Download Membrane Bound: C2-Domain Abscisic Acid

The Plant Cell, Vol. 26: 4566, December 2014, www.plantcell.org ã 2014 American Society of Plant Biologists. All rights reserved.
IN BRIEF
Membrane Bound: C2-Domain Abscisic Acid-Related Proteins Help
Abscisic Acid Receptors Get Where They Need to Go
We’ve learned so much about abscisic acid
(ABA) since its discovery in 1963. This phytohormone, whose levels increase under
osmotic stress conditions and fluctuate
during seed development, plays a vital
role in regulating plant stress responses,
transpiration, seed germination, and embryogenesis. In order to function, ABA must
bind to soluble PYRABACTIN RESISTANCE
(PYR)/PYR1-LIKE (PYL) ABA receptors. Following perception of ABA, these receptors
form ternary complexes with clade A PP2C
phosphatases (such as ABA INSENSITIVE1
and HYPERSENSITIVE TO ABA1), thereby
inactivating them (Park et al., 2009) and
unleashing a cascade of ABA responses.
These ABA responses involve ion transporters
and membrane-associated enzymes, which
generate second messengers that function
in ABA signaling. How do the cytosolic PYR/
PYL ABA receptors get to these membraneassociated enzymes? Diffusion alone is
unlikely to accomplish this task. In general,
cytosolic proteins can reside on the periphery
of vesicles, and they can transiently interact
with membranes for trafficking or signaling
purposes; these associations are driven
by protein modules that recognize specific
features of proteins or membranes (Cho
and Stahelin, 2005). The identities of the
proteins that help PYR/PYL receptors arrive
at their destination in the membrane have
only recently been revealed.
In their search for interacting partners of
PYR/PYL receptors in Arabidopsis thaliana,
Rodriguez et al. (2014) discovered a family
of proteins that interact with PYR/PYLs and
positively regulate ABA sensitivity. The authors began their study by performing yeast
two-hybrid screening in the absence of ABA
using the highly expressed PYL4 as bait.
This analysis led to the discovery of the
10-member family of C2-domain ABA-related
(CAR) proteins. CAR proteins interact with
several PYR/PYLs and are also found in crops
such as tomato (Solanum lycopersicum)
and rice (Oryza sativa). The presence of
www.plantcell.org/cgi/doi/10.1105/tpc.114.134411
a C2-domain in these proteins is intriguing,
as these calcium binding motifs are involved
in targeting proteins to cell membranes.
Indeed, high local calcium levels are found
at the cell membrane in response to various
stimuli (ABA, abiotic stress, and pathogen
attack), suggesting that the C2-domain in
CAR proteins enables them to translocate to
cell membranes in response to calcium
oscillations. Bimolecular fluorescence complementation assays, transient expression
analysis in tobacco (Nicotiana benthamiana)
leaves, and coimmunoprecipitation experiments revealed that CAR1 and PYL4 interact in plant cells. The authors transiently
expressed various combinations of CAR
proteins fused to green fluorescent protein
in tobacco cells, which they coexpressed
with a plasma membrane marker. This analysis, as well as biochemical experiments,
revealed that CAR proteins localize to the
plasma membrane, while further analysis
revealed that membrane recruitment of
PYL4 by CAR1 affects only a fraction of
the total receptor pool. Crystallographic
analysis demonstrated that, in addition
to the C2-domain, CAR4 contains a specific
CAR signature that is likely responsible for
the interaction with PYR/PYL receptors and
their recruitment to phospholipid vesicles.
The authors then used gain-of-function and
loss-of function approaches to investigate
whether CAR genes affect ABA signaling.
While CAR1 overexpressors were hypersensitive to the inhibition of seedling establishment and shoot growth by ABA, triple car
mutant seedlings were less sensitive to
ABA-mediated inhibition of seedling establishment and root growth than the wild type
(see figure). Moreover, car5 car9 pyr1 pyl4
pyl8 mutants exhibited reduced sensitivity
to ABA-mediated inhibition of primary root
growth and salt-induced inhibition of lateral
root growth, indicating that CAR and PYR/
PYL proteins additively regulate root sensitivity to ABA, as both are needed for
a proper ABA response. After all, PYR/PYL
Wild-type and triple car mutant seedlings 10 d after
transfer from Murashige and Skoog medium to
Murashige and Skoog medium supplemented with
10 mM ABA. (Reprinted from Rodriguez et al.
[2014], Figure 5D.)
ABA receptors can’t get where they need to
go without CAR proteins.
Jennifer Lockhart
Science Editor
[email protected]
ORCID ID: 0000-0002-1394-8947
REFERENCES
Cho, W., and Stahelin, R.V. (2005). Membraneprotein interactions in cell signaling and membrane trafficking. Annu. Rev. Biophys. Biomol.
Struct. 34: 119–151.
Park, S.Y., et al. (2009). Abscisic acid inhibits
type 2C protein phosphatases via the PYR/
PYL family of START proteins. Science 324:
1068–1071.
Rodriguez, L., et al. (2014). C2-domain abscisic
acid-related proteins mediate the interaction
of PYR/PYL/RCAR abscisic acid receptors
with the plasma membrane and regulate
abscisic acid sensitivity in Arabidopsis. Plant
Cell 26: 4802–4820.
Membrane Bound: C2-Domain Abscisic Acid-Related Proteins Help Abscisic Acid Receptors Get
Where They Need to Go
Jennifer Lockhart
Plant Cell 2014;26;4566; originally published online December 2, 2014;
DOI 10.1105/tpc.114.134411
This information is current as of July 31, 2017
References
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