The CUB domains of procollagen C-proteinase enhancer control collagen assembly solely by their effect on procollagen C-proteinase/bone morphogenetic protein-1

Procollagen C-proteinase enhancer (PCPE) is a 55 kDa glycoprotein that increases the activity of procollagen C-proteinase (PCP)/bone morphogenetic protein-1 (BMP-1) during C-terminal processing of fibrillar collagen precursors. Here we show that the 36 kDa, active fragment of PCPE enhances the activity of both the short (mouse) and long (chick) forms of PCP/BMP-1. The activity of PCPE is not associated with the formation of sedimentable procollagen aggregates. In addition, PCPE (36 kDa) has no effect in vitro on N-terminal procollagen processing by highly purified procollagen N-proteinase. Finally, when the amount of PCP is adjusted so that the rate of C-terminal processing remains constant, PCPE (36 kDa) has no effect on the assembly of collagen or pN-collagen in vitro following C-terminal processing of the corresponding precursors.