MICROHETEROGENEITY OF N-GLYCOSYLATION ON A STYLAR SELF-INCOMPATIBILITY GLYCOPROTEIN OF NICOTIANA-ALATA

Gametophytic self-incompatibility, a mechanism that prevents inbreedin g in some families of flowering plants, is mediated by the products of a single genetic locus, the S-locus. The products of the S-gene in th e female sexual tissues of Nicotiana alata are an allelic series of gl ycoproteins with RNase activity. In this study, we report on the micro heterogeneity of N-linked glycosylation at the four potential N-glycos ylation sites of the S-2-glycoprotein. The S-glycoproteins from N.alat a contain from one to five potential N-glycosylation sites based on th e consensus sequence Asn-Xaa-Ser/Thr. The S-2-glycoprotein contains fo ur potential N-glycosylation sites at Asn(27), Asn(37), Asn(138) and A sn(150), designated sites I, II, TV and V, respectively. Site LII is a bsent from the S-2-glycoprotein. Analysis of glycopeptides generated f rom the S-2-glycoprotein by trypsin and chymotrypsin digestions reveal ed the types of glycans and the degree of microheterogeneity present a t each site. Sites I (Asn(27)) and IV (Asn(138)) display microheteroge neity, site II (Asn(37)) contains only a single type of N-glycan, and site V (Asn(150)) is not glycosylated. The microheterogeneity observed at site I on the S-2-glycoprotein is the same as that observed at the only site, site I, on the S-1-glycoprotein (Woodward et al., Glycobio logy, 2, 241-250, 1992), Since the N-glycosylation consensus sequence at site I is conserved in all S-glycoproteins from other species of se lf-incompatible solanaceous plants, glycosylation at this site may be important to their function. No other post-translational modifications (e.g. O-glycosylation, phosphorylation) were detected on the S-2-glyc oprotein.