Members of the bone morphogenetic protein-1/tolloid (BMP-1/Tld) family of metalloproteinases, also known as procollagen C-proteinases (PCPs), control multiple biological events (including matrix assembly, cross-linking, cell adhesion/migration and pattern formation) through enzymatic processing of several extracellular substrates. PCP activities on fibrillar procollagens can be stimulated by another family of extracellular proteins, PCP enhancers (PCPE-1, PCPE-2), which lack intrinsic enzymatic activity. While PCPs have multiple substrates, the extent to which PCPEs is involved in the processing of proteins other than fibrillar procollagens is unknown. In the experiments reported here, PCPE-1 was found to have no effect on the in vitro BMP-1 processing of procollagen VII, the procollagen V N-propeptide, the laminin 5 ␥2 chain, osteoglycin, prolysyl oxidase, or chordin. In contrast, PCPE-1 enhanced C-terminal processing of human fibrillar procollagen III but only when this substrate was in its native, disulfidebonded conformation. Surprisingly, processing of procollagen III continued to be enhanced when essentially all the triple-helical region was removed. These and previous results (Ricard-Blum, S., Bernocco, S., Font, B., Moali, C., Eichenberger, D., Farjanel, J., Burchardt, E. R., van der Rest, M., Kessler, E., and Hulmes, D. J. S. In recent years, it has become clear that members of the tolloid family of metalloproteinases (BMP-1, 1 mTld, mTLL-1, mTLL-2) are key players in morphogenesis through their ability to control and synchronize the processing of multiple extracellular substrates (1-25). Also called procollagen C-proteinases, tolloid proteinases trigger collagen fibril formation in the extracellular matrix (26) by cleaving the C-propeptide regions from the major fibrillar procollagens (I, II, and III). They are also involved in precursor processing of the minor fibrillar collagens V and XI, necessary for the regulation of heterotypic fibril assembly, in both the N-and C-propeptide regions (8,15,17,19,23,27). Tolloid proteinases also cleave precursor forms of small leucine-rich proteoglycans such as biglycan (13) and osteoglycin (24). While some of these proteoglycans have been shown to modulate the kinetics of assembly as well as fibril diameter (28), functional differences between mature and precursor forms are not yet clearly established (24). Biglycan is also important for bone formation (29) and more recently, dentin matrix protein-1, a protein involved in mineralization, has been found to be a substrate for tolloid proteinases (22).Another important function early attributed to tolloid proteinases is the maturation of the precursor form of lysyl oxidase (LOX) (5, 30), the enzyme responsible for the stabilization of collagen and elastin networks through the formation of covalent cross-links. This tolloid-dependent processing has recently gained in significance with the demonstration that the cleaved propeptide of proLOX accounts for the ras-recision gene activity of this protein (31). In addition, ...