Intermediates in the Conversion of Procollagen to Collagen. Evidence for Stepwise Limited Proteolysis of the COOH-terminal Peptide Extensions

Intermediates in the conversion of procollagen to collagen were isolated from radioactively labeled chick cranial bones by ion-exchange chromatography. Cleavage of these proteins with vertebrate collagenase revealed that each of the several forms of these intermediates lacked NH2-terminal but retained COOH-terminal extensions. The chain composition of each intermediate was resolved by two-dimensional slab gel electrophoresis. The intermediates differed from each other in having sustained cleavages in zero, one or two pcalpha chains. The relative proportions of intermediates with different intact pcalpha chains, observed in conversion of procollagen, have enabled us to construct a detailed model of the stepwise limited proteolysis of procollagen.