BIOSYNTHESIS AND PROCESSING OF TYPE-XVI COLLAGEN IN HUMAN FIBROBLASTSAND SMOOTH-MUSCLE CELLS

The alpha 1(XVI) collagen chain, recently identified by cDNA cloning, exhibits structural similarity to a subgroup of collagens that associa te with collagen fibrils. Recombinant alpha 1(XVI) collagen chains pro duced in embryonic kidney cells are able to form stable homotrimers, w hich are rapidly converted into smaller polypeptides after secretion i nto the culture medium. In this study, we investigated the biosynthesi s of native type XVI collagen by immunoprecipitation of metabolically labeled human cells. Dermal fibroblasts and arterial smooth muscle cel ls were precipitated with three antibodies raised against distinct reg ions in the N- and C-terminal part of the human alpha 1(XVI) collagen chain. A disulfide-bonded polypeptide of 220 kDa was obtained from the culture medium, cells and extracellular matrix with all three antibod ies. This polypeptide is sensitive to bacterial collagenase digestion and partially resistant to pepsin digestion, suggesting that it is the endogenous alpha 1(XVI) collagen chain. Pulse/chase experiments showe d that the newly synthesized alpha 1(XVI) chains are secreted into the medium and deposited in the extracellular matrix in a time-dependent manner. Unlike the recombinant chain, the native type XVI collagen doe s not undergo extensive proteolytic processing upon secretion. Both ce ll types deposit a substantial amount of the newly synthesized alpha 1 (XVI) chain into the extracellular matrix, in which the 220-kDa polype ptide is the only product immunoprecipitated. There is little evidence for the presence of another constituent chain. The data are consisten t with a homotrimeric chain composition for type XVI collagen. No appa rent difference exists in the rate of synthesis and secretion between fibroblasts and smooth muscle cells. Indirect immunofluorescence micro scopy showed an extracellular distribution of type XVI collagen, which is located close to cells but not associated with fibrillar structure s.