Protein Conformation Ensembles Monitored by HDX Reveal a Structural Rationale for Abscisic Acid Signaling Protein Affinities and Activities

Summary Plants regulate growth and respond to environmental stress through abscisic acid (ABA) regulated pathways, and as such these pathways are of primary interest for biological and agricultural research. The ABA response is first perceived by the PYR/PYL/RCAR class of START protein receptors. These ABA activated receptors disrupt phosphatase inhibition of Snf1-related kinases (SnRKs), enabling kinase signaling. Here, insights into the structural mechanism of proteins in the ABA signaling pathway (the ABA receptor PYL2, HAB1 phosphatase, and two kinases, SnRK2.3 and 2.6) are discerned through hydrogen/deuterium exchange (HDX) mass spectrometry. HDX on the phosphatase in the presence of binding partners provides evidence for receptor-specific conformations involving the Trp385 "lock" that is necessary for signaling. Furthermore, kinase activity is linked to a more stable "closed" conformation. These solution-based studies complement the static crystal structures and provide a more detailed understanding of the ABA signaling pathway.