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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:19933100</ref><ref>PMID:23265948</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 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.  
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:19933100</ref><ref>PMID:23265948</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 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 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 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]].


==Pyr/Pyl/RCAR family of ABA receptors==
==Pyr/Pyl/RCAR family of ABA receptors==
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PYR1/RCAR11 and PYL9/RCAR1  
PYR1/RCAR11 and PYL9/RCAR1  
PYR1, PYL1, PYL2, and PYL3 show ABA-dependent inhibition of PP2Cs such as ABI1, HYPERSENSITIVE TO ABA 1 (HAB1), HAB2, and ABA-HYPERSENSITIVE GERMINATION 3 (AHG3, also named PP2CA)<ref>PMID:23265948</ref>
PYR1, PYL1, PYL2, and PYL3 show ABA-dependent inhibition of PP2Cs such as ABI1, HYPERSENSITIVE TO ABA 1 (HAB1), HAB2, and ABA-HYPERSENSITIVE GERMINATION 3 (AHG3, also named PP2CA)<ref>PMID:23265948</ref>
"Upon binding to ABA or pyrabactin, the gate
(Melcher et al., 2009) of PYL undergoes a pronounced conformational
change, and the PYL protomers in the cis-homodimer
separate slightly, decreasing the area of interface and creating
a binding surface for protein phosphatase 2C (PP2C)." "Upon ligand binding, the apo-PYL3 cis-homodimer is
converted to a trans-homodimer, in which two ligand-bound
PYLs bind to each other in a reverse direction"<ref>"PMID:22579247</ref>




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|-
|-
|<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='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/1'/><br  clear='both'>'''3klx scene'''<Br><scene name='56/564059/Apopyl3cisdimer/1'>Apo PYL3 ''cis'' dimer </scene>
|<applet load='4dsc' size='300' frame='true' align='left' caption='4dsc - PYL3.ABA ''trans'' dimer' scene = '56/564059/Pylabatransdimer/1'/><br  clear='both'>'''4dsc scene'''<Br><scene name='56/564059/Pylabatransdimer/1'>PYL3.ABA ''trans'' dimer'</scene>
|}  
|}  


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[[3kay]], apo AtPYL1 <br>
[[3kay]], apo AtPYL1 <br>
[[3kdh]], [[3kaz]], [[3kl1]] apo AtPYL2<br>
[[3kdh]], [[3kaz]], [[3kl1]] apo AtPYL2<br>
[[3klx]], apo AtPYL3<br>
[[3klx]], Apo AtPYL3<br>
[[4jdl]], Apo AtPYL5<br>
[[4jdl]], Apo AtPYL5<br>
[[3rt2]], [[3uqh]] apo AtPYL10<br>
[[3rt2]], [[3uqh]] apo AtPYL10<br>
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