The S locus receptor kinase gene encodes a soluble glycoprotein corresponding to the SRK extracellular domain in Brassica oleracea

Self-incompatibility in Brassica is controlled by the S locus which contains at least two genes. SLG encodes a secreted S locus glycoprotein whilst SRK encodes a putative S locus receptor kinase which consists of three domains: an extracellular domain sharing extensive sequence identity with SLG, a transmembrane region, and a cytoplasmic domain exhibiting a serine/threonine protein kinase activity. Here, the existence of truncated forms of the SRK protein corresponding to the extracellular domain of the putative receptor is reported. These proteins were detected by an antibody which recognizes the N-terminus of SRK(3) and, in an F-2 progeny segregating for the S-3 haplotype, were only expressed in plants possessing the S-3 haplotype. The truncated SRK proteins were expressed specifically in stigmas but, unlike the membrane-spanning SRK(3) protein, were soluble and occurred as four different glycoforms sharing the same amino acid backbone as shown by deglycosylation experiments. Several SRK(3) transcripts that may code for these truncated SRK(3) proteins have been identified by RACE PCR, stigma cDNA library screening and RNA blot analysis. These transcripts are apparently generated by a combination of alternative splicing and the use of alternative polyadenylation signals. The existence of truncated forms of the S locus receptor kinase highlights some similarities between plant and animal receptor kinases. In animals, soluble extracellular domains of receptors have been described and, in some cases, have been shown to play a role in the modulation of signal transduction. By analogy, the soluble, truncated SRK proteins may play a similar role in the self-incompatibility response.