Homologous recombination in embryonic stem cells was used to prepare transgenic mice with an inactivated Col2a1 gene for collagen 11, the major protein component of the extracellular matrix of cartilage. Heterozygous mice had a minimal phenotype. Homozygous mice developed into fetuses that were delivered vaginally but died either just before or shortly after birth. The cartilage in the mice consisted of highly disorganized chondrocytes with a complete lack of extracellular fibrils discernible by electron microscopy. There was no endochondrial bone or epiphyseal growth plate in long bones. However, many skeletal structures such as the cranium and ribs were normally developed and mineralized. The results demonstrate that a well-organized cartilage matrix is required as a primary tissue for development of some components of the vertebrate skeleton, but it is not essential for others.

Bone morphogenic protein-1 (BMP-1) was originally identified as one of several BMPs that induced new bone formation when implanted into ectopic sites in rodents. BMP-1, however, differed from other BMPs in that it its structure was not similar to transforming growth factor 13. Instead, it had a large domain homologous to a metalloendopeptidase isolated from crayfish, an epidermal growthfactor-like domain, and three regions of internal sequence homology referred to as CUB domains. Therefore, BMP-1 was a member of the "astacin families" of zinc-requiring endopeptidases. Many astacins have been shown to play critical roles in embryonic hatching, dorsal/ventral patterning, and early developmental decisions. Here, we have obtained amino acid sequences and isolated cDNA clones for procollagen Cproteinase (EC 3.4.24.19), an enzyme that is essential for the processing of procollagens to fibrillar collagens. The results demonstrate that procollagen C-proteinase is identical to BMP-1.Bone morphogenic protein (BMP) was originally identified by Urist (1) and then by Reddi and Huggins (2) as an activity found in extracts of demineralized bone that induced new bone formation when implanted into ectopic sites in rodents. In 1988, Wozney and coworkers (3) obtained partial amino acid sequences of several components with bone morphogenic activity and used the sequence information to isolate three cDNA clones that they named BMP-1, BMP-2A, and BMP-3. On the basis of their structural similarity, BMP-2A and BMP-3 were identified as members of the transforming growth factor f3 (TGF-13) superfamily. Subsequently, five additional BMPs that were also similar to the TGF-,B superfamily were identified (4, 5). BMP-1, however, was apparently isolated as a complex with the other BMPs because it differed in structure from TGF-f3. It contained a large domain homologous to a metalloendopeptidase isolated from crayfish (6); an epidermal growth factor (EGF)-like domain; and three regions of internal sequence homology referred to as CUB domains because they are found in the complement components Clr/Cls, the sea urchin protein Uegf, and BMP-1 (7). The metalloendopeptidase from crayfish is now recognized as one of the simplest members of the "astacin family" of over 17 similar zinc-requiring endopeptidases (7-10).