An alternative transcript of the S locus glycoprotein gene in a class II pollen-recessive self-incompatibility haplotype of Brassica oleracea encodes a membrane-anchored protein.

Recent reports have shown that SLG, one of two genes linked to the S locus of Brassica, encodes a secreted glycoprotein. We have used RNA gel blot analysis, genomic and cDNA clone analysis, expression in transgenic plants, and immunodetection to characterize SLG2, the SLG gene derived from the S2 haplotype. This haplotype belongs to the class II group of S haplotypes that exhibit a weak incompatibility phenotype and are pollen recessive. We showed that SLG2 produces two transcript forms: the expected 1.6-kb transcript that predicts a secreted glycoprotein and an alternative 1.8-kb transcript that predicts a membrane-anchored protein. Stigmas of the S2 haplotype and pistils of transgenic tobacco plants transformed with the SLG2 gene produce a membrane-associated 62-kD protein as well as soluble 57- and 58-kD glycoforms. Because of the sequence similarity between SLG2 and the extracellular domain of the S Locus Receptor Kinase (SRK2) gene, the membrane-anchored form of SLG2 may be viewed as a naturally occurring truncated form of the receptor that lacks the kinase catalytic domain. The occurrence of this protein has potential implications for the activity of the full-length receptor. Furthermore, the underlying structure of the SLG2 gene suggests the evolution of SLG from an ancestral SRK-like gene.