The Brassinosteroid-Activated BRI1 Receptor Kinase Is Switched off by Dephosphorylation Mediated by Cytoplasm-Localized PP2A B' Subunits.

Brassinosteroid (BR) binding activates the receptor kinase BRI1 by inducing heterodimerization with its co-receptor kinase BAK1; however, the mechanisms that reversibly inactivate BRI1 remain unclear. Here we show that cytoplasm-localized protein phosphatase 2A (PP2A) B' regulatory subunits interact with BRI1 to mediate its dephosphorylation and inactivation. Loss-of-function and overexpression experiments showed that a group of PP2A B' regulatory subunits, represented by B'η, negatively regulate BR signaling by decreasing BRI1 phosphorylation. BR increases the expression levels of these B' subunits, and B'η interacts preferentially with phosphorylated BRI1, suggesting that the dynamics of BR signaling are modulated by the PP2A-mediated feedback inactivation of BRI1. Compared with PP2A B'α and B'β, which promote BR responses by dephosphorylating the downstream transcription factor BZR1, the BRI1-inactivating B' subunits showed similar binding to BRI1 and BZR1 but distinct subcellular localization. Alteration of the nuclear/cytoplasmic localization of the B' subunits revealed that cytoplasmic PP2A dephosphorylates BRI1 and inhibits the BR response, whereas nuclear PP2A dephosphorylates BZR1 and activates the BR response. Our findings not only identify the PP2A regulatory B subunits that mediate the binding and dephosphorylation of BRI1, but also demonstrate that the subcellular localization of PP2A specifies its substrate selection and distinct effects on BR signaling.

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