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==Pyr/Pyl/RCAR family of ABA receptors==
[[Image:ABA model 2 med.jpg|left]]
 
==ABA signaling pathway==
Abscisic acid (ABA,)[[Image:Abscisic acid.svg.png|thumb| Abscisic Acid]] is a plant hormone that regulates seed development, dormancy and germination, stomatal closure, and responses to drought stress, and its core signaling pathway has recently been identified.<ref>PMID:15012233</ref><ref>PMID:12045268</ref><ref>PMID:20192755</ref> In Arabidopsis, this pathway involves families of ABA receptors (PYR1/RCAR11, PYL1/RCAR12, PYL2/RCAR14, PYL3/RCAR13, PYL8/RCAR3, PYL9/RCAR1)<ref>PMID:19407143</ref><ref>PMID:19407142</ref><ref>"PMID:22579247</ref><ref>PMID:23370718</ref>, protein phosphatase 2Cs (ABI1, ABI2, HAB1, HAB2, PP2CA/AHG3)<ref>PMID:19407143</ref><ref>PMID:19407142</ref><ref>PMID:16339800</ref>, SNRK2 protein kinases ([[SnRK2.6/OST1/SRK2E]], SNRK2.2/SRK2D and SnRK2.3/SRK2IA).<ref name = "Mustilli2002">PMID:12468729</ref><ref>PMID:12514244</ref><ref name = "Nakashima2009">PMID:19541597</ref><ref name = "Fujii2007">PMID:17307925</ref>
 
The core ABA signaling pathway is shown in the figure to the left. In unstimulated cells, the ABA receptor (R) is an unliganded dimer<ref>PMID:19898420</ref><ref>PMID:19933100</ref><ref>PMID:23265948</ref><ref>"PMID:22579247</ref> (monomer shown) in the cytosol and nucleus, and the SNRK2 protein kinase (K) is bound to a protein phosphatase 2C (P) in a complex (K-P) in which the kinase is dephosphorylated and inactivated by the phosphatase.  When the ABA level rises ABA binds to the ABA receptor<ref>PMID:19893533</ref><ref>PMID:19407143</ref><ref>PMID:19407142</ref>. The activated receptor (R<sup>.</sup>ABA) binds to a protein phosphatase 2C<ref>PMID:19624469</ref> (R<sup>.</sup>ABA-P), and this sequestration of the phosphatase frees the protein kinase to be activated by autophosphorylation or phosphorylation by another protein kinase. Activation of SnRK2.6/OST1/SRK2E leads to phosphorylation of: 1) ion channels SLAC1<ref>PMID: 19955405</ref> and KAT1<ref>PMID:19785574</ref> in guard cells and stomatal closure; 2)  transcription factor ABI5<ref name = "Nakashima2009"/> in seeds/seedlings and dormancy/growth arrest; or 3) phosphorylation of transcription factor AREB/ABF <ref>PMID: 16446457</ref><ref name = "Fujii2007"/> in vegetative tissue and stress tolerance and growth regulation.
 
Structures in the figure are: R, apo PYL2, [[3kdh]];  R<sup>.</sup>ABA, PYL2<sup>.</sup>ABA, [[3kdi]]; K-P; SnRK2.6-HAB1, [[3ujg]]; R<sup>.</sup>ABA-P, PYL2<sup>.</sup>ABA-ABI2, [[3ujl]]; K, SnRK2.6,[[3uc4]].
 
For further information about proteins involved in this pathway see [[PYR/PYL/RCAR family of ABA receptors]], [[ABA-regulated Protein Phosphatase 2C]], [[ABA-regulated SNRK2 Protein Kinase]].
 
 
==PYR/PYL/RCAR family of ABA receptors==
 
 


{|
{|
|[[Image:Abscisic acid.svg.png|thumb|]]
|'''Left panel''' - apo PYL2 [[3kdh]]
|[[Image:ABA model 2 med.jpg|right]]
|'''Middle panel''' - ABA bound to PYL2 [[3kdi]] or PYR1 [[3k3k]]
|}
|'''Right panel''' - PYL<sup>'''.'''</sup>ABA bound to HAB1[[3ujl]]
|-
| <applet load='3kdh' size='300' frame='true' align='left' caption='3kdh - apo-Pyl2' scene = '56/564059/Apopyl2/1'/><br  clear='both'>'''3kdh scenes''' <Br><scene name='56/564059/Apopyl2/1'>1. Default scene </scene> PYL2 is shown as a monomer. See also Scene 3 of the middle panel, which shows the receptor dimer. <br><scene name='56/564059/Apopyl2/3'>2. Open gate</scene> The entrance to binding pocket for ABA is regulated by a "latch" shown in orchid and a "gate" shown in blue. Proline 92 is shown in ball and stick. Here the gate and entrance to the binding site are open.
| <applet load='3kdi' size='300' frame='true' align='left' caption='3kdi - ABA bound to PYL2' scene = '56/564059/Abapyl2/1' /><Br clear='both'>'''3kdi scenes'''<Br><scene name='56/564059/Abapyl2/1'>1. Default scene </scene> ABA (CPK spheres) binds to a water-filled (not shown) pocket of PYL2. <br><scene name='56/564059/Abapyl2/2'>2. Closed gate </scene> The gate folds over ABA and interacts with the latch. <br>
|<applet load='3ujl' size='300' frame='true' align='left' caption='3ujl - PYR2-HAB1' scene = '56/564059/Pyl2hab1/2' /><Br clear='both'>'''3ujl scenes'''<Br><scene name='56/564059/Pyl2hab1/1'>1. Default scene</scene> Complex between PYL2 (blue) with bound ABA (CPK spheres)and HAB1 (gold), a protein phosphatase 2C. Magnesium ions in the active site of HAB1 are shown as green spheres. <br><scene name='56/564059/Pyl2hab1/5'>2. Closed gate locked by interaction with HAB1</scene> Gate residue proline 92 (blue ball and stick) interacts with typtophan 290 (gold ball and stick and residues in a hydrophobic loop (dark gold ball and stick) of HAB1. The gate also interacts with residues surrounding the phosphatase's active site, which is marked by magnesium ions (small green spheres).
|-
|<applet load='3k3k' size='300' frame='true' align='left' caption='3k3k - PYR1 dimer' scene = '56/564059/Pyr1dimer/2'/><br  clear='both'>'''3k3k scene'''<Br><scene name='56/564059/Pyr1dimer/2'>3. PYR1 dimer</scene> in which one monomer is bound to ABA. The native form of the receptor is a dimer<ref>PMID:19933100</ref><ref>PMID:23265948</ref>.
|<applet load='3klx' size='300' frame='true' align='left' caption='3klx - PYL3 ''cis'' dimer' scene = '56/564059/Apopyl3cisdimer/2'/><br  clear='both'>'''3klx scene'''<Br><scene name='56/564059/Apopyl3cisdimer/2'>Apo PYL3 ''cis'' dimer </scene>
|<applet load='4dsc' size='300' frame='true' align='left' caption='4dsc - PYL3.ABA ''trans'' dimer' scene = '56/564059/Pylabatransdimer/2'/><br  clear='both'>'''4dsc scene'''<Br><scene name='56/564059/Pylabatransdimer/2'>PYL3.ABA ''trans'' dimer'</scene>
|}  


==ABA signaling pathway==
Melcher K, Ng LM, Zhou XE, Soon FF, Xu Y, Suino-Powell KM,Park SY, Weiner JJ, Fujii H, Chinnusamy V, Kovach A, Li
Abscisic acid (ABA;structure shown above at left) is a plant hormone that regulates seed dormancy and germination, stomatal closure, and responses to drought stress. As shown in the right figure, ABA binds to an ABA receptor (R)<ref>PMID:19893533</ref><ref>PMID:19407143</ref><ref>PMID:19407142</ref>. The activated receptor (R<sup>.</sup>ABA) binds to a protein phosphatase 2C (R<sup>.</sup>ABA-P), which was formerly in a complex with a SnRK2 protein kinase. The protein kinase is now free to be activated by autophosphorylation or phosphorylation by another protein kinase. Activation of SnRK2.6/OST1/SRK2E leads to phosphorylation of: 1) ion channels SLAC1<ref>PMID: 19955405</ref> and KAT1<ref>PMID: 19785574</ref> in guard cells and stomatal closure; 2)  transcription factor ABI5<ref name = "Nakashima2009"/> in seeds/seedlings and dormancy/growth arrest; or 3) phosphorylation of transcription factor AREB/ABF <ref>PMID: 16446457</ref><ref name = "Fujii2007"/> in vegetative tissue and stress tolerance and growth regulation.  
J, Wang Y, Peterson FC, Jensen DR, Yong EL, Volkman BF,Cutler SR, Zhu JK, Xu HE(2009) A gate-latch-lock mechanism for
hormone signalling by abscisic acid receptors.Nature 462, 602–608.<ref>PMID:19898420</ref><br>
Melcher K, Zhou XE, Xu HE (2010) Thirsty plants and beyond: Structural mechanisms of abscisic acid perception and signaling. Curr. Opin. Struct. Biol. 20, 722–729.<ref>PMID:20951573</ref><br>
Miyazono K, Miyakawa T, Sawano Y, Kubota K, Kang HJ, Asano A, Miyauchi Y, Takahashi M, Zhi Y, Fujita Y, Yoshida T, Kodaira KS, Yamaguchi-Shinozaki K, Tanokura M (2009) Structural basis of abscisic acid signalling. Nature 462, 609–614<ref>PMID:19855379</ref><br>
Nishimura N, Hitomi K, Arvai AS, Rambo RP, Hitomi C, Cutler SR,
Schroeder JI, Getzoff ED (2009) Structural mechanism of abscisic acid binding and signaling by dimeric PYR1. Science 326, 1373–1379<ref>PMID:19933100</ref><br>
Santiago J, Rodrigues A, Saez A, Rubio S, Antoni R, Dupeux F, Park SY, Marquez JA, Cutler SR, Rodriguez PL (2009b)
Modulation of drought resistance by the abscisic acid receptor PYL5 through inhibition of clade A PP2Cs.
Plant J. 60, 575–588<ref>PMID:19624469</ref><br>
YinP,FanH,HaoQ,YuanX,WuD,PangY,YanC,LiW,WangJ,Yan N (2009) Structural insights into the mechanism of abscisic acid signaling by PYL proteins. Nat. Struct. Mol. Biol. 16, 1230–1236<ref>PMID:19893533</ref><br>
Structural insights into PYR/PYL/RCAR ABA receptors and PP2Cs.<br>
Santiago J, Dupeux F, Betz K, Antoni R, Gonzalez-Guzman M, Rodriguez L, Márquez JA, Rodriguez PL.Plant Sci. 2012 182:3-11<ref>PMID:22118610</ref>
 
===PYR/PYL/RCAR structures===
At is ''Arabidopsis thaliana''
 
'''Apo structures'''<br>
 
[[3k3k]], AtPYR1 dimer, one monomer is bound to ABA and the other unliganded<br>
[[3kay]], apo AtPYL1 <br>
[[3kdh]], [[3kaz]], [[3kl1]] apo AtPYL2<br>
[[3klx]], Apo AtPYL3<br>
[[4jdl]], Apo AtPYL5<br>
[[3rt2]], [[3uqh]] apo AtPYL10<br>
 
 
'''Structures with (+)-ABA'''<br>
[[3k90]], AtPYR1.ABA<br>
[[3k3k]], AtPyr1 dimer, one monomer is bound to ABA and the other unliganded<br>
[[3jrs]], AtPYL1.ABA<br>
[[3kdi]], [[3kb0]] AtPYL2.ABA<br>
[[4dsb]], [[4dsc]] AtPYL3 with ABA<br>
[[3oqu]], AtPYL9.ABA<br>
[[3r6p]], AtPYL10.ABA<br>
 
 
'''Structures with (-)-ABA'''<br>
[[4jda]], AtPYL3 with (-)-ABA<br>
 
 
'''Structures with pyrabactin'''<br>
[[3njo]], AtPYR1.Pyrabactin<br>
[[3nef]], [[3neg]], [[3nr4]] AtPYL1.pyrabactin<br>
[[3nj0]], [[3ns2]] AtPYL2.Pyrabactin<br>
[[3nj1]], AtPYL2 V114I mutant.Pyrabactin <br>
[[3nmh]], AtPYL2 in complex with pyrabactin<br>
[[3nmp]], AtPYL2 mutant A93F in complex with pyrabactin<br>
[[3oji]], AtPYL3 with pyrabactin(?)<br>
 
 
'''Structures with (+)-ABA or  homolog and a PP2C'''<br>
[[3qn1]], AtPYR1.ABA - AtHAB1<br>
[[3zvu]], AtPYR1 H60P mutant .ABA - AtHAB1<br>
[[3kb3]], AtPYL1.ABA - HAB1<br>
[[3jrq]], [[3kdj]] AtPYL1.ABA - ABI1<br>
[[3ujl]], AtPYL2.ABA - AtABI2<br>
[[4lga]], [[4lgb]] AtPYL2.ABA mimic - AtHAB1<br>
[[4ds8]], AtPYL3.ABA complex with AtHAB1
[[3rt0]], AtPYL10.ABA - AtHAB1<br>


The ABA receptor family includes PYR1, PYR2, PYL1, PYL2, and RCAR. The ABA-responsive protein phosphatase 2Cs are in clade C and they include ABI1, ABI2, and HAB1. The ABA-activated protein kinases are in subclass III of SnRK2s and they include ==SnRK2.6/OST1/SRK2E==, and SNRK2.2/SRK2D and SnRK2.3/SRK2I


Structures in the figure are: R, apo PYR2, [[3kdh]];  R<sup>.</sup>ABA, PYR2<sup>.</sup>ABA, [[3kdi]]; K-P; SnRK2.6-HAB1, [[3ujg]]; R<sup>.</sup>ABA-P, PYR2<sup>.</sup>ABA-ABI2, [[3ujl]]; K, SnRK2.6,[[3uc4]].
'''Structures with pyrabactin or homolog and a PP2C'''<br>
[[4la7]], [[4lg5]] AtPYL2.Quinabactin - AtHAB1<br>
[[3nmn]], AtPYL1.pyrabactin in complex with AtABI1<br>
[[3nmt]], AtPYL2 mutant A93F.pyrabactin in complex with type 2C protein phosphatase AtHAB1<br>
[[3nmv]], AtPYL2 mutant A93F.pyrabactin in complex with type 2C protein phosphatase AtABI1<br>


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