Characterization of the S locus genes, SLG and SRK, of the Brassica S3 haplotype: identification of a membrane-localized protein encoded by the S locus receptor kinase gene.

The S locus, which controls the self-incompatibility response in Brassica, has been shown to contain at least two genes. SLG encodes a secreted S locus glycoprotein whilst SRK encodes a putative S locus receptor kinase. SRK has been shown potentially to encode a functional kinase and genetic evidence indicates that this gene is essential for the self-incompatibility response. Here the characterization of the SRK and SLG genes of a Brassica line homozygous for the S3 haplotype is described. A 120 kDa glycoprotein was identified in stigmas and several lines of evidence indicated that this protein is encoded by the SRK3 gene. First, the 120 kDa glycoprotein was recognized by antibodies raised against peptides based on the SRK3 gene sequence. Secondly, this protein is polymorphic and, in an F2 population segregating for the S3 haplotype, was expressed only in plants possessing the S3 haplotype. Thirdly, the 120 kDa protein was expressed specifically in stigmas. Finally, the 120 kDa protein was only extracted from stigmas in the presence of detergent indicating that it is anchored in the membrane. SRK has been predicted to encode a transmembrane glycoprotein based on the deduced amino acid sequence. Located on the membrane, SRK is in a position to interface between an extracellular recognition event between pollen and pistil and an intracellular signal transduction pathway which initiates the self-incompatibility response.