Distinct patterns of glycosylation of colligin, a collagen-binding glycoprotein, and SPARC (osteonectin), a secreted Ca2+-binding glycoprotein. Evidence for the localisation of colligin in the endoplasmic reticulum.

Mouse parietal endoderm PYS cells were labelled with [2-3H]mannose for 16-24 h. Colligin, an Mr-47000 collagen-binding protein, and SPARC, a Mr-43000 protein, highly homologous to the Ca2+-binding protein osteonectin, were isolated from labelled cell extracts and culture medium respectively. Glycopeptides obtained by exhaustive digestion with pronase were analysed by lectin-affinity, ion-exchange, and gel-filtration chromatography and by paper chromatography of high-mannose oligosaccharides after endo H release. The results show that the N-linked carbohydrate chains of colligin are exclusively the high-mannose type, of which (Man)8(GlcNAc)2 and (Man)9(GlcNAc)2 make up 77%. This carbohydrate structure provides strong evidence that colligin is a component of the endoplasmic reticulum, and argues against a role in cell-surface interactions. By contrast to colligin, SPARC secreted by PYS cells contains predominantly a diantennary complex type of chain containing a variable number of sialic acid and core-substituted fucose residues. Similar glycosylation patterns to those discussed above were seen in colligin isolated from primary mouse embryonic parietal endoderm cells and the murine 3T3 cell line, and in SPARC secreted by bovine corneal endothelial cells. Unlike the type-IV-collagen-binding glycoprotein studied by Dennis, J., Waller, C. and Schirrmacher, V. [J. Cell Biol. 99, 1416-1423 (1984)], removal of N-linked oligosaccharides from colligin had no effect on its binding to native type IV collagen.

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Author Brigid L. M. Hogan Cell Biology