Analysis Of Rat And Humancdnas Encoding Colligin, A Collagen-binding Protein

Several cDNA clones for both human and rat colligin, a collagen-binding protein, were isolated from human skin fibroblasts and rat skeletal myoblasts. The full-length cDNAs for both human and rat colligin encoded a 17-amino acid signal peptide and a 400-amino acid mature protein containing several potential N-linked oligosaccharide binding sites and a COOH-terminal RDEL sequence. This COOH-terminal tetrapeptide has been shown to act as an endoplasmic reticulum retention sequence in other systems. The derived amino acid sequence of human colligin shows 98.6% identity with rat colligin (previously called gp46 from L6 myoblasts), 80% identity with hsp47 from chick embryo fibroblasts, and 96.8% identity with mouse J6 protein from F9 embryonal carcinoma cells. Both gp46 and hsp47, have been shown to be heat-inducible, transformation-sensitive glycoproteins which bind collagen I and IV, and are similar to colligin described earlier in 3T3 fibroblasts and keratinocytes. In light of the similarity of the deduced amino acid sequence of human and rat (gp46) colligin presented here and that of hsp47 and J6, we suggest that these proteins all be considered synonymous with colligin.;Human colligin, as well as rat gp46, chick hsp47 and mouse J6, show sequence similarity with the serine protease inhibitor family (Serpins). Interestingly, all four proteins show the greatest conservation of residues in the pentapeptide neodomain recognized by the serpin enzyme complex (SEC) receptor.;By using a variety of deletions of the rat colligin cDNA, a major collagen-binding region was localized to the NH{dollar}\sb2{dollar}-terminus of colligin. This region exhibited no homology with known collagen-binding proteins, yet was consistent with the localization of protein binding regions present on other serpins.;These data allow for the presentation of a model which suggests that colligin binds to procollagen I and IV in the endoplasmic reticulum, in order to protect them from endogenous proteases. As the colligin-procollagen complex is transported to the Golgi apparatus for further processing, the decrease in pH causes colligin to be released. The RDEL tetrapeptide sequence at the COOH-terminus facilitates the recycling of colligin to the endoplasmic reticulum by retrograde transport.;Several key amino acid residues and sequences have been identified to be the basis for future site-directed mutagenesis experiments to verify this hypothesis