| Summary: | Cellular involvement in collagen fibril organisation is a poorly understood phenomenon. The localisation of procollagen to the cell membrane coincident with secretion, the identification of cell surface receptors for collagen, and the assembly of collagen fibrils within extracellular compartments suggest a role for plasma membranes in the assembly and organisation of collagen fibrils. Cell-collagen interaction may be mediated by collagen receptors, cell surface proteoglycans and also membrane lipids. The interactions of type I collagen with phosphatidylcholine have been described and there have been several reports of lipid association with collagen. The interactions of [<SUP>3</SUP>H]procollagen I with various phospholipids were studied by density gradient centrifugation. At physiological conditions of pH, ionic strength and temperature, there was no evidence of procollagen binding to phosphatidylcholine phosphatidylethanolamine, phosphatidylinositol or phosphatidylserine liposomes. In contrast type I procollagen bound strongly to sphingomyelin lipsomes, in a reversible and saturable manner, with an apparent dissociation constant (K<SUB>d</SUB>) of 2.6 nM. Binding occurred over a range of temperatures (4<SUP>o</SUP>C to 35<SUP>oN</SUP>) and was relatively unaffected by salt concentrations up to 1.2 M NcCl. Binding was observed in phosphate buffers but not in the presence of high concentrations of Tris or Hepes. Bovine serum albumin had no effect on procollagen binding to sphingomyelin, neither did unlabeled type I collagen, with or without the non-helical telopeptides. Type II procollagen and denatured type I procollagen also bound to sphingomyelin. Procollagen binding to sphingomyelin at 35<SUP>o</SUP>C was considerably reduced when small amounts of phosphatidylcholine were present, though binding was partially restored when the temperature was reduced below the corresponding phase transition temperature. Purified, unlabelled procollagen C-propeptides successfully competed for binding, and [<SUP>125</SUP>I]C-propeptides bound to sphingomyelin in the absence of procollagen. Weaker binding to sphingomyelin, mediated by the collagen triple helical region, was also observed, but this was dominated by the sphingomyelin C-propeptide interaction. The data suggest a novel mechanism for matrix vesicle mediated biomineralisation.
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