baumann - 2010
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8/3/2019 Baumann - 2010
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S T O C K B Y T E
Abscisic acid (ABA) is a ubiquitousplant horone that regulates arious
aspects of plant groth and deelop-ent, including seed aturation anddorancy, and has a central role in
the adaptation of egetatie tissuesto enironental stresses, notably drought. Although any coponents
of the ABA signalling pathay areknon, the identity and signalling
echaniss of ABA receptors haereained elusie. No, six researchgroups report the echanis by
hich the pyrabactin resistance(PYR)/PYR-like (PYL)/regulatory coponent of ABA receptors (RCAR)
faily of START proteins, preiously identified as ABA receptors, bindABA and positiely regulate ABA
signalling.Preious findings hae reported
that the Arabidopsis thaliana PYR/
PYL/RCAR proteins function as ABAreceptors that, folloing ABA binding,inhibit the actiity of knon negatie
regulators of ABA signalling — thetype 2C protein phosphatases (PP2Cs)
ABI1, ABI2, HAB1, HAB2 andPP2CA — in both seeds and egeta-
tie tissues. The suggested odelpredicts that, in the presence of ABA,PYR/PYL/RCARs bind to PP2Cs and
induce the release of SNRK2 Ser/Thrkinases fro PP2Cs, hich ouldotherise keep SNRK2s in an inactie
state. SNRK2s can then phosphorylatedonstrea substrates, including
ABA responsie eleent-bindingprotein (AREB) and ABA responsieeleent-binding factor (ABF)
bZIP transcription factors, hichactiate ABA-responsie genes andABA-related responses.
In vitro studies by Fuii et al. sho that cobining PYR1, ABI1,SNRK2.6 (also knon as SRK2E) and ABF2 is sufficient for ABA-triggered ABF2 phosphorylation.This indicates that these four proteins
are the core coponents of the ABAsignalling pathay.
The other fie groups carried out
structural analyses of PYL1, PYL2,PYR1 and PYR2 alone, bound to
ABA or in coplex ith both ABAand PP2Cs. The data reeal ho ABA
binding induces a conforationalchange that allos the receptors to sta-bly bind to PP2Cs. PYR and PYL hae
a seen-stranded cured β-sheet thatfors a central caity resebling thatof a folded hand. The surface of the
caity also coprises α-helices, hich,together ith the inner side of theβ-sheet, constitute the ABA-binding
site. To highly consered β-loops actas caity ‘lids’, hich, in the absence
of ABA, are in an open conforationthat allos ABA to access the caity.On ABA binding, the β-loops adopt aclosed-lid conforation. Iportantly,
this conforational change odifiesthe lid to create a binding site forPP2C. The receptor–PP2C interaction
results in the β-loops coering thePP2C actie site, thereby inhibiting itsactiity. This suggests that ABA recep-
tors function as PP2C inhibitors in anABA-dependent anner.
Understanding the structural
basis of ABA receptor interactionshas iportant iplications as it paes
the ay for the design of agonistolecules that could increase theresistance of crops to ater stress.
Kim Baumann
ORIGINAL RESEARCH PAPERS Fujii, H. et al.
In vitro reconstitution of an abscisic acid
signalling pathway. Nature 462, 660–664 (2009) |
Nishimura, N. et al. Structural mechanism of
abscisic acid binding and signaling by dimeric
PYR1. Science326, 1373–1379 (2009) |
Miyazono, K. et al. Structural basis of abscisic acid
signalling. Nature 462, 609–614 (2009) | Melcher, K.
et al. A gate-latch-lock mechanism for hormone
signalling by abscisic acid receptors. Nature 462,
602–608 (2009) | Santiago, J. et al. The abscisic acidreceptor PYR1 in complex with abscisic acid.
Nature 462, 665–668 (2009) | Yin, P. et al. Structural
insights into the mechanism of abscisic acid
signaling by PYL proteins. Nature Struct. Mol. Biol.
16, 1230–1236 (2009)
S I G N A L L I N G
ABA’s atst hts
ABA receptors
function
as PP2C
inhibitors
in an ABA-dependent
manner.
R e s e a R c h h i g h l i g h t s
NATURE REvIEwS | MoleculAr cell Biology vOLUmE 11 | jANUARY 2010
© 2009 Macmillan Publishers Limited. All rights reserved