Characterization of O-linked glycosylation motifs in the glycopeptide domain of bovine κ-casein

κ-Casein is the major glycoprotein in bovine milk. It has a proteinase-sensitive (chymosin) site which cleaves the glycoprotein into two segments: N-terminal para-κ-casein domain and the C-terminal κ-casein macroglycopeptide domain which is highly heterogeneous in oligosaccharide content. We have identified six sites of O-glycosylation on the macroglycopeptide by solid-phase Edman degradation: Thr121, Thr131, Thr133, Thr136 (A variant only), Thr142 and Thr165. No Ser residues are glycosylated. The glycosylation status of 15 of 17 potential O-glycosylation sites in the B variant was accurately predicted using the four peptide motifs previously proposed for the glycosylation of human glycophorin A (Pisano,A., Redmond,J.W., Williams,K.L. and Gooley,A.A., Glycobiology, 3, 429-435, 1993), provided one additional assumption is made concerning an inhibitory role for a nearby Ile.