Structural Basis of ABA Perception by PYR/PYL/RCAR Receptors - Abscisic Acid: Metabolism, Transport and Signaling - page 117

C-terminal ʱ -helix is also involved in the gate closing. In PYR1 and PYL10, the
front parts of the C-terminal ʱ -helix alter conformation after binding ( + )-ABA
(see Fig. 7.8 a). These conformational alterations facilitate the latch loop lock the
closed gate loop by hydrogen bonds and van der Waals contacts, refers to open-to-
closed gating mechanism. Altogether, the gate-and-latch loops in apo-PYLs adopt
an open conformation that allows the entry of ABA into the binding pocket. Upon
binding ABA, the gate-and-latch loops fold over ABA to complete ABA enclosure
by conformation changes.
Interestingly, the published PYR1, PYL1, PYL2, and PYL3 structures were
homodimers, whereas PYL5, PYL9, and PYL10 were monomers. From gel filtra-
tion chromatography, we know that PYR1, PYL1-3 are dimers in solution, PYL4-
10 are monomers in solution (Hao et al. 2011 ). The structural changes by ABA
perception mentioned above only consider one protomer of PYLs. Thus we should
consider the oligomeric state of PYLs. For homodimeric PYLs, the above mecha-
nisms also can be applied. For PYR1, PYL1, PYL2, except PYL3, the apo-PYLs
and ABA-bound PYLs form cis-homodimers by the gate loop and the C-terminal
ʱ -helix. Upon binding ABA, the gate loop and the C-terminal ʱ -helix change their
conformation just like those in the monomeric PYLs, which alter the relative ori-
entation of the two promoters and weaken the dimeric assembly to form complex
with ABA. Similar dimerization of PYL3 (cis-homodimer) has also been found in
the apo-PYL3 structure (see Fig. 7.4 a), but ABA-bound PYL3 transforms into the
trans-homodimer by a protomer rotation of almost 135° compared with the cis-
homodimer (see Fig. 7.4 b). The gate-and-latch loops in the trans-homodimer are
more exposed into the solvent (Zhang et al. 2012 ) (see Fig. 7.4 b). Therefore, based
on the solved structures of PYLs-( + )-ABA, the PYLs could be divided into three
major subclasses, cis-homodimer for PYR1, PYL1, and PYL2, trans-homodimer
for PYL3, monomer for PYL4-PYL13 (Zhang et al. 2012 ).
The above-mentioned ABA is a naturally S-( + )-ABA form (also referred to
as ( + )-ABA), with one asymmetric carbon atom at C-1' position. However, the
synthetic enantiomer (-)-ABA (also referred to as R-(-)-ABA) has been exten-
sively investigated since the discovery of ABA (Cutler et al. 2010 ) (see Fig. 7.5 a).
Previous studies showed that in some assays, the (-)-ABA has same or ever higher
activity compared with ( + )-ABA (Milborrow 1974 ). Two hypothesized mecha-
nisms were mentioned for explaining the bioactivity of (-)-ABA; one is the same
occupied site between these two ABA molecules by flipping the cyclohexene plane
(Milborrow 1974 ), the other is the dual selectivity of ABA receptors (Nambara
et al. 2002 ). From the reported assay (Zhang et al. 2013 ), all PYLs inhibited the
phosphatase activity of PP2Cs in the presence of ( + )-ABA except PYL7, PYL11,
and PYL12 that denied soluble expression in Escherichia coli . However, the situ-
ation for (-)-ABA in all PYLs was different. The inhibition of HAB1 by mono-
meric PYLs was stronger than that by cis-dimeric PYLs, except monomeric
PYL10. Interestingly, trans-dimeric PYL3 showed a powerful inhibition of PP2Cs
like monomeric PYLs. PYL5 bound (-)-ABA very strong, while PYL9 did not
inhibit PP2Cs activity in the presence of (-)-ABA. Due to the highly sequence
conservation, further structural investigations of apo-PYL5, PYL3-(-)-ABA and