Glycosylation sites identified by detection of glycosylated amino acids released from Edman degradation: the identification of Xaa-Pro-Xaa-Xaa as a motif for Thr-O-glycosylation.

Here we report the use of automated Edman degradation of covalently linked glycopeptides to identify positively the sites of O- and N-glycosylation. The O-glycosidic linkage of carbohydrate to the hydroxy amino acids Ser and Thr is a major form of post-translational modification. However, unlike Asn-linked glycosylation, which is identified by the consensus sequence Asn-Xaa-Thr/Ser, no simple motif conferring O-linkage to Thr and Ser has been described. After sequencing glycopeptides derived from two cell surface glycoproteins, a Thr-O-glycosylation motif of Xaa-Pro-Xaa-Xaa, where at least one Xaa = Thr(Sac), has been defined. This motif predicts the site(s) of Pro- associated Thr-O-glycosylation in O-glycosylated proteins, although it is clear that there are also other forms of Thr-O-glycosylation not associated with Pro.

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