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 l ocus of Brassica, encodes a secreted glycoprotein. We have used RNA ge l blot analysis, genomic and cDNA clone analysis, expression in transg enic plants, and immunodetection to characterize SLG2, the SLG gene de rived from the S2 haplotype. This haplotype belongs to the class II gr oup of S haplotypes that exhibit a weak incompatibility phenotype and are pollen recessive. We showed that SLG2 produces two transcript form s: the expected 1.6-kb transcript that predicts a secreted glycoprotei n and an alternative 1.8-kb transcript that predicts a membrane-anchor ed protein. Stigmas of the S2 haplotype and pistils of transgenic toba cco plants transformed with the SLG2 gene produce a membrane-associate d 62-kD protein as well as soluble 57- and 58-kD glycoforms. Because o f the sequence similarity between SLG2 and the extracellular domain of the S Locus Receptor kinase (SRK2) gene, the membrane-anchored form o f SLG2 may be viewed as a naturally occurring truncated form of the re ceptor 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 sugg ests the evolution of SLG from an ancestral SRK-like gene.