Rate of helix formation by intracellular procollagen and protocollagen. Evidence for a role for disulfide bonds

Matrix-free cells from embryonic tendons were incubated under conditions in which they synthesized and accumulated protocollagen, the unhydroxylated form of procollagen, which is non-helical at 37°. Limited digestion with pepsin demonstrated that when the accumulated protocollagen was hydroxylated intracellularly to procollagen, or when the cells were cooled below the T_m of protocollagen, the protein became triple-helical in about 5 min, or in a fraction of the time required for isolated α chains to become helical. When disulfide bonds in the NH₂-terminal extensions of protocollagen were reduced by treating the cells with dithiothreitol, the rate of helix formation was markedly decreased. The results demonstrated that the NH₂-terminal extensions found in protocollagen and procollagen play an important role in formation of the triple-helix during biosynthesis.