These precursors contain amino- and carboxyl-terminal peptide extensions known as N- and C-propeptides, respectively, which are cleaved, upon secretion of procollagen from the cell, to yield the mature triple helical, highly structured fibrils. In the mouse, procollagen C-proteinase depends for full expression upon the presence of either a 55-kD glycoprotein or smaller 36- and 34-kD proteolytically processed forms of the same protein. None of the 3 enhancer proteins exhibits intrinsic procollagen processing activity, but all are capable of enhancing the activity of the C-proteinase by approximately one order of magnitude. type I procollagen COOH-terminal proteinase enhancer, a glycoprotein that binds to the COOH-terminal propeptide of type I procollagen and enhances C-proteinase activity, from mouse fibroblast culture medium.