Basement Membrane Procollagen is Not Converted to Collagen in Organ Cultures of Parietal Yolk Sac Endoderm

Basement membrane procollagen biosynthesis was studied in organ cultures of embryonic rat parietal yolk sac endoderm by following [14C]proline incorporation into nondialyzable proteins. After reduction with 2-mercaptoethanol the 14C-proteins synthesized were characterized by agarose gel filtration and disc electrophoresis in the presence of sodium dodecyl sulfate. The labeled procollagen was identified by its content of hydroxy[14C]proline, its sensitivity to digestion with bacterial collagenase, and its resistance to digestion with pepsin. In cultures which were continuously labeled for periods from 6 hours to 4 days, the pro-alpha chains consistently eluted as a single peak with an apparent molecular weight of 160,000. After pepsin digestion the resultant alpha chains had an apparent molecular weight between 125,000 and 140,000. This suggests that basement membrane procollagen either contains non-triple helical pepsin-resistant regions or a triple helical region which is larger than the corresponding region of interstitial procollagen. Two experiments were performed to determine whether the chains of newly synthesized basement membrane procollagen were cleaved to a smaller molecular species. In the first, the hydroxylation and secretion of procollagen were blocked with alpha, alpha'-dipyridyl, and the resulting intracellular chains of basement membrane protocollagen were found to co-elute with fully hydroxylated and secreted pro-alpha chains. In the second, cultures were labeled for 1 day and chased for 3 days with unlabeled medium. Autoradiography had shown that most of the label was chased into new basement membrane. Agarose chromotography showed that after 3-day chase the pro-alpha chains still eluted with an apparent molecular weight of 160,000. Thus, the data indicated that basement membrane procollagen was deposited in new basement membrane without undergoing a time-dependent extracellular conversion.