Time of occurrence of disulfide linking between procollagen chains

The disulfide bonds that link together single collagen pro-alpha chains into trimers, prior to their secretion, were found to be synthesized after the pro-alpha chains were completed and released from polysomes and not while the pro-alpha chains were still nascent. After a 4- or 8-min incubation of cell cultures derived from embryonic chick sternum with [3H]proline, the nascent chains isolated from polysome preparations contained negligible amounts of disulfide-linked chains. After a 20-min incubation, some very large disulfide-linked material was found associated with the polysome preparation, but this material proved to be noncollagenous and also appeared to represent completed, and not nascent, chains. The presence of this proline-labeled species was confirmed in analyses of whole cell lysates, in which it is present as a mixture of monomers of approximately 290,000 molecular weight and disulfide-linked dimers. Pulse-chase experiments also gave results that were consistent with the conclusion that released single pro-alpha chains are the precursors of released disulfide-linked pro-alpha chains. Such experiments also showed that pro-alpha chains could continue to form disulfide-linked species in the presence of inhibitors of peptide chain elongation.