Formation of the triple helix of type I procollagen in cellulo. A kinetic model based on cis-trans isomerization of peptide bonds.

The kinetics of the triple helix formation of type I procollagen within chick embryo fibroblasts were measured in a pulse-chase experiment in which the appearance of fully-aligned triple-helical molecules was assayed by proteolytic digestion. Production of triple-helical molecules required 8-9 min after complete synthesis of the pro alpha chains, an observation which was inconsistent with helix formation being a co-translational process. The experimental data were in good agreement with the predictions derived from the following model: triple helix formation is initiated immediately after the completion of the synthesis of the polypeptide chains and after the formation of the interchain disulfide bonds within the C-propeptide; folding of the protein starts from a single nucleus located at one end of the three polypeptide chains, probably the carboxyl terminus, and propagates throughout the chain by a stepwise mechanism limited by the cis-trans isomerization of peptide bonds.