Biosynthesis of chick type VI collagen. I. Intracellular assembly and molecular structure.

A monospecific rabbit antiserum to pepsin-extracted chick gizzard type VI collagen was used to characterize the intact forms of type VI collagen in tissues and cultured cells. Immunoblotting of gizzard extracts revealed polypeptides of Mr ranging from 260,000 to 140,000. Components of about Mr = 260,000, 150,000, and 140,000, each with a different peptide profile, were immunoprecipitated from labeled matrix-free chick embryo cells. Cleavage of the immunoprecipitated polypeptides with pepsin generated pepsin-resistant fragments of about Mr = 70,000, 55,000, and 45,000 that represent the alpha 1(VI), alpha 2(VI), and alpha 3 (VI) fragments. Immunoblotting with affinity-purified antibodies indicated that the Mr = 150,000 is the intact parent polypeptide of the alpha 1(VI) pepsin; the Mr = 140,000 of the alpha 2(VI) pepsin, and the Mr = 260,000 of the alpha 3(VI) pepsin. Association of the three parent chains was studied by pulse-chase experiments and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis under nonreduced conditions. A complex of Mr = 500,000 is already present intracellularly at the end of a 7-min pulse and increases considerably with time while the three unassembled chains show a comparable decrease. After 5-15 min of chase larger forms appeared along with small amounts of aggregated material that did not enter the gel. Analysis of the immunoprecipitate by diagonal electrophoresis indicated that the component of Mr = 500,000 and the larger forms dissociated into the Mr = 260,000, 150,000, and 140,000 polypeptides. Sedimentation profile of a labeled cell extract on a 5-20% sucrose gradient under nondenaturing conditions confirmed the presence of three different peptides in the complex.