Structural Basis of ABA Perception by PYR/PYL/RCAR Receptors - Abscisic Acid: Metabolism, Transport and Signaling - page 116

in other PYLs abrogate completely this steric clash (Li et al. 2013 ). Second, the
isoprene moiety and the cyclohexene ring (see Fig. 7.2 d) form a lot of hydropho-
bic interactions and water-mediated hydrogen bonds with the side chains in PYLs.
Among these, the interactions between ABA and residues in the gate or latch loops
are most important in stabilizing its closure conformation.
Through the superposition of apo-PYLs and ( + )-ABA-bound PYLs structures,
it reveals notable conformation changes in the gate-and-latch loops. In apo-form,
the proline residue in the gate (such as P88 in PYR1, P115 in PYL1, P92 in PYL2,
P112 in PYL3) is flipped out of the binding pocket. Upon binding ( + )-ABA, the
proline moves toward the ligand to cover the cavity. In addition, the imidazole ring
of histidine on the latch (such as H115 in PYR1, H142 in PYL1, H119 in PYL2,
H139 in PYL3) also turns into the binding pocket to form van der Waals contacts
with the cyclohexene moiety of ABA (see Fig. 7.2 a). Besides, the front part of the
Fig. 7.2 ( + )-ABA binding in the PYL3 pocket. a Superposition of apo-PYL3 (pdb ID:3KLX,
cyan ) and PYL3-( + )-ABA (pdb ID:4DSC, green ). Pro112 in gate loop is flipped out of the bind-
ing pocket in the apo-form. Residues Pro112 and His139 in latch loop turn into the cavity after
binding ABA. b ABA ( cyan ) buried in the PYL3 cavity, Lys79 is located at the bottom of the cav-
ity, establishes an electrostatic interaction between its amine group and the carboxylate group of
the ABA. c 2D map of interactions between ( + )-ABA and PYL3. Hemispheres represent hydro-
phobic interactions whereas polar interactions are represented by lines . Cyan spheres indicate
water molecules. Mg 2 + indicates magnesium ions. d The chemical structure of ( + )-ABA