Abstract
ABI1 encodes a protein phosphatase 2C that functions early in the Abscisic acid (ABA) signaling cascade. To address the mechanism of ABI1-mediated ABA signaling, we generated tagged ABI1 Arabidopsis expression lines and performed affinity column purification. Interestingly, the most robust in planta ABI1-interacting proteins were nine of the 14 PYR/PYL/RCAR proteins, which were recently reported as ABA receptors, demonstrating in vivo PYR/PYL/RCAR interactions with ABI1 in Arabidopsis. To investigate the biological relevance of the PYR/PYLs, we analyzed pyr1pyl1pyl2pyl4 quadruple mutant plants and found strong insensitivity in ABA-induced stomatal closure.
We investigated the structural mechanisms by which PYR/PYL/RCAR proteins mediate signaling. PYR1 crystallographic structures reveal an α/β helix-grip fold and homodimeric assembly. The homodimeric assembly was verified in planta by co-immunoprecipitation. ABA binding within a large internal cavity switches structural motifs distinguishing ABA-free "open-lid" from ABA-bound "closed-lid" conformations. Site-directed PYR1 mutants designed to disrupt hormone binding lose ABA-triggered interactions with ABI1 in planta.
