Structural Basis of ABA Perception by PYR/PYL/RCAR Receptors - Abscisic Acid: Metabolism, Transport and Signaling - page 125

Zhang et al. 2012 ). Pyrabactin binds to PYL1 and PYR1 in productive mode as
an agonist, whereas to PYL2 or PYL3 in non-productive mode as an antagonist,
revealing that pyrabactin can adopt different conformations within the highly con-
served ligand-binding pocket of PYLs (Zhang et al. 2012 ).
Structural analyses have revealed that pyrabactin occupies the similar bind-
ing pocket to ABA, although their chemical structures are different. From the
superimposition of the structures of PYLs-ABA and PYLs-pyrabactin, several
key interactions are conserved between them. The pyridyl nitrogen or the sulfon-
amide functional group of pyrabactin occupies the position of carboxylate oxy-
gen of ABA, forming hydrogen bonds with Lys86 and Glu121 in PYL1 (Lys64
and Glu98 in PYL2). The amine group of the sulfonamide moiety of pyrabactin
occupies the position of the hydroxyl group of ABA, forming a hydrogen bond
with Asn197 in PYL1 (Asn173 in PYL2). The naphthalene ring of pyrabactin
mimics the 2,6,6-trimethyl-cyclohexene ring of ABA, forming hydrophobic
interactions with the gate loop of PYLs and promoting the closure of gate (see
Fig. 7.10 a).
Moreover, there are also some obvious differences between the solved four
PYLs-pyrabactin complexes. Pyrabactin can induce the closure of the gate in
PYR1 and PYL1, PYL3 as an agonist or antagonist, but not in PYL2, although
it is bound in the same binding pocket. Compared with those in PYL1 or PYR1,
the naphthalene and pyridine rings of pyrabactin bound in PYL2 rotate almost
Fig. 7.10 Structural basis for pyrabactin selectivity. a Structure superimposition of structures
PYL1-pyrabactin (3NEF, green ), PYR1-pyrabactin (3NJO, yellow ), PYL2-pyrabactin (3NR4,
cyan ) and PYL3-pyrabactin (3OJI, magenta ). The lower panel shows the chemical structures of
ABA and pyrabactin. b The molecular determinant affects the agonism and antagonism of pyra-
bactin toward PYR1, PYL1 and PYL2. c Structure superimposition of PYL2-pyrabactin and
PYL3-pyrabactin. The residues affect the gate loop closure are shown in sticks