GLYCOSYLATION SITES IDENTIFIED BY SOLID-PHASE EDMAN DEGRADATION - O-LINKED GLYCOSYLATION MOTIFS ON HUMAN GLYCOPHORIN-A

The human red blood cell sialoglycoprotein, glycophorin A (GpA), conta ins a 'mucin-like' extensively O-glycosylated extracellular domain whi ch carries the MN blood group antigens. We have revised the sites of O -glycosylation in the extracellular domain of GpA by automated solid-p hase Edman degradation, which allowed positive identification and quan titation of O-glycosylated Ser and Thr residues, as well as the single N-glycosylation site. One N-linked and 16 O-linked sites were identif ied. Carbohydrate was absent on Ser1, Ser14, Ser15, Ser23, Thr28 and T hr58 in GpA. We propose that the glycosyltransferases present in eryth rocytes recognize specific flanking sequences around potential O-glyco sylation sites. All 16 O-glycosylation sites are explained on the basi s of four motifs. Three motifs are associated with Thr-glycosylation: Xaa-Pro-Xaa-Xaa where at least one Xaa = Thr; Thr-Xaa-Xaa-Xaa where at least one Xaa = Thr; Xaa-Xaa-Thr-Xaa where at least one X = Arg or Ly s. The fourth motif is associated with Ser-glycosylation: Ser-Xaa-Xaa- Xaa where at least one Xaa = Ser. These simple rules explain the glyco sylation (or lack of it) on 21 of 22 Ser/Thr in the extracellular doma in of GpA.