Maintaining basal B-RAF kinase activity for abscisic acid signaling via reciprocal phosphoregulation of a single serine residue.

Abstract

The phytohormone abscisic acid (ABA) regulates plant responses to environmental stresses, development, and immunity. Under unfavorable conditions, ABA forms a complex with its receptor proteins Pyrabactin Resistance 1 (PYR1)/PYR1-likes (PYLs)/Regulatory Component of ABA Receptors (RCARs), inhibiting Clade A Protein Phosphatases Type 2C (PP2Cs) and releasing Sucrose Non-Fermenting-1-Related Protein Kinase 2s (SnRK2s) from PP2C-mediated inhibition. Rapidly Accelerated Fibrosarcoma (RAF) kinases from the B1, B2, and B3 subgroups phosphorylate and reactivate SnRK2s, initiating ABA responses. While ABA does not significantly activate B-RAFs, their basal activity is essential for initiating ABA signaling. However, the mechanisms sustaining this basal B-RAF activity are not fully understood. In this study, we revealed that Clade A PP2Cs interact with and dephosphorylate a certain number of B3 subgroup RAFs at a conserved serine residue, corresponding to Ser619 in RAF3, within the phosphate-binding loop. A phosphomimicking mutation at this residue, RAF3^S619D, failed to bind ATP and exhibited diminished kinase activity in vitro and in vivo. Ser619 in RAF3 is an autophosphorylation site, phosphorylated by recombinant RAF3-KD but not by its substrate SnRK2.6. The RAF3^S619A mutant, abolishing Ser619 autophosphorylation, displayed increased kinase activity in vitro. The B-RAF high-order mutant OK¹⁰⁰-B3 carrying RAF3^S619A showed enhanced ABA sensitivity compared with those with wild-type RAF3. Thus, PP2C-mediated dephosphorylation and the autophosphorylation of this unique serine residue dynamically regulate ATP binding affinity and tightly control RAF3 activity during various ABA signaling phases. This intricate mechanism ensures rapid RAF-SnRK2 cascade activation during stress while promptly desensitizing RAFs once stress signaling commences.