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of approximately 40 nM, similar to the value reported for GCR2 (Pandey et al.
2009 ). However, they showed an unusual type of G protein signaling, in which
the GTP-bound G ʱ protein 'turns off' signaling while the GDP-bound form allows
ABA binding to the GTG receptor and the initiation of a signaling cascade, differ-
ent from the conventional model for signaling by G proteins. However, the above
experiments were unable to show evidence for at least one binding site per GTG
protein molecule (Klingler et al. 2010 ).
Another possible ABA receptor protein in Arabidopsis is CHLH (Mg-chelatase
H subunit), a chloroplast protein involved in chlorophyll biosynthesis. With the K d
value of 30-40 nM, a lot of overexpression, RNAi, insertion, and point mutations
experiments have shown the affinity between CHLH and ABA (Shen et al. 2006 ).
Compared with those above ABA receptors, PYLs are widely studied with crys-
tallographic studies, site-specific mutations, and so on. Also, ABA-PYLs signal-
ing pathway are investigated in detail. Studies of PYLs give more information
for understanding of ABA signaling and give more means to identify other ABA
receptors.
Structural studies of other phytohormone receptors have been identified in
recent years. Auxin binds to TIR1 and mediates contacts between TIR1 and the
transcriptional repressor Aux/IAA, but auxin binding does not induce significant
conformational changes in the receptor, so auxin is called a molecule glue (Tan
et al. 2007 ). However, gibberellin induces conformational changes in GID1, pro-
viding the binding interface for the transcriptional repressor DELLA, functioned
as allosteric effector (Murase et al. 2008 ; Shimada et al. 2008 ). Similar to gib-
berellin, ABA induces conformational changes in PYLs by binding to the pocket,
creating a binding interface for PP2Cs. So, ABA regulates PYLs as an allosteric
effector. Although the common action of phytohormone toward receptors is to
introduce interaction to the downstream proteins, the mechanisms of the action are
quite different for each receptors (Umezawa et al. 2010 ).
These structural investigations have led to major progress in understanding hor-
mone signaling; the mechanistic basis of them provides a rational framework for
future design of alternative ligands.
This chapter gives a detailed structural analysis of ABA perception by
PYR/PYL/RCAR receptors and the PYLs-ABA-PP2Cs ternary complex struc-
tures, explains the gate-latch-lock mode, and elucidates its molecular mecha-
nisms. However, many questions are still unknown, why the ABA signaling
pathway in Arabidopsis involves so many different PYLs receptors. The structural
mechanism of the individual receptors differs in their sensitivity and selectivity in
response to ABA, how can we develop new selective ABA agonists and antago-
nists to use in agriculture? Finally, this is just a beginning to understand the ABA
signaling pathway. Much work needs to be done to gain a global mechanistic syn-
thesis of the complete signaling network.
Acknowledgments The work in author's laboratory was supported by National Basic Research
Program of China (973 Program, 2011CB965304), and National Natural Science Foundation of
China (31370720 and 31222032).
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