Differential recognition by conglutinin and mannan-binding protein of N-glycans presented on neoglycolipids and glycoproteins with special reference to complement glycoprotein C3 and ribonuclease B.

Conglutinin and mannan-binding protein are serum proteins that have similar carbohydrate binding specificities toward high mannose-type oligosaccharides, and yet only conglutinin binds the complement glycoprotein iC3b, which contains oligosaccharides of this type. In the present study, the interactions of conglutinin and mannan-binding protein were evaluated with the complement glycoprotein C3, including various physiologically derived fragments of this glycoprotein, and neoglycolipids prepared from oligosaccharides released from C3 and its isolated alpha and beta chains. Several conclusions can be drawn. First, the interaction of conglutinin is profoundly influenced by the state of the protein moiety of the alpha chain in the vicinity of the glycosylation site Asn-917. Second, the binding to the C3-derived glycoprotein iC3b appears to be exclusively mediated through the Man8 or Man9 oligosaccharide on the alpha chain; there is no evidence for other N-linked oligosaccharides on C3 that are uniquely bound by conglutinin. Third, although conglutinin shows a more restricted binding relative to mannan-binding protein toward the oligosaccharides free of protein, it has a broader binding pattern toward the oligosaccharides as presented on C3-derived glycoproteins. From these and additional observations with RNase B, which contains high mannose-type oligosaccharides at Asn-34, it is clear that the protein moieties of these glycoproteins markedly influence the presentation of the oligosaccharides such that biological specificity is mediated by the commonly occurring high mannose-type oligosaccharides in the context of specific carrier proteins.