Precursors of collagen secreted by cultured human fibroblasts

Soluble precursors of collagen ("procollagens") in the culture medium of human diploid fibroblasts were characterized by molecular sieve chromatography on 6% agarose and by sodium dodecyl sulfate-acrylamide gel electrophoresis. Under the denaturing conditions for gels, the following molecular species were identified: a major species of three procollagen chains, a procollagen dimer, and free procollagen chains. Reduction with 2-mercaptoethanol completely dissociated the trimer and dimer, releasing procollagen alpha1 and procollagen alpha2 chains with molecular weights of about 120,000. Both pro alpha1 and pro alpha2 chains could be labeled with [(35)S]cystine. In addition, free procollagen chains with molecular weights of less than 120,000, but heavier than native alpha chains, were identified in gels. Radioactivity in all of the above molecules could be chased into native alpha chains extracted from extracellular fibers of the cell layers. The data indicate that: (i) collagen is secreted as a disulfidestabilized procollagen trimer with the composition, (pro alpha1)(2).pro alpha2; (ii) noncovalent interactions maintain the three-chain assembly during stepwise enzymatic excisions of N-terminal, cysteine-containing procellagen peptides; and (iii) after such excisions, native helical molecules, (alpha1)(2).alpha2, precipitate as fibers in the cell layers.