CHARACTERIZATION OF THE S-LOCUS GENES, SLG AND SRK, OF THE BRASSICA S-3 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 Brass ica, has been shown to contain at least two genes. SLG encodes a secre ted S locus glycoprotein whilst SRK encodes a putative S locus recepto r kinase. SRK has been shown potentially to encode a functional kinase and genetic evidence indicates that this gene is essential for the se lf-incompatibility response. Here the characterization of the SRK and SLG genes of a Brassica line homozygous for the S-3 haplotype is descr ibed. A 120 kDa glycoprotein was identified in stigmas and several lin es of evidence indicated that this protein is encoded by the SRK(3) ge ne. First, the 120 kDa glycoprotein was recognized by antibodies raise d against peptides based on the SRK(3) gene sequence. Secondly, this p rotein is polymorphic and, in an F-2 population segregating for the S- 3 haplotype, was expressed only in plants possessing the S-3 haplotype . Thirdly, the 120 kDa protein was expressed specifically in stigmas. Finally, the 120 kDa protein was only extracted from stigmas in the pr esence of detergent indicating that it is anchored in the membrane. SR K has been predicted to encode a transmembrane glycoprotein based on t he deduced amino acid sequence. Located on the membrane, SRK is in a p osition to interface between an extracellular recognition event betwee n pollen and pistil and an intracellular signal transduction pathway w hich initiates the self incompatibility response.