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Baumann - 2010

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     S      T      O      C      K      B      Y      T      E Abscisic acid (ABA) is a ubiquitous plant horone that regulates arious aspects of plant groth and deelop- ent, including seed aturation and dorancy, and has a central role in the adaptation of egetatie tissues to enironental stresses, notably drought. Although any coponents of the ABA signalling pathay are knon, the identity and signalling echaniss of ABA receptors hae reained elusie. No, six research groups report the echanis by hich the pyrabactin resistance (PYR)/PYR-like (PYL)/regulatory coponent of ABA receptors (RCAR) faily of ST ART proteins, preiously identified as ABA receptors, bind ABA and positiely regulate ABA signalling. Preious findings hae reported that the Arabidopsis thaliana PYR/ PYL/RCAR proteins function as ABA receptors that, folloing ABA binding, inhibit the actiity of knon negatie regulators of ABA signalling — the type 2C protein phosphatases (PP2Cs) ABI1, ABI2, HAB1, HAB2 and PP2CA — in both seeds and egeta- tie tissues. The suggested odel predicts that, in the presence of ABA, PYR/PYL/RCARs bind to PP2Cs and induce the release of SNRK2 Ser/Thr kinases fro PP2Cs, hich ould otherise keep SNRK2s in an inactie state. SNRK2s can then phosphorylate donstrea substrates, including ABA responsie eleent-binding protein (AREB) and ABA responsie eleent-binding factor (ABF) bZIP transcription factors, hich actiate ABA-responsie genes and ABA-related responses. In vitro studies by Fuii et al. sho that cobining PYR1, ABI1, SNRK2.6 (also knon as SRK2E)  and ABF2 is suffi cient for ABA- triggered ABF2 phosphorylation. This indicates that these four proteins are the core coponents of the ABA signalling pathay . The other fie groups carried out structural analyses of PYL1, PYL2, PYR1 and PYR2 alone, bound to ABA or in coplex ith both ABA and PP2Cs. The data reeal ho ABA binding induces a conforation al change that allos the receptors to sta- bly bind to PP2Cs. PYR and PYL hae a seen-stranded cured β-sheet that fors a central caity resebling that of a folded hand. The surface of the caity also coprises α-helices, hich, together ith the inner side of the β-sheet, constitute the ABA-binding site. T o highly consered β-loops act as caity ‘lids’, hich, in the absence of ABA, are in an open conforation that allos ABA to access the caity . On ABA binding, the β-loops adopt a closed-lid conforation . Iportantly , this conforational change odifies the lid to create a binding site for PP2C. The receptor–P P2C interaction results in the β-loops coering the PP2C actie site, thereby inhibiting its actiity . This suggests that ABA recep- tors function as PP2C inhibitors in an ABA-dependent anner. Unders tanding the structural basis of ABA receptor interactions has iportant iplications as it paes the ay for the design of agonist olecules that could increase the resistance of crops to ater stress. Kim Baumann ORIGINAL RESEARCH PAPE RS 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. Science 326, 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 acid receptor 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) SIGNALLING ABA’ s at st h ts  ABA receptors function as PP2C inhibitors in an ABA- dependent manner. ReseaRch highlights NATURE REvIEwS | MoleculAr cell Biology vOLUmE 11 | jANUARY 2010 © 2009 Macmillan Publishers Limited. All rights reserved
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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


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