Most Bovine Milk Fat Globule Membrane Glycoproteins Contain Asparagine-Linked Sugar Chains with GalNAcβ1→4GlcNAc Groups

Abstract

N-Acetylgalactosamine is usually not a constitutive monosaccharide of Asn-linked sugar chains. Our previous study showed that the Asn-linked sugar chains of bovine CD 36 prepared from milk fat globule membranes (MFGM) contain this unique monosaccharide as the Ga1NAcβ1→4GlcNAc group [Nakata et al. (1993) Biochemistry 32, 4369-4383]. Western blot analysis of bovine MFGM glycoproteins with Wistaria floribunda agglutinin (WFA), which binds oligosaccharides terminating with either an α- or β-N-acetylgalactosamine residue, showed that WFA binding is observed for most of the protein bands as detected with Coomassie Brilliant Blue staining. However, no WFA binding was observed for protein bands after treatment of MFGM glycoproteins with N-glycanase. Structural analyses of the sugar chains released by hydrazinolysis from the MFGM glycoproteins by sequential exoglycosidase digestion and by methylation analysis revealed that oligosac-charides, which bound to a WFA-agarose column, are bi-, tri-, and tetraantennary complex-type and hybrid-type sugar chains with the GalNAcβ1→4GlcNAc group in their outer chain moieties, while oligosaccharides, which passed through the column, were of high-mannose-type, hybrid-type, and complex-type, of which the latter two groups contained the Galβ1→4GlcNAc groups. These results indicated that many bovine MFGM glycoproteins contain Asn-linked sugar chains with the GalNAcβ1→4GlcNAc group.