BRS1, a serine carboxypeptidase, regulates BRI1 signaling in Arabidopsis thaliana

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Abstract
Brassinosteroid-insensitive 1 ( BRI1 ) of Arabidopsis thaliana encodes a cell surface receptor for brassinosteroids. Mutations in BRI1 severely affect plant growth and development. Activation tagging of a weak bri1 allele ( bri1 - 5 ) resulted in the identification of a new locus, brs1 - 1D. BRS1 is predicted to encode a secreted carboxypeptidase. Whereas a brs1 loss-of-function allele has no obvious mutant phenotype, overexpression of BRS1 can suppress bri1 extracellular domain mutants. Genetic analyses showed that brassinosteroids and a functional BRI1 protein kinase domain are required for suppression. In addition, overexpressed BRS1 missense mutants, predicted to abolish BRS1 protease activity, failed to suppress bri1 - 5 . Finally, the effects of BRS1 are selective: overexpression in either wild-type or two other receptor kinase mutants resulted in no phenotypic alterations. These results strongly suggest that BRS1 processes a protein involved in an early event in the BRI1 signaling.