The S-locus receptor kinase is inhibited by thioredoxins and activated by pollen coat proteins

The self-incompatibility response in Brassica allows recognition and reject ion of self-pollen by the stigmatic papillae. The transmembrane S-locus rec eptor kinase (SRK), a member of the receptor-like kinase superfamily in pla nts, mediates recognition of self-pollen on the female side(1), whereas the S-locus cysteine-rich protein (SCR) is the male component of the self-inco mpatibility response(2). SCR is presumably located in the pollen coat, and is thought to be the SRK ligand(2,3). Although many receptorlike kinases ha ve been isolated in plants, the mechanisms of signal transduction mediated by these molecules remain largely unknown. Here we show that SRK is phospho rylated in vivo within one hour of self-pollination. We also show that, in vitro, autophosphorylation of SRK is prevented by the stigma thioredoxin TH L1 in the absence of a ligand. This inhibition is released in a haplotype-s pecific manner by the addition of pollen coat proteins. Our data indicate t hat SRK is inhibited by thioredoxins and activated by pollen coat proteins.