Structural Basis of ABA Perception by PYR/PYL/RCAR Receptors - Abscisic Acid: Metabolism, Transport and Signaling - page 120

PYL binding to ABA enantiomers, such as steric hindrance by the two bulk side
chains of I112 and L165 in PYL9 (see Fig. 7.5 c). Moreover, hydrophobic interac-
tion through indirect interaction with 8′, 9′ methyl groups of (-)-ABA also con-
tributes the stereospecificity of PYLs to ABA enantiomers, because V66I mutation
increases PYL9's inhibitory ability on PP2Cs (see Fig. 7.5 c). This study on PYLs
preference to (-)-ABA might provide novel insights into the design of selective
ABA agonists.
7.3 The Architecture of Ternary Complexes
PYLs-ABA-PP2Cs
As we all know, the important role of PYLs in ABA signaling pathway is to
inhibit the phosphatase activity of PP2Cs in an ABA-dependent manner. The crys-
tal structures of four ternary complexes have been reported, PYL1-ABA-ABI1,
PYL2-ABA-HAB1, PYR1-ABA-HAB1, and PYL3-ABA-HAB1 (Melcher et al.
2009 ; Yin et al. 2009 ; Dupeux et al. 2011 ; Zhang et al. 2012 ). Moreover, the struc-
tures of PYL10-HAB1 and PYL13-PP2CA in the absence of ABA have also been
solved recently (Hao et al. 2011 ; Li et al. 2013 ).
In these structures, the PP2Cs catalytic cores (residues: ABI1 125-429, HAB1
172-511) adopt a folding with two central five-stranded ʲ -sheets sandwiched by
two pairs of ʱ -helices, and the catalytic site located at the edge of the two central
ʲ -sheets (see Fig. 7.6 a). The antiparallel ʲ -sheets of PP2Cs provide the binding
interface with PYLs, so the activity of PP2Cs is blocked by the closed gate loop of
PYLs when binding to each other. Other than the blocked active sites, the PP2Cs
contact with the PYLs through a small protruding region containing an important
Trp residue (Trp300 in ABI1, Trp385 in HAB1). This Trp residue points into the
ABA-binding pocket through the gate-and-latch loops covering ABA molecule,
and forms a water-mediated hydrogen bond to the ketone group of ABA. Upon
the insertion of the Trp in PP2Cs into the ABA-binding pocket, the gate loop and
latch loop undergo conformational changes to close the pocket, generate additional
contacts between PYLs and ABA molecule. Thus, the Trp residue acts as a lock to
keep the gate-and-latch loops in a more closed conformation, which is referred to
gate-latch-lock mechanism. Apart from these interactions, a conserved arginine
residue of PP2Cs stacks its guanidinium group with a conserved proline residue
on the gate loop (Arg389 in HAB1, Pro112 in PYL3). Moreover, the gate loop of
PYLs directly packs against the PP2Cs active site with a conserved serine residue,
which exposed upon ABA binding and formed two hydrogen bonds with the con-
served residues of PP2Cs (metal-stabilizing residue Glu203 in HAB1), and with
the conserved glycine residue on the active site loop of PP2Cs (Gly180 in ABI1,
Gly246 in HAB1) (see Fig. 7.6 b). Thus, the phosphatase active center of PP2Cs
is competitively blocked by PYLs bound. Overall, PP2Cs use their active sites to
interact with the gate-and-latch loops of PYLs and induce some conformational
changes; these PP2Cs-induced conformational changes make a higher binding