076.
De Veylder L, Larkin JC, Schnittger A. 2011. Molecular control and
function of endoreplication in development and physiology. Trends in
Plant Science 16, 624–634.
del Pozo JC, Diaz-Trivino S, Cisneros N, Gutierrez C. 2006. The
balance between cell division and endoreplication depends on E2FCDPB, transcription factors regulated by the ubiquitin-SCFSKP2A
pathway in Arabidopsis. Plant Cell 18, 2224–2235.
d’Erfurth I, Cromer L, Jolivet S, Girard C, Horlow C, Sun Y,
To JP, Berchowitz LE, Copenhaver GP, Mercier R. 2010. The
cyclin-A CYCA1;2/TAM is required for the meiosis I to meiosis II
transition and cooperates with OSD1 for the prophase to first meiotic
division transition. PLoS Genetics 6, e1000989.
d’Erfurth I, Jolivet S, Froger N, Catrice O, Novatchkova M,
Mercier R. 2009. Turning meiosis into mitosis. PLoS Biology 7,
e1000124.
Downes BP, Stupar RM, Gingerich DJ, Vierstra RD. 2003. The
HECT ubiquitin-protein ligase (UPL) family in Arabidopsis: UPL3 has a
specific role in trichome development. The Plant Journal 35, 729–742.
Edgar BA, Orr-Weaver TL. 2001. Endoreplication cell cycles: more
for less. Cell 4, 297–306.
El Refy A, Perazza D, Zekraoui L, Valay JG, Bechtold N, Brown
S, Hülskamp M, Herzog M, Bonneville JM. 2003. The Arabidopsis
Genschik P, Sumara I, Lechner E. 2013. The emerging family
of CULLIN3-RING ubiquitin ligases (CRL3s): cellular functions and
disease implications. EMBO Journal 32, 2307–2320.
Guardavaccaro D, Kudo Y, Boulaire J, Barchi M, Busino L,
Donzelli M, Margottin-Goguet F, Jackson PK, Yamasaki L,
Pagano M. 2003. Control of meiotic and mitotic progression by the F
box protein beta-Trcp1 in vivo. Developmental Cell 4, 799–812.
Gusti A, Baumberger N, Nowack M, Pusch S, Eisler H, Potuschak
T, De Veylder L, Schnittger A, Genschik P. 2009. The Arabidopsis
thaliana F-box protein FBL17 is essential for progression through the
second mitosis during pollen development. PLoS One 4, e4780.
Hase Y, Trung KH, Matsunaga T, Tanaka A. 2006. A mutation in
the uvi4 gene promotes progression of endo-reduplication and confers
increased tolerance towards ultraviolet B light. The Plant Journal 46,
317–326.
Havens CG, Walter JC. 2011. Mechanism of CRL4Cdt2, a PCNAdependent E3 ubiquitin ligase. Genes and Development 25,
1568–1582.
Heyman J, Cools T, Vandenbussche F, et al. 2013. ERF115
controls root quiescent center cell division and stem cell
replenishment. Science 342, 860–863.
Selective protein degradation to modulate the cell cycle | 2613
Heyman J, De Veylder L. 2012. The anaphase-promoting complex/
cyclosome in control of plant development. Molecular Plant 5,
1182–1194.
Heyman J, Van den Daele H, DeWit K, et al. 2011. Arabidopsis
ULTRAVIOLET-B-INSENSITIVE4 maintains cell division activity by
temporal inhibition of the Anaphase-Promoting Complex/Cyclosome.
Plant Cell 23, 4394–4410.
Hua Z, Viestra R. 2011. The cullin-RING ubiquitin-protein ligases.
Annual Review of Plant Biology 62, 299–334.
Imai KK, Ohashi Y, Tsuge T, Yoshizumi T, Matsui M, Oka A,
Aoyama T. 2006. The A-type cyclin CYCA2;3 is a key regulator of
ploidy levels in Arabidopsis endoreduplication. Plant Cell 18, 3823–96.
Iwabuchi M, Ohsumi K, Yamamoto TM, Sawada W, Kishimoto T.
2000. Residual Cdc2 activity remaining at meiosis I exit is essential for
meiotic M-M transition in Xenopus oocyte extracts. EMBO Journal 19,
4513–4523.
Iwata E, Ikeda S, Abe N. 2012. Roles of GIG1 and UVI4 in
genome duplication in Arabidopsis thaliana. Plant Signal Behavior 7,
1079–1081.
Iwata E, Ikeda S, Matsunaga S, Kurata M, Yoshioka Y, Criqui
MC, Genschik P, Ito M. 2011. GIGAS CELL1, a novel negative
regulator of the Anaphase-Promoting Complex/Cyclosome, is
required for proper mitotic progression and cell fate determination in
Arabidopsis. Plant Cell 23, 4382–4393.
Jin L, Williamson A, Banerjee S, Philipp I, Rape M. 2008.
Mechanism of ubiquitin-chain formation by the human anaphasepromoting complex. Cell 133, 653–665.
Jurado S, Abraham Z, Manzano C, López-Torrejón G, Pacios LF,
Del Pozo JC. 2010. The Arabidopsis cell cycle F-box protein SKP2A
binds to auxin. Plant Cell 22, 3891–3904.
Kasili R, Walker JD, Simmons LA, Zhou J, De Veylder L, Larkin
JC. 2010. SIAMESE cooperates with the CDH1-like protein CCS52A1
to establish endoreplication in Arabidopsis thaliana trichomes.
Genetics 185, 257–268.
Kevei Z, Baloban M, Da Ines O, Tiricz H, Kroll A, Regulski K,
Mergaert P, Kondorosi E. 2011. Conserved CDC20 cell cycle
functions are carried out by two of the five isoforms in Arabidopsis
thaliana. PLoS One 6, e20618.
Kim HJ, Oh SA, Brownfield L, Hong SH, Ryu H, Hwang I,
Twell D, Nam HG. 2008b. Control of plant germline proliferation
by SCF(FBL17) degradation of cell cycle inhibitors. Nature 455,
1134–1137.
Kim S, Yu H. 2011. Mutual regulation between the spindle checkpoint
and APC/C. Seminars in Cell and Developmental Biology 22,
551–558.
Kim Y, Starostina NG, Kipreos ET. 2008a. The CRL4Cdt2 ubiquitin
ligase targets the degradation of p21Cip1 to control replication
licensing. Genes and Development 22, 2507–2519.
Kimata Y, Trickey M, Izawa D, Gannon J, Yamamoto M, Yamano
H. 2008. A mutual inhibition between APC/C and its substrate Mes1
required for meiotic progression in fission yeast. Developmental Cell
14, 446–454.
Kwee HS, Sundaresan V. 2003. The NOMEGA gene required for
female gametophyte development encodes the putative APC6/CDC16
component of the Anaphase Promoting Complex in Arabidopsis. The
Plant Journal 36, 853–866.
Lammens T, Boudolf V, Kheibarshekan L, et al. 2008. Atypical
E2F activity restrains APC/CCCS52A2 function obligatory for
endocycle onset. Proceedings of the National Academy of Sciences,
USA 105, 14721–14726.
Lee HO, Davidson JM, Duronio RJ. 2009. Endoreplication:
polyploidy with purpose. Genes and Development 23, 2461–2477.
Lindsay DL, Bonham-Smith PC, Postnikoff S, Gray GR,
Harkness TA. 2011. A role for the anaphase promoting complex in
hormone regulation. Planta 233, 1223–1235.
Liu J, Zhang Y, Qin G, et al. 2008. Targeted degradation of the
cyclin-dependent kinase inhibitor ICK4/KRP6 by RING-type E3 ligases
is essential for mitotic cell cycle progression during Arabidopsis
gametogenesis. Plant Cell 20, 1538–1554.
Madgwick S, Hansen DV, Levasseur M, Jackson PK, Jones KT.
2006. Mouse Emi2 is required to enter meiosis II by reestablishing
cyclin B1 during interkinesis. Journal of Cell Biology 174, 791–801.
Maerki S, Olma MH, Staubli T, Steigemann P, Gerlich DW,
Quadroni M, Sumara I, Peter M. 2009. The Cul3-KLHL21
E3 ubiquitin ligase targets aurora B to midzone microtubules in
anaphase and is required for cytokinesis. Journal of Cell Biology
187, 791–800.
Magyar Z, De Veylder L, Atanassova A, Bakó L, Inzé D,
Bögre L. 2005. The role of the Arabidopsis E2FB transcription
factor in regulating auxin-dependent cell division. Plant Cell 17,
2527–2541.
Mansfeld J, Collin P, Collins MO, Choudhary JS, Pines J. 2011.
APC15 drives the turnover of MCC-CDC20 to make the spindle
assembly checkpoint responsive to kinetochore attachment. Nature
Cell Biology 13, 1234–1243.
Margottin-Goguet F, Hsu JY, Loktev A, Hsieh HM, Reimann JD,
Jackson PK. 2003. Prophase destruction of Emi1 by the SCF(βTrCP/
Slimb) ubiquitin ligase activates the anaphase promoting complex
to allow progression beyond prometaphase. Developmental Cell 4,
813–826.
Marrocco K, Bergdoll M, Achard P, Criqui MC, Genschik P.
2010. Selective proteolysis sets the tempo of the cell cycle. Current
Opinion in Plant Biology 13, 631–639.
Marrocco K, Thomann A, Parmentier Y, Genschik P, Criqui
MC. 2009. The APC/C E3 ligase remains active in most post-mitotic
Arabidopsis cells and is required for proper vasculature development
and organization. Development 136, 1475–1485.
Mathieu-Rivet E, Gévaudant F, Sicard A, et al. 2010. Functional
analysis of the anaphase promoting complex activator CCS52A
highlights the crucial role of endo-reduplication for fruit growth in
tomato. The Plant Journal 62, 727–741.
Miller JJ, Summers MK, Hansen DV, Nachury MV, Lehman NL,
Loktev A, Jackson PK. 2006. Emi1 stably binds and inhibits the
anaphase-promoting complex/cyclosome as a pseudosubstrate
inhibitor. Genes and Development 20, 2410–2420.
Mocciaro A, Rape M. 2012. Emerging regulatory mechanisms in
ubiquitin-dependent cell cycle control. Journal of Cell Science 125,
255–263.
2614 | Genschik et al.
Musacchio A, Ciliberto A. 2012. The spindle-assembly checkpoint
and the beauty of self-destruction. Nature Structural and Molecular
Biology 19, 1059–1061.
Nakayama K, Nakayama KI. 2004. Cytoplasmic ubiquitin ligase
KPC regulates proteolysis of p27Kip1 at G1 phase. Nature Cell Biology
6, 1229–1235.
Narbonne-Reveau K, Senger S, Pal M, Herr A, Richardson HE,
Asano M, Deak P, Lilly MA. 2008. APC/CFzr/Cdh1 promotes cell
cycle progression during the Drosophila endocycle. Development 135,
1451–1461.
Nurse P. 2000. A long twentieth century of the cell cycle and beyond.
Cell 100, 71–78.
Ohe M, Inoue D, Kanemori Y, Sagata N. 2007. Erp1/Emi2 is
essential for the meiosis I to meiosis II transition in Xenopus oocytes.
Developmental Biology 303, 157–164.
Okaz E, Argüello-Miranda O, Bogdanova A, Vinod PK, Lipp
JJ, Markova Z, Zagoriy I, Novak B, Zachariae W. 2012. Meiotic
prophase requires proteolysis of M phase regulators mediated by the
meiosis-specific APC/CAma1. Cell 151, 603–618.
Peres A, Churchman ML, Hariharan S, et al. 2007. Novel plantspecific cyclin-dependent kinase inhibitors induced by biotic and
abiotic stresses. Journal of Biological Chemistry 282, 25588–25596.
Pérez-Pérez JM, Serralbo O, Vanstraelen M, González C, Criqui
MC, Genschik P, Kondorosi E, Scheres B. 2008. Specialization
of CDC27 function in the Arabidopsis thaliana anaphase-promoting
complex (APC/C). The Plant Journal 53, 78–89.
Pesin JA, Orr-Weaver TL. 2008. Regulation of APC/C activators
in mitosis and meiosis. Annual Review of Cell and Developmental
Biology 24, 475–499.
Peters JM. 2006. The anaphase promoting complex/cyclosome: a
machine designed to destroy. Nature Reviews Molecular Cell Biology
7, 644–656.
Petroski MD, Deshaies RJ. 2005. Function and regulation of cullinRING ubiquitin ligases. Nature Reviews Molecular Cell Biology 6,
9–20.
Pines J. 2006. Mitosis: a matter of getting rid of the right protein at
the right time. Trends in Cell Biology 16, 55–63.
Primorac I, Musacchio A. 2013. Panta rhei: the APC/C as steady
state. Journal of Cell Biology 201, 177–189.
Ren H, Santner A, del Pozo JC, Murray JA, Estelle M. 2008.
Degradation of the cyclin-dependent kinase inhibitor KRP1 is
regulated by two different ubiquitin E3 ligases. The Plant Journal 53,
705–716.
Rieder C, Maiato H. 2004. Stuck in division or passing through: what
happens when cells cannot satisfy the spindle assembly checkpoint.
Developmental Cell 7, 637–651.
Rojas CA, Eloy NB, Lima Mde F, Rodrigues RL, Franco LO,
Himanen K, Beemster GT, Hemerly AS, Ferreira PC. 2009.
Overexpression of the Arabidopsis anaphase promoting complex
subunit CDC27a increases growth rate and organ size. Plant
Molecular Biology 71, 307–318.
Roodbarkelari F, Bramsiepe J, Weinl C, Marquardt S, Novák B,
Jakoby MJ, Lechner E, Genschik P, Schnittger A. 2010. Cullin
4-ring finger-ligase plays a key role in the control of endoreplication
cycles in Arabidopsis trichomes. Proceedings of the National
Academy of Sciences, USA 107, 15275–15280.
Saze H and Kakutani T. 2007. Heritable epigenetic mutation of a
transposon-flanked Arabidopsis gene due to lack of the chromatinreodeling factor DDM1. EMBO Journal 26, 3641–3652.
Schwob E, Bohm T, Mendenhall MD, Nasmyth K. 1994. The
B-type cyclin kinase inhibitor p40SIC1 controls the G1 to S transition
in S. cerevisiae. Cell 79, 233–244.
Serralbo O, Pérez-Pérez JM, Heidstra R, Scheres B. 2006. Noncell-autonomous rescue of anaphase-promoting complex function
revealed by mosaic analysis of HOBBIT, an Arabidopsis CDC27
homolog. Proceedings of the National Academy of Sciences, USA
103, 13250–13255.
Starostina NG, Kipreos ET. 2012. Multiple degradation pathways
regulate versatile CIP/KIP CDK inhibitors. Trends in Cell Biology 22,
33–41.
Su’udi M, Cha JY, Jung MH, Ermawati N, Han CD, Kim MG,
Woo YM, Son D. 2012. Potential role of the rice OsCCS52A gene in
endoreduplication. Planta 235, 387–397.
Tarayre S, Vinardell JM, Cebolla A, Kondorosi A, Kondorosi
E. 2004. Two classes of the CDh1-type activators of the anaphasepromoting complex in plants: novel functional domains and distinct
regulation. Plant Cell 16, 422–434.
Thornton BR, Toczyski DP. 2006. Precise destruction: an emerging
picture of the APC. Genes and Development 20, 3069–3078.
Torres Acosta JA, Fowke LC, Wang H. 2011. Analyses of
phylogeny, evolution, conserved sequences and genome-wide
expression of the ICK/KRP family of plant CDK inhibitors. Annals of
Botany 107, 1141–1157.
Uzunova K, Dye BT, Schutz H, et al. 2012. APC15 mediates
CDC20 autoubiquitylation by APC/CMCC and disassembly of the
mitotic checkpoint complex. Nature Structural and Molecular Biology
19, 1116–1123.
Van Leene J, Hollunder J, Eeckhout D, et al. 2010. Targeted
interactomics reveals a complex core cell cycle machinery in
Arabidopsis thaliana. Molecular Systems Biology 6, 397.
van Leuken R, Clijsters L, Wolthuis R. 2008. To cell cycle, swing
the APC/C. Biochimica and Biophysica Acta 1786, 49–59.
Vanstraelen M, Baloban M, Da Ines O, Cultrone A, Lammens
T, Boudolf V, Brown SC, De Veylder L, Mergaert P, Kondorosi
E. 2009. APC/C-CCS52A complexes control meristem maintenance
in the Arabidopsis root. Proceedings of the National Academy of
Sciences, USA 106, 11806–11811.
Verkest A, Manes CL, Vercruysse S, Maes S, Van Der Schueren E,
Beeckman T, Genschik P, Kuiper M, Inze D, De Veylder L. 2005b.
The cyclin-dependent kinase inhibitor KRP2 controls the onset of the
endoreduplication cycle during Arabidopsis leaf development through
inhibition of mitotic CDKA;1 kinase complexes. Plant Cell 17, 1723–1736.
Verkest A, Weinl C, Inzé D, De Veylder L, Schnittger A. 2005a.
Switching the cell cycle. Kip-related proteins in plant cell cycle control.
Plant Physiology 139, 1099–1106.
Vierstra RD. 2009. The ubiquitin-26S proteasome system at the
nexus of plant biology. Nature Reviews Molecular Cell Biology 10,
385–397.
Selective protein degradation to modulate the cell cycle | 2615
Wang W, Kirschner MW. 2013. Emi1 preferentially inhibits ubiquitin
chain elongation by the anaphase-promoting complex. Nature Cell
Biology 15, 797–806.
Wang Y, Hou Y, Gu H, Kang D, Chen Z, Liu J, Qu LJ. 2012. The
Arabidopsis APC4 subunit of the anaphase-promoting complex/
cyclosome (APC/C) is critical for both female gametogenesis and
embryogenesis. The Plant Journal 69, 227–240.
Wang Y, Hou Y, Gu H, Kang D, Chen ZL, Liu J, Qu LJ. 2013. The
Arabidopsis anaphase-promoting complex/cyclosome subunit 1 is
critical for both female gametogenesis and embryogenesis(F). Journal
of Integrative Plant Biology 55, 64–74.
Yamano H, Tsurumi C, Gannon J, Hunt T. 1998. The role of the
destruction box and its neighbouring lysine residues in cyclin B for
anaphase ubiquitin-dependent proteolysis in fission yeast: defining the
D-box receptor. EMBO Journal 17, 5670–5678.
Zhao X, Harashima H, Dissmeyer N. et al. 2012. A general G1/Sphase cell-cycle control module in the flowering plant Arabidopsis
thaliana. PLoS Genetics 8, e1002847.
Zheng B, Chen X, McCormick S. 2011. The anaphase-promoting
complex is a dual integrator that regulates both MicroRNA-mediated
transcriptional regulation of cyclin B1 and degradation of Cyclin B1 during
Arabidopsis male gametophyte development. Plant Cell 23, 1033–46.
Zielke N, Querings S, Rottig C, Lehner C, Sprenger F. 2008.
The anaphase-promoting complex/cyclosome (APC/C) is required
for rereplication control in endoreplication cycles. Genes and
Development 22, 1690–1703.