Bovine glomerular basement membrane. Location and structure of the asparagine-linked oligosaccharide units and their potential role in the assembly of the 7 S collagen IV tetramer.

Collagen IV contains an amino-terminal tetramerization domain (7 S) that is involved in aggregation and cross-linking as part of the process of self-assembly of the collagen IV matrix of basement membranes. We determined the structure and location of the Asn-linked oligosaccharides of the 7 S tetramer. Two glycopeptides, GP-1 and GP-2, were isolated from tryptic digests of the 7 S tetramer and were characterized. GP-1 and GP-2 are derived from the alpha 1(IV) chain and the alpha 2(IV) chain, respectively. Each glycopeptide contained one sequence, -Asn-Xaa-Thr-, which was shown to be N-glycosylated at Asn, corresponding to position 126 of the alpha 1 chains and 138 of the alpha 2 chain. 1H NMR spectroscopic analysis of the oligosaccharide is a biantennary N-acetyllactosamine type of N-linked oligosaccharide with a broad heterogeneity in the presence of the sugar residues at their nonreducing termini as indicated. [formula: see text] The location of the Asn-linked oligosaccharide units and Hyl-linked disaccharide units and their orientation with respect to the surface of the triple helix were calculated using two models. We conclude that both units are important determinants in the assembly of the 7 S tetramer.