Plant Kelch phosphatases are Ser/Thr phosphatases involved in cell cycle regulation

ABSTRACT

Brassinosteroids (BRs) are plant steroid hormones sensed at the cell surface by the membrane receptor kinase BRI1. Activation of BRI1 leads to the dephosphorylation of BZR1/BES1 transcription factors through inactivation of the glycogen synthase kinase 3 BIN2. Overexpression of the Kelch phosphatase BRI1 SUPPRESSOR 1 (BSU1) rescued the growth defects of bri1 loss-of-function mutants. Subsequent studies identified BSU1 as a protein tyrosine phosphatase, which promotes BR signaling by dephosphorylating a phosphotyrosine in BIN2 critical for kinase activity. Crystal structures of the BSU1 phosphatase domain now reveal a high degree of structural similarity to protein phosphatase 1 (PP1), a eukaryotic Ser/Thr phosphatase. Consistently, BSU1 efficiently dephosphorylates phosphothreonine- and phosphoserine-containing substrate peptides, but shows no detectable activity toward phosphotyrosine substrates. A catalytically inactive BSU1 phosphatase domain suppresses the growth phenotypes of the Arabidopsis bri1-5 mutant and binds the BSU1 homologs BSL1-3. bsu1 and bsu1 bsl1 bsl2/3 loss-of-function mutants display wild type-like BR responses, but show stomatal patterning and fertility defects. Importantly, the PP1-like C-terminal tail of BSU1 is phosphorylated at Thr785 by a cyclin-dependent kinase complex. The phosphorylated tail binds to the BSU1 substrate binding grooves and blocks access to the active site. Mutation of Thr785 to alanine activates BSU1, suggesting that Kelch phosphatases and PP1 share a common regulatory mechanism. Deletion of the Marchantia polymorpha Kelch phosphatase MpBSLM results in an undifferentiated cell mass phenotype, associated with the over-activation of a cell cycle reporter. Taken together, our work identifies Kelch phosphatases as PP1-like cell cycle regulators in plants.