Download Composition of the plant nuclear envelope: theme and variations

Journal of Experimental Botany, Vol. 58, No. 1, pp. 27–34, 2007
Intracellular Compartmentation: Biogenesis and Function Special Issue
doi:10.1093/jxb/erl009 Advance Access publication 27 June, 2006
SPECIAL ISSUE PAPER
Composition of the plant nuclear envelope: theme and
variations
Iris Meier*
Plant Biotechnology Center and Department of Plant Cellular and Molecular Biology, The Ohio State University,
244 Rightmire Hall, 1060 Carmack Road, Columbus, OH 43210, USA
Received 20 February 2006; Accepted 30 March 2006
The nuclear envelope is the hallmark of all eukaryotic
cells, separating the nucleoplasm from the cytoplasm.
At the same time, the nuclear envelope allows for the
controlled exchange of macromolecules between the
two compartments through nuclear pores and presents
a surface for anchoring and organizing cytoskeletal
components and chromatin. Although our molecular
understanding of the nuclear envelope in higher plants
is only just beginning, fundamental differences from
the animal nuclear envelope have already been found.
This review provides an updated investigation of these
differences with respect to nuclear pore complexes,
targeting of Ran signalling to the nuclear envelope,
inner nuclear envelope proteins, and the role and fate
of the nuclear envelope during mitosis.
Key words: Lamin, NMCP1, nuclear envelope, nuclear export,
nuclear import, nuclear pore, Ran cycle.
Introduction
The nuclear envelope (NE) is the hallmark of all eukaryotic
cells, separating the nucleoplasm from the cytoplasm
during interphase and allowing for the controlled exchange
of macromolecules between the two compartments. Nuclear pores are large multiprotein complexes of 8-fold
symmetry that span the inner and outer nuclear membranes.
The outer nuclear membrane (ONM) is continuous with the
endoplasmic reticulum, while the inner nuclear membrane
(INM) contains a number of INM-specific integral and
peripheral membrane proteins and is lined by the nuclear
lamina in metazoan cells (reviewed by Holaska et al.,
2002). While the protein composition of the NE has been
studied in detail in animal cells, present knowledge of the
components of the plant NE is still at an early stage.
Recently, putative plant nucleoporins (Nups) have been
identified through specific mutant phenotypes related to
plant–microbe interactions. Most components of nucleocytoplasmic trafficking have been identified in plants. Their
over- and underexpression reveals surprising connections
to known plant regulatory pathways.
Plant nuclear envelope composition
The inner nuclear envelope
A number of specific proteins are associated with the INM,
either by direct interaction with the membrane or by
protein–protein interactions with membrane-associated
proteins. The lamins, a subgroup of the intermediatefilament proteins, form the protein meshwork of the nuclear
lamina in animal cells. They are involved in attaching
chromatin to the inner surface of the NE and in a number of
additional functions such as NE assembly, DNA synthesis,
transcription, and apoptosis (Goldman et al., 2002; Holaska
et al., 2002). Mutations in A-type lamins or associated
proteins have been linked to at least six different inherited
diseases in humans affecting skeletal and cardiac muscle
and/or the loss and redistribution of white fat (Mounkes
et al., 2003), demonstrating the importance of NE function
in certain types of animal cells.
Electron microscopy (EM) studies revealed that a structure similar to the vertebrate nuclear lamina is also found in
the nuclei of higher plant cells (Galcheva-Gargova et al.,
1988; Moreno Dı́az de la Espina et al., 1991). However,
there is no evidence for the existence of bona fide lamins in
plants. Immunohistochemical studies suggested the existence of lamin-like proteins in plant nuclei (Li and Roux,
1992; McNulty and Saunders, 1992; Mı́nguez and Moreno
Dı́az de la Espina, 1993), but no sequence information from
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Abstract
28 Meier
Fig. 1. Comparison of localization and structural organization of human
lamin B and Arabidopsis nuclear matrix constituent proteins (NMCPs).
While almost twice the size of lamins, NMCPs have a comparable
domain organization of short head, coiled-coil centre, and longer tail
domain. The gene identifiers of the four Arabidopsis homologues of
DcNMCP1 are shown.
of the mammalian lamin B receptor (LBR) was targeted
successfully to the NE in tobacco cells, consistent with its
animal cell localization pattern (Irons et al., 2003). LBR is
an integral membrane protein of the INM, and protein and
DNA database analysis showed no evidence for LBR
homologues in plants. The N-terminal portion of LBR
comprises the nucleoplasmic domain containing a bipartite
NLS and a transmembrane domain. This domain is
sufficient to anchor the protein in the INM (Smith and
Blobel, 1993; Soullam and Worman, 1993) and can be used
as an NE marker in mammalian cells (Ellenberg et al.,
1997). LBR not only targets to the NE in mammalian and
plant cells, but also in yeast (Smith and Blobel, 1994),
indicating the existence of a common mechanism for the
targeting of INM proteins in eukaryotic cells.
The outer nuclear envelope
Spectrin-related proteins of the a-actinin superfamily have
been known to be associated with the plasma membrane,
but have recently been found associated with the nuclear
envelope, as well (reviewed in Worman and Gundersen,
2006). Spectrin-repeat-containing nuclear envelope proteins have been named nesprins. Nesprin-1 in the INM
interacts with the lamina components lamin A and emerin
(Mislow et al., 2002). Nesprin-2, also called NUANCE,
is a 796 kDa protein located at the ONM, where it might
function in connecting the nucleus with the cytoskeleton
through its interaction with actin (Zhen et al., 2002).
Attachment of nesprin-2 and nesprin-1 to the ONM and
INM, respectively, is achieved by the presence of a Cterminal transmembrane domain (Zhang et al., 2001; Zhen
et al., 2002). An antiserum directed against chicken
spectrin detects an antigenic component at the plant NE,
the only current evidence that this class of proteins might
exist in plants (de Ruijter et al., 2000). In vertebrates, there
is evidence for a role of nesprins in positioning the nucleus
within the cell (Mosley-Bishop et al., 1999; Apel et al.,
2000; Worman and Gundersen, 2006). In this context, it is
worthwhile to recall earlier evidence for the regulated,
actin-dependent rapid migration of plant nuclei to the site of
fungal infection (Gross et al., 1993), which is mechanistically still not understood (Schmelzer, 2002). Nesprins,
which are actin-binding proteins located on the ONM,
could be prime candidates for anchoring the plant nucleus
to a signal-perceiving apparatus at the plasma membrane.
The next step here will be to identify plant equivalents
of nesprins and to test (e.g. by loss-of-function approaches)
if they affect plant nuclear migration.
Microtubule organizing centres
During nuclear division, the genetic material inside the
nucleus is distributed into two daughter nuclei by the action
of the microtubular spindle apparatus. The microtubule
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the antigens detected in these studies is available, and no
lamin-coding genes seem to be present in the fully
sequenced Arabidopsis and rice genomes. This might
indicate that plants, like yeast, do not have this group of
nuclear coiled-coil proteins. The question remains, which
components make up the structures detected by EM and
which plant proteins are detected by the anti-lamin antisera.
NMCP1 is a 134 kDa carrot nuclear matrix protein found
exclusively at the periphery of the nucleus during interphase and is associated with the spindle during mitosis
(Masuda et al., 1997, 1999). Using BLAST sequence
similarity searches, four NMCP1-like proteins have been
identified in the Arabidopsis genome (Fig. 1). Like lamins,
they have a central coiled-coil domain flanked by a nonhelical short head and a larger tail domain and a pI of 5.6–
5.8. Although they are roughly twice the size of lamins,
they are currently the best candidate for lamin-like proteins
in plants. Their investigation by immunolocalization and
loss-of function analysis will indicate if they are indeed
functional plant inner nuclear envelope proteins.
Another class of plant proteins with a possible role as
nuclear envelope structural proteins are the filament-like
plant proteins (FPP1–FPP7; Gindullis et al., 2002). Like
lamins and the NMCP1-related proteins, they contain
extended coiled-coil domains. Two indirect lines of evidence connect them to the nuclear envelope. First, the
tomato homologue LeFPP was identified as a yeast-two
hybrid interactor of LeMAF1, a NE-associated protein
(Gindullis et al., 1999, 2002). Second, a pea protein was
identified with an antibody against lamin B which has
similarity to AtFPP3 (Blumenthal et al., 2004). Interestingly, the AtFPP family stands out as a plant-specific family
of long coiled-coil proteins in a cluster analysis of long
coiled-coil proteins of 22 genomes (Rose et al., 2005) and is
characterized by four highly conserved sequence motifs of
unknown function (Gindullis et al., 2002). The subcellular
location of the Arabidopsis proteins is currently not known.
To date, no integral membrane proteins of the NE have
been cloned in plants. However, an N-terminal domain
Plant nuclear envelope
The nuclear pore
The physical barrier of the NE needs to be permeable to
a variety of macromolecules and signals in order for the cell
to function. The only known gateways for the transport
of macromolecules across the NE are the nuclear pore
complexes (NPCs). The NPC is a 125 MDa multiprotein
complex with a 9 nm aqueous pore through which
molecules smaller than 30 kDa can readily diffuse (Stoffler
et al., 1999; Gasiorowski and Dean, 2003). The proteins
forming the NPCs are called nucleoporins (Nups). Significant progress in the identification of animal and yeast Nups
has been made recently by the use of proteomics (Allen
et al., 2001; Cronshaw et al., 2002). It was assumed that the
animal NPC would contain a larger number of distinct
subunits than the yeast NPC because of its larger size.
However, the proteomic analysis of mammalian NPC
composition showed that it contains only 30 different
nucleoporins, similar to the results from yeast and less
complex than expected (Cronshaw et al., 2002).
The composition of the plant NPC is largely unknown.
With a few exceptions, homologues to most animal and
yeast Nups cannot be identified in plants using sequence
homology searches. A putative 215 kDa orthologue of
gp210, one of the integral membrane proteins of the NPC,
has been identified in Arabidopsis and is predicted to
contain a transmembrane domain at its C-terminus, consistent with its animal counterpart (Cohen et al., 2001). A
number of yeast and vertebrate Nups have FG repeats
(FXFG in vertebrates and GLFG in yeast), which are
thought to provide transient, low-affinity binding sites for
transport receptors (Mattaj and Englmeier, 1998). A
nucleoporin-like FG-repeat-containing protein can be identified by sequence similarity searches with human Nup98
in the Arabidopsis database. Its FG repeats appear to be
conserved with its animal Nup counterpart, indicating
a possible role for this motif also in plant NPCs (Rose
et al., 2004).
Interestingly, two putative nucleoporins and a component
of the nuclear transport machinery (see below) have been
identified very recently as mutants in plant–bacterial
interactions. snc1 (suppressor of npr1-1, constitutive 1) is
a mutation in a TOLL Interleucine 1 receptor (TIR)-like
protein with a C-terminal leucine-rich repeat (LRR). The
mutant is a gain-of function, leading to constitutive PR
expression. The mutation is located in the spacer between
the N-terminal TIR domain and the LRR of SNC1,
presumably disrupting the interaction with an unknown
negative regulator of SNC1 (Zhang and Li, 2005). In
a mutant screen for a suppressor of scn1, a mutant was
uncovered which abolishes the constitutive expression of
PR genes and leads to increased pathogen susceptibility.
This mutation, mos3-1, was now found to be a loss-offunction allele of putative Arabidopsis nucleoporin Nup96.
Another mutant derived from this suppressor screen is
mos6. MOS6 was found to be one of the eight copies of
Arabidopsis importin alpha, impa3 (Palma et al., 2005).
Together the two reports indicate a connection between
nucleo-cytoplasmic trafficking and plant R-mediated and
basal disease resistance. It remains to be seen whether
Nup96 and importin a3 have a specific function in the
investigated pathways, or whether an overall reduction
of protein import or RNA export lead to the observed
suppressor phenotypes.
In genetic screens using the model legume Lotus
japonicus, six loci have been defined that are required for
mycorrhizal colocalization and rhizobial root nodule development. One of the nodule-deficient mutants was now
identified by map-based cloning and is a truncation of the
reading frame of a protein with similarity to the human
nucleoporin Nup133 (Kanamori et al., 2006). Interestingly,
this nucleoporin is located in the same nuclear pore subcomplex as Nup96.
While it is astonishing that both a plant–pathogen
interaction and a nodulation mutant affect a nucleoporin,
it is even more astonishing that these mutations do not lead
to much more severe phenotypes. In the mammalian and
yeast nuclear pore, Nup133 and Nup96 and their yeast
equivalents are components of the conserved Nup107–160
complex (Walther et al., 2003b). Partial in vivo depletion of
Nup133 has been shown to lead to nuclei with a reduced
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organizing centres (MTOCs) of spindle microtubules are
the cytoplasmic centrosomes in animal cells and the NEembedded spindle pole bodies in yeast cells. Higher plant
cells appear to lack well-defined centrosomes or spindle
pole bodies. Instead, the plant NE acts as the main site for
plant microtubule nucleation. Stoppin et al. (1994) showed
that isolated maize nuclei are capable of nucleating microtubules in vitro. While not the only site for microtubule
nucleation, the nuclear surface is especially active in the G2
phase, before NE breakdown (Schmit et al., 1994, 1996;
Canaday et al., 2000), suggesting cell-cycle control for its
microtubule-nucleating activity. Microtubule-associated
proteins (MAPs) modulate the nucleating activity of the
plant nuclear surface (Stoppin et al., 1996). Microtubule
nucleation in plant cells depends on the presence of both ctubulin and the homologue of the yeast spindle pole
proteins Spc98p (Erhardt et al., 2002). GFP-labelled plant
Spc98p localizes within the nucleus, at the NE and close to
the plasma membrane, suggesting its involvement in the
multiple MTOCs in plant cells (Seltzer et al., 2003).
Recently, c-tubulin has also been found associated with
the nuclear envelope during microsporogenesis in Ginkgo
biloba (Brown and Lemmon, 2005) and mitosis in two
liverworts, Marchantia polymorpha (Brown et al., 2004)
and Conocephalum japonicum (Shimamura et al., 2003),
suggesting conservation of the nuclear envelope’s role as
cell-cycle-dependent MTOC in land plants.
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Nuclear import and export receptors
The NPC allows passive diffusion of molecules up to about
60 kDa, molecules over 50 kDa at a very slow rate. Most
proteins transported across the nuclear envelope contain
a nuclear localization signal (NLS). The NLS is recognized
by a nuclear import complex, enabling active transport to
the nucleus. Even the transport of small proteins that can
diffuse through the nuclear pore is increased by an NLS
(Gasiorowski and Dean, 2003). Proteins trafficking back
and forth between nucleus and cytoplasm often also contain
a nuclear export signal (NES). Animal and yeast NLS and
NES sequences are functional in plants (Smith et al., 1997;
Ward and Lasarowitz, 1999; Merkle, 2001). Likewise,
endogenous NLS and NES motifs have been identified on
a variety of plant proteins such as the movement protein
BR1 of the squash leaf curl virus, which contains one NES
and two NLSs (Ward and Lazarowitz, 1999). The NES on
AtRanBP1a is functionally indistinguishable from the NES
on the HIV-1 Rev protein (Haasen et al., 1999).
NLS sequences are recognized by the import receptor
importin a, a member of the karyopherin family. Several
importin a variants have been characterized in plants
(Smith et al., 1997; Jiang et al., 1998a, 2001; Shoji
et al., 1998; Hübner et al., 1999). Plant importin a appears
to be strongly associated with the NE (Hicks et al., 1996;
Smith et al., 1997). In contrast to animal systems where the
importin a-mediated import is dependent on importin b,
Arabidopsis importin At-IMPa is capable in vitro of
mediating nuclear import in the absence of importin b
(Hübner et al., 1999). The multiple isoforms of importin a
typically found in plants could play an important role in
plant signal transduction if specific importin a-substrate
interactions could be verified. While evidence of such
specificity is sparse, at least a few examples point in this
direction. Rice importin a1b selectively binds to a number
of plant NLSs and is differentially expressed in different
tissues and in response to light (Jiang et al., 2001). It is
involved in the nuclear import of constitutive photomorphogenic 1 (COP1), a repressor of photomorphogenesis
(Jiang et al., 2001). The above-mentioned mutant in an
Arabidopsis impa with specific effects on plant defence
also indicates a yet to be discovered functional specificity of
the plant adaptor importins.
Another member of the karyopherin family that plays an
important role in nuclear import is importin b. It facilitates
the interaction of the importin/cargo complex with FG
repeat components of the NPC (Bayliss et al., 2002). Plant
importin b homologues were isolated in rice by Jiang et al.
(1998b) and are involved in NE docking of NLS-containing
proteins and their subsequent nuclear localization. The
Arabidopsis genome encodes at least 17 predicted importin
b-like proteins (Bollman et al., 2003).
NES sequences interact with another member of the
karyopherin family, the nuclear export receptor CRM1/
exportin 1. An Arabidopsis homologue of exportin 1,
AtXPO1, has been identified and functionally characterized
(Haasen et al., 1999). Similar to the animal exportin 1, it is
inhibited by the antifungal antibiotic leptomycin B (Haasen
et al., 1999). The Arabidopsis homologue of exportin 5
(involved in double-stranded RNA export), HASTY, has
also been identified (Bollman et al., 2003). HASTY protein
is located at the nuclear periphery and interacts with the
Ran GTPase. Its loss causes a variety of developmental
phenotypes (Bollman et al., 2003). PAUSED, the Arabidopsis homologue of exportin T (involved in tRNA export),
is able to rescue a tRNA export defective yeast mutant,
indicating conservation of its function in plants (Hunter
et al., 2003). Like HASTY, mutants of PAUSED have
pleiotropic effects in plant development (Hunter et al.,
2003; Li and Chen, 2003).
The Ran cycle
The small GTPase Ran (Ras related nuclear protein) is
required for nucleocytoplasmic transport. Ran exists in two
forms, Ran-GTP and Ran-GDP, which are transformed into
each other by the action of accessory proteins of the Ran
cycle. The low intrinsic GTPase activity of Ran is enhanced
by the cofactors Ran GTPase Activating Protein (RanGAP)
and Ran Binding Proteins (RanBPs) on the cytoplasmic
side of the NE, which leads to the transformation of
RanGTP to RanGDP outside the nucleus. In the nucleus,
the chromatin-bound Ran nucleotide exchange factor
RCC1 (regulator of chromosome condensation 1) converts
RanGDP to RanGTP. Thus, a RanGDP versus RanGTP
gradient over the NE is established by the spatial sequestering of the Ran accessory proteins, which is involved in
maintaining the directionality of nucleocytoplasmic transport. The association of RanGTP with karyopherins inside
the nucleus causes import cargos to be released and export
cargos to be bound. By contrast, RanGTP hydrolysis in the
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number of nuclear pores, and complete depletion of the
Nup107–160 complex leads to nuclei entirely lacking pores
(Walther et al., 2003b). The molecular architecture of the
yeast equivalent yNup84 complex has been determined by
modelling of protein folds (Devos et al., 2004). The data
are consistent with it having a ‘scaffolding’ function
sandwiched between the membrane-associated Nups and
the FG-repeat Nups that line the pore channel (Devos et al.,
2006). Thus, combined current evidence suggests a structural role in nuclear pore assembly for the Nup107–160
complex. It might be enlightening to investigate the NPC
appearance and abundance in the putative Nup133 and
Nup96 mutants (e.g. by SEM or AFM studies of isolated
nuclei), and to design experiments targeting the degree of
specificity of the mutant phenotypes.
Plant nuclear envelope
suggesting a possible role for plant RanBP1 inside the
nucleus (Kim and Roux, 2003). More recently, overexpression of wheat Ran in transgenic rice and Arabidopsis
has also been shown to alter sensitivity to auxin, as well as
meristem size and mitotic index (Wang et al., 2006). As for
the mutations in importin a and nucleoporins discussed
above, it remains to be shown whether these effects are
indeed ‘regulatory’, or rather indirect by affecting a general
aspect of nucleocytoplasmic transport that leads to a modulation of the abundance of one or more critical components of the observed pathways.
Some components of the Ran cycle still await characterization in plants. Recently, two functional homologues
of the Ran nuclear import factor NTF2 were identified in
Arabidopsis (Zhao et al., 2006). They appear structurally
and functionally highly similar to NTF2 from humans and
yeast. By contrast, the Ran-specific guanine nucleotide
exchange factor RCC1 appears to be significantly more
diverged and has escaped identification by sequence
similarity searches. Assuming functional conservation of
such a crucial component of the Ran cycle, a yeast
complementation screen might lead to the identification
of plant RCC1.
Outlook: theme and variations
Similarities as well as deviations are found in the composition and possibly function of the nuclear envelope and
nuclear pore in plants, compared with other eukaryotes.
One of the most striking differences is the apparent absence
of plant equivalents of any of the currently identified inner
nuclear envelope proteins. To ask for the relevance of this
finding means to ask for the function of the INM proteins in
metazoans. In the past few years, a striking number of
human diseases have been found to be associated with
mutations in INM and lamina components (Burke and
Stewart, 2002; Worman and Courvalin, 2004). The two
current models for this connection are (i) that the mutations
lead to an increase in ‘fragility’ of the nucleus, and (ii) that
they affect an aspect of gene expression ultimately leading
to the disease phenotype (Hetzer et al., 2005). Evidence for
the former model comes from studies comparing nuclear
envelope integrity between the wild type and the lamin
mutants associated with muscular dystrophy (Sullivan
et al., 1999) and from the finding that the nuclear lamina
has elasticity and compressibility compatible with features
of a ‘molecular shock absorber’ (Dahl et al., 2004). If
indeed the INM proteins are mainly involved in protecting
the nucleus from physical forces, for example, in muscle
tissue, it is quite conceivable that plants lack the entire
complement of proteins involved in this function.
However, equal or even stronger evidence exists for the
second model. It has been known for several years that
actively transcribed genes are rarely associated with the
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cytoplasm triggers the release of Ran from the karyopherins
and thereby dissociates export cargo complexes (for more
specific reviews see Merkle, 2001; Gasiorowski and Dean,
2003). During the cell cycle, the Ran cycle plays a critical
role in the regulation of spindle and NE assembly (for
review see Quimby and Dasso, 2003). In addition, Ran is
implied in altering the conformation of NPCs (Goldberg
et al., 2000) and in NPC assembly (Walther et al., 2003a).
Most components of the Ran cycle have now been
identified in plants. Ran has been found in a variety of plant
species (Ach and Gruissem, 1994; Merkle et al., 1994;
Saalbach and Christov, 1994) and is encoded by a family of
three genes in Arabidopsis (Haizel et al., 1997). RanGAP
also has been identified (Meier, 2000) and can complement
a temperature-sensitive mutant of the yeast RanGAP
homologue rna1p (Pay et al., 2002). Arabidopsis RanGAP
is concentrated at the plant NE during interphase (Rose and
Meier, 2001; Pay et al., 2002), consistent with the NE
localization of its mammalian homologue, and is associated
with spindle and phragmoplast microtubules during mitosis
(Pay et al., 2002) and the cell plate during cytokinesis
(Jeong et al., 2005). These findings indicate a role for the
Ran cycle in plant cell division that might differ from the
roles reported for animal Ran (Jeong et al., 2005).
Plant RanGAP contains an N-terminal domain termed
WPP-domain, after a conserved tryptophan-proline-proline
motif that appears to be unique to plants (Meier, 2000; Rose
and Meier, 2001). Mutation of the conserved WPP motif
within this domain abolishes NE targeting (Rose and Meier,
2001) and cell-plate association (Jeong et al., 2005).
Vertebrate RanGAPs lack the WPP-domain, but possess
a C-terminal domain required for targeting mammalian
RanGAP to the nuclear pore. The small ubiquitin-like
modifier SUMO is attached to this domain, and the
SUMOylated C-terminus of mammalian RanGAP binds
to the nucleoporin RanBP2/Nup358 (Matunis et al., 1998).
This domain is not present on plant RanGAPs, and no
homologues of RanBP2/Nup358 appear to be encoded by
the Arabidopsis genome. Interestingly, human RanGAP1
does not localize to the NE in tobacco and Arabidopsis cells
and Arabidopsis RanGAP1 does not localize to the NE in
HeLa cells (Jeong et al., 2005). This suggests that plant and
mammalian RanGAP not only have different targeting
domains, but also utilize different binding partners for
retention at the NE.
Haizel et al. (1997) isolated genes encoding RanBPs
(AtRanBP1a and AtRanBP1b) from Arabidopsis. Kim and
Roux (2003) cloned RanBP1c and showed that suppression
of its expression resulted in altered root development and
hypersensitivity to auxin. They hypothesized that
AtRanBP1c, by maintaining the RanGDP/RanGTP cycle,
is involved in the control of proteins that regulate auxin
sensitivity. AtRanBP1c stabilizes the RanGTP conformation and is a co-activator of RanGAP. Localization studies
indicated that it is present in the cytoplasm and the nucleus,
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recapitulate findings from other systems. Even with the
limited knowledge currently at hand, there are already
a striking number of differences in organization and
function of the plant nuclear periphery. There is much to
do now to investigate all the aspects of plant nuclear
envelope biology that are currently so actively researched
in other model systems, such as the composition of the
inner nuclear envelope, its function in chromatin association and gene regulation, the function of the Ran cycle in
mitosis and cytokinesis, and the role of the nuclear
envelope as a MTOC. These examples are probably only
the tip of an iceberg of highly rewarding investigation into
plants’ version of one of the most important landmarks
of a eukaryotic cell, its semi-permeable barrier between
chromatin and cytoplasm.
References
Ach R, Gruissem W. 1994. A small nuclear GTP-binding protein
from tobacco suppresses a Schizosaccharomyces pombe cell-cycle
mutant. Proceedings of the National Academy of Sciences, USA
91, 5863–5867.
Allen NP, Huang L, Burlingame A, Rexach M. 2001. Proteomic
analysis of nucleoporin interacting proteins. Journal of Molecular
Biology 276, 29268–29274.
Apel ED, Lewis RM, Grady RM, Sanes JR. 2000. Syne-1,
a dystrophin- and Klarsicht-related protein associated with synaptic nuclei at the neuromuscular junction. Journal of Biological
Chemistry 275, 31986–31995.
Bayliss R, Littlewood T, Strawn LA, Wente SR, Stewart M. 2002.
GLFL and FxFG nucleoporins bind to overlapping sites on
importin b. Journal of Biological Chemistry 277, 50597–50606.
Blumenthal SS, Clark GB, Roux SJ. 2004. Biochemical and
immunological characterization of pea nuclear intermediate filament proteins. Planta 218, 965–975.
Bollman KM, Aukerman MJ, Park MY, Hunter C, Berardini TZ,
Poethig RS. 2003. HASTY, the Arabidopsis ortholog of exportin
5/MSN5, regulates phase change and morphogenesis. Development 130, 1493–1504.
Brown RC, Lemmon BE, Horio T. 2004. Gamma-tubulin localization changes from discrete polar organizers to anastral spindles
and phragmoplasts in mitosis of Marchantia polymorpha L.
Protoplasma 224, 187–193.
Brown RC, Lemmon BE. 2005. Gamma-tubulin and microtubule
organization during microsporogenesis in Ginkgo biloba. Journal
of Plant Research 118, 121–128.
Burke B, Stewart CL. 2002. Life at the edge: the nuclear envelope
and human disease. Nature Review of Molecular and Cell Biology
3, 575–585.
Canaday J, Stoppin-Mellet V, Mutterer J, Lambert AM,
Schmit AC. 2000. Higher plant cells: c-tubulin and microtubule
nucleation in the absence of centrosomes. Microscopy Research
and Technique 49, 487–495.
Cohen M, Wilson KL, Gruenbaum Y. 2001. Membrane proteins of
the nuclear pore complex: Gp210 is conserved in Drosophila,
C. elegans and A. thaliana. Gene Therapy and Molecular Biology
6, 47–55.
Cronshaw JM, Krutchinsky AN, Zhang W, Chait BT,
Matunis MJ. 2002. Proteomic analysis of the mammalian
nuclear pore complex. Journal of Cell Biology 158, 915–927.
Dahl KN, Kahn SM, Wilson KL, Discher DE. 2004. The nuclear
envelope lamina network has elasticity and a compressibility limit
Downloaded from http://jxb.oxfordjournals.org/ at Pennsylvania State University on February 21, 2013
nuclear periphery, which is preferentially occupied by
gene-poor areas of chromatin (Taddei et al., 2004). Several
interactions between lamins, INM proteins and chromatin,
gene-regulatory and signalling proteins have been identified (reviewed by Zastrow et al., 2004). If INM proteins are
indeed involved in the spatial organization of chromatin
and gene expression, it is unlikely that plants entirely lack
an equivalent mechanism. In this case, it is postulated that
equivalents of the known INM proteins exist in plants, but
that they have diverged too far from their animal counterparts to be recognizable by the currently available computational methods. The NMCPs and FPPs discussed above
are, in this case, probably the most promising starting point
to identify plant proteins involved in chromatin–NE
interactions.
An area that is clearly under-investigated in plants is the
nuclear pore. While hampered in the past by a lack of
obvious sequence similarity of yeast and mammalian
nucleoporins with sequences in plant databases, proteomic
approaches should now allow for an unbiased investigation
of the Arabidopsis NPC. The recent identification of
nucleoporin mutations as the cause of specific phenotypes
will probably create more interest in this field in plants. It
will be crucial to investigate whether the identified proteins
indeed have a ‘regulatory’ role, or whether a general feature
of the nuclear pore has been disturbed that leads to shifts in
the abundance of components involved in the observed
pathways.
While the Ran cycle in plants appears to utilize the same
components as in animals, their physical tethering is not
conserved. Specifically, the association of RanGAP with
the nuclear envelope is established by different means in
plants and animals. This finding draws additional relevance
from the emerging picture that nuclear pore components
and nucleocytoplasmic trafficking factors play important
additional roles during mitosis (Fahrenkrog et al., 2004).
RanGAP in both kingdoms migrates between an interphase
and a mitotic location. While there is increasing molecular
evidence for the specific mitotic functions of the Ran cycle
in animal mitosis, the association of plant RanGAP with the
growing cell plate implies an additional, possibly plantspecific function of the Ran cycle during cytokinesis.
If plant RanGAP uses a nuclear envelope protein as an
‘anchor’ for this migration, then the available data imply
that this anchor has evolved differently in plants and
animals. It is important to recall in this context that the
plant NE comprises an MTOC in higher plant cells, a striking
difference to the organization of microtubule nucleation in
other eukaryotic cells. The identity of the plant anchor of
RanGAP, and its functions both in mitotic RanGAP
trafficking and beyond RanGAP binding, might enlighten
us both about nucleocytoplasmic trafficking and about the
still enigmatic process of open mitosis in higher plants.
In summary, the investigation of the plant nuclear
envelope and nuclear pore appears by no means simply to
Plant nuclear envelope
localization sequences with high affinity and can mediate nuclear
import independent of importin b. Journal of Biological Chemistry
274, 22610–22617.
Hunter CA, Aukerman MJ, Sun H, Fokina M, Poethig RS. 2003.
PAUSED encodes the Arabidopsis Exportin-t ortholog. Plant
Physiology 132, 2135–2143.
Irons SL, Evans DE, Brandizzi F. 2003. The first 238 amino acids
of the human lamin B receptor are targeted to the nuclear envelope
in plants. Journal of Experimental Botany 54, 943–950.
Jeong SY, Rose A, Joseph J, Dasso M, Meier I. 2005. Plantspecific mitotic targeting of RanGAP requires a functional WPP
domain. The Plant Journal 42, 270–282.
Jiang CJ, Imamoto N, Matsuki R, Yoneda Y, Yamamoto N.
1998a. Functional characterization of a plant importin a homologue. Nuclear localization signal (NLS)-selective binding and
mediation of nuclear import of NLS proteins in vitro. Journal of
Biological Chemistry 273, 24083–24087.
Jiang CJ, Imamoto N, Matsuki R, Yoneda Y, Yamamoto N.
1998b. In vitro characterization of rice importin a1: molecular
interaction with nuclear transport factors and mediation of nuclear
protein import. FEBS Letters 437, 127–130.
Jiang CJ, Shoji K, Matsuki R, et al. 2001. Molecular cloning of
a novel importin a homologue from rice, by which constitutive
photomorphogenic 1 (COP1) nuclear localization signal (NLS)protein is preferentially nuclear imported. Journal of Biological
Chemistry 276, 9322–9329.
Kanamori N, Madsen LH, Radutoiu S, et al. 2006. A nucleoporin
is required for induction of Ca2+ spiking in legume nodule development and essential for rhizobial and fungal symbiosis. Proceedings of the National Academy of Sciences, USA 103, 359–364.
Kim SH, Roux SJ. 2003. An Arabidopsis Ran-binding protein,
AtRanBP1c, is a co-activator of Ran GTPase-activating protein
and requires the C-terminus for its cytoplasmic localization. Planta
216, 1047–1052.
Li J, Chen X. 2003. PAUSED, a putative exportin-t, acts pleiotropically in Arabidopsis development but is dispensable for viability.
Plant Physiology 132, 1913–1924.
Li H, Roux SJ. 1992. Casein kinase II protein kinase is bound to
lamina-matrix and phosphorylates lamin-like protein in isolated
pea nuclei. Proceedings of the National Academy of Sciences, USA
89, 8434–8438.
Masuda K, Haruyama S, Fujino K. 1999. Assembly and
disassembly of the peripheral architecture of the plant cell nucleus
during mitosis. Planta 210, 165–167.
Masuda K, Xu ZJ, Takahashi S, Ito A, Ono M, Nomura K,
Inoue M. 1997. Peripheral framework of carrot cell nucleus
contains a novel protein predicted to exhibit a long a-helical
domain. Experimental Cell Research 232, 173–181.
Mattaj IW, Englmeier L. 1998. Nucleocytoplasmic transport: the
soluble phase. Annual Review of Biochemistry 67, 256–306.
Matunis MJ, Wu J, Blobel G. 1998. SUMO-1 modification and its
role in targeting the Ran GTPase-activating protein, RanGAP1, to
the nuclear pore complex. Journal of Cell Biology 140, 499–509.
McNulty AK, Saunders MJ. 1992. Purification and immunological
detection of pea nuclear intermediate filaments: evidence for plant
nuclear lamins. Journal of Cell Science 103, 407–414.
Meier I. 2000. A novel link between Ran signal transduction
and nuclear envelope proteins in plants. Plant Physiology 124,
1507–1510.
Merkle T. 2001. Nuclear import and export of proteins in plants:
a tool for the regulation of signaling. Planta 213, 499–517.
Merkle T, Haizel T, Matsumoto T, Harter K, Dallmann G,
Nagy F. 1994. Phenotype of the fission yeast cell cycle regulatory
mutant pim1-46 is suppressed by a tobacco cDNA encoding a small,
Ran-like GTP-binding protein. The Plant Journal 6, 555–565.
Downloaded from http://jxb.oxfordjournals.org/ at Pennsylvania State University on February 21, 2013
suggestive of a molecular shock absorber. Journal of Cell Science
117, 4779–4786.
de Ruijter NCA, Ketelaar T, Blumenthal SSD, Emons AM,
Schel JHN. 2000. Spectrin-like proteins in plant nuclei. Cell
Biology International 24, 427–438.
Devos D, Dokudovskaya S, Alber F, Williams R, Chait BT, Sali A,
Rout MP. 2004. Components of coated vesicles and nuclear pore
complexes share a common molecular architecture. PLoS Biology
2, e380.
Devos D, Dokudovskaya S, Williams R, Alber F, Eswar N,
Chait BT, Rout MP, Sali A. 2006. Simple fold composition and
modular architecture of the nuclear pore complex. Proceedings of
the National Academy of Sciences, USA 103, 2172–2177.
Ellenberg J, Siggia ED, Moreira JE, Smith CL, Presley JF,
Worman HJ, Lippincott-Schwartz J. 1997. Nuclear membrane
dynamics and reassembly in living cells: Targeting of an inner
nuclear membrane protein in interphase and mitosis. Journal of
Cell Biology 138, 1193–1206.
Erhardt M, Stoppin-Mellet V, Campagne S, et al. 2002. The plant
Spc98p homologue colocalizes with c-tubulin at microtubule
nucleation sites and is required for microtubule nucleation. Journal
of Cell Sciences 115, 2423–2431.
Fahrenkrog B, Koser J, Aebi U. 2004. The nuclear pore complex:
a jack of all trades? Trends in Biochemical Science 29, 175–182.
Galcheva-Gargova ZL, Marinova EL, Koleva ST. 1988. Isolation
of nuclear shells from plant cells. Plant, Cell and Environment 11,
819–825.
Gasiorowski JZ, Dean DA. 2003. Mechanisms of nuclear transport
and interventions. Advances in Drug Development Review 55,
703–716.
Gindullis F, Peffer NJ, Meier I. 1999. MAF1, a novel plant protein
interacting with matrix attachment region binding protein MFP1, is
located at the nuclear envelope. The Plant Cell 11, 1755–1767.
Gindullis F, Rose A, Patel S, Meier I. 2002. Four signature motifs
define the first class of structurally related large coiled-coil proteins
in plants. BMC Genomics 3, 9.
Goldberg MW, Rutherford SA, Hughes M, Cotter LA, Bagley S,
Kiseleva E, Allen TD, Clarke PR. 2000. Ran alters nuclear
pore complex conformation. Journal of Molecular Biology 300,
519–529.
Goldman RD, Gruenbaum Y, Moir RD, Shumaker DK,
Spann TP. 2002. Nuclear lamins: building blocks of nuclear architecture. Genes and Development 16, 533–547.
Gross P, Julius C, Schmelzer E, Hahlbrock K. 1993. Translocation
of cytoplasm and nucleus to fungal penetration sites is associated
with depolymerization of microtubules and defence gene activation
in infected, cultured parsley cells. EMBO Journal 12, 1735–1744.
Haasen D, Köhler C, Neuhaus G, Merkle T. 1999. Nuclear export
of proteins in plants: AtXPO1 is the export receptor for leucinerich nuclear export signals in Arabidopsis thaliana. The Plant
Journal 20, 695–705.
Haizel T, Merkle T, Pay A, Fejes E, Nagy F. 1997. Characterization
of proteins that interact with the GTP-bound form of the regulatory
GTPase Ran in Arabidopsis. The Plant Journal 11, 93–103.
Hetzer MW, Walther TC, Mattaj IW. 2005. Pushing the envelope:
structure, function, and dynamics of the nuclear periphery. Annual
Review of Cell Development Biology 21, 347–380.
Hicks GR, Smith HM, Lobreaux S, Raikhel NV. 1996. Nuclear
import in permeabilized protoplasts from higher plants has unique
features. The Plant Cell 8, 1337–1352.
Holaska JM, Wilson KL, Mansharamani M. 2002. The nuclear
envelope, lamins and nuclear assembly. Current Opinion in Cell
Biology 14, 357–364.
Hübner S, Smith HMS, Hu W, Chan CK, Rihs HP, Paschal BM,
Raikhel NV, Jans DA. 1999. Plant importin a binds nuclear
33
34 Meier
Smith S, Blobel G. 1993. The first membrane spanning region of the
lamin B receptor is sufficient for sorting to the inner nuclear
membrane. Journal of Cell Biology 120, 631–637.
Smith S, Blobel G. 1994. Colocalization of vertebrate lamin B and
lamin B receptor (LBR) in nuclear envelopes and in LRB-induced
membrane stacks of the yeast Saccharomyces cerevisiae. Proceedings of the National Academy of Sciences, USA 91, 10124–10128.
Soullam B, Worman HJ. 1993. The amino-terminal domain of the
lamin B receptor is a nuclear envelope targeting signal. Journal of
Cell Biology 120, 1093–1100.
Stoffler D, Fahrenkrog B, Aebi U. 1999. The nuclear pore complex:
from molecular architecture to functional dynamics. Current
Opinion in Cell Biology 11, 391–401.
Stoppin V, Lambert AM, Vantard M. 1996. Plant microtubuleassociated proteins (MAPs) affect microtubule nucleation and
growth at plant nuclei and mammalian centrosomes. European
Journal of Cell Biology 69, 11–23.
Stoppin V, Vantard M, Schmit AC, Lambert AM. 1994. Isolated
plant nuclei nucleate microtubule assembly: the nuclear surface
in higher plants has centrosome-like activity. The Plant Cell 6,
1099–1106.
Sullivan T, Escalante-Alcalde D, Bhatt H, Anver M, Bhat N,
Nagashima K, Stewart CL, Burke B. 1999. Loss of A-type lamin
expression compromises nuclear envelope integrity leading to
muscular dystrophy. Journal of Cell Biology 147, 913–920.
Taddei A, Hediger F, Neumann FR, Gasser SM. 2004. The
function of nuclear architecture: a genetic approach. Annual
Review of Genetics 38, 305–345.
Walther TC, Askjaer P, Gentzel M, Habermann A, Griffiths G,
Wilm M, Mattaj IW, Hetzer M. 2003a. RanGTP mediates
nuclear pore complex assembly. Nature 424, 689–694.
Walther TC, Alves A, Pickersgill H, et al. 2003b. The conserved
Nup107–160 complex is critical for nuclear pore complex
assembly. Cell 113, 195–206.
Wang X, Xu Y, Han Y, Bao S, Du J, Yuan M, Xu Z, Chong K.
2006. Overexpression of RAN1 in rice and Arabidopsis alters
primordial meristem, mitotic progress, and sensitivity to auxin.
Plant Physiology 140, 91–101.
Ward BM, Lazarowitz SG. 1999. Nuclear export in plants. Use of
geminivirus movement proteins for a cell-based export assay. The
Plant Cell 11, 1267–76.
Worman HJ, Courvalin JC. 2004. How do mutations in lamins
A and C cause disease? Journal of Clinical Investigation 113,
349–351.
Worman HJ, Gundersen GG. 2006. Here come the SUNs:
a nucleocytoskeletal missing link. Trends in Cell Biology [Epub
ahead of print; PMID: 16406617]
Zastrow MS, Vlcek S, Wilson KL. 2004. Proteins that bind A-type
lamins: integrating isolated clues. Journal of Cell Science 117,
979–987.
Zhang Q, Skepper JN, Yang F, Davies JD, Hegyi L, Roberts RG,
Weissberg PL, Ellis JA, Shanahan CM. 2001. Nesprins: a novel
family of spectrin-repeat-containing proteins that localize to the
nuclear membrane in multiple tissues. Journal of Cell Science 114,
4485–4498.
Zhang Y, Li X. 2005. A putative nucleoporin 96 is required for both
basal defense and constitutive resistance mediated by suppressor of
npr1-1, constitutive 1. The Plant Cell 17, 1306–1316.
Zhao Q, Leung S, Corbett AH, Meier I. 2006. Identification and
characterization of the Arabidopsis orthologs of NTF2, the nuclear
import factor of Ran. Plant Physiology [Epub ahead of print;
PMID: 16428596].
Zhen YY, Libotte T, Munck M, Noegel AA, Korenbaum E. 2002.
NUANCE, a giant protein connecting the nucleus and actin
cytoskeleton. Journal of Cell Science 115, 3207–3222.
Downloaded from http://jxb.oxfordjournals.org/ at Pennsylvania State University on February 21, 2013
Mı́nguez A, Moreno Dı́az de la Espina S. 1993. Immunological
characterization of lamins in the nuclear matrix of onion cells.
Journal of Cell Science 106, 431–439.
Mislow JM, Holaska JM, Kim MS, Lee KK, Segura-Totten M,
Wilson KL, McNally EM. 2002. Nesprin-1a self-associates and
binds directly to emerin and lamin A in vitro. FEBS Letters 525,
135–140.
Moreno Dı́az de la Espina S, Barthelemy L, Cerezuela MA. 1991.
Isolation and ultrastructural characterization of the residual nuclear
matrix in a plant cell system. Chromosoma 100, 110–117.
Mosley-Bishop KL, Li Q, Patterson L, Fischer JA. 1999.
Molecular analysis of the klarsicht gene and its role in nuclear
migration within differentiating cells of the Drosophila eye.
Current Biology 9, 1211–1220.
Mounkes L, Kozlov S, Burke B, Steward CL. 2003. The
laminopathies: nuclear structure meets disease. Current Opinion
in Genetic Development 13, 223–230.
Palma K, Zhang Y, Li X. 2005. An importin alpha homolog, MOS6,
plays an important role in plant innate immunity. Current Biology
15, 1129–1135.
Pay A, Resch K, Frohnmeyer H, Fejes E, Nagy F, Nick P. 2002.
Plant RanGAPs are localized at the nuclear envelope in interphase
and associated with microtubules in mitotic cells. The Plant
Journal 30, 699–709.
Quimby BB, Dasso M. 2003. The small GTPase Ran: interpreting
the signs. Current Opinion in Cell Biolotgy 15, 338–344.
Rose A, Meier I. 2001. A domain unique to plant RanGAP is
responsible for its targeting to the plant nuclear rim. Proceedings of
the National Academy of Sciences, USA 98, 15377–15382.
Rose A, Manikantan S, Schraegle SJ, Maloy MA, Stahlberg EA,
Meier I. 2004. Genome-wide identification of Arabidopsis coiledcoil proteins and establishment of the ARABI-COIL database.
Plant Physiology 134, 927–939.
Rose A, Schraegle SJ, Stahlberg EA, Meier I. 2005. Coiled-coil
protein composition of 22 proteomes: differences and common
themes in subcellular infrastructure and traffic control. BMC
Evolutionary Biology 5, 66.
Saalbach G, Christov V. 1994. Sequence of a plant cDNA from
Vicia faba encoding a novel Ran-related GTP-binding protein.
Plant Molecular Biology 24, 969–972.
Schmelzer E. 2002. Cell polarization, a crucial process in fungal
defence. Trends in Plant Science 7, 411–415.
Schmit AC, Endle MC, Lambert AM. 1996. The perinuclear
microtubule-organizing center and the synaptonemal complex
of higher plants share a common antigen: its putative transfer
and role in meiotic chromosomal ordering. Chromosoma 104,
405–413.
Schmit AC, Stoppin V, Chevrier V, Job D, Lambert AM. 1994.
Cell cycle dependent distribution of a centrosomal antigen at the
perinuclear MTOC or at the kinetochores of higher plant cells.
Chromosoma 103, 343–351.
Seltzer V, Pawlowski T, Campagne S, Canaday J, Erhardt M,
Evrard JL, Herzog E, Schmit AC. 2003. Multiple microtubule
nucleation sites in higher plants. Cell Biology International 27,
267–269.
Shimamura M, Brown RC, Lemmon BE, et al. 2004. Gammatubulin in basal land plants: characterization, localization, and
implication in the evolution of acentriolar microtubule organizing
centers. The Plant Cell 16, 45–59.
Shoji K, Iwasaki T, Matsuki R, Miyao M, Yamamoto N. 1998.
Cloning of a cDNA encoding an importin-a and down-regulation
of the gene by light in rice leaves. Gene 212, 279–286.
Smith HM, Hicks GR, Raikhel NV. 1997. Importin a from
Arabidopsis thaliana is a nuclear import receptor that recognizes
three classes of import signals. Plant Physiology 114, 411–417.