Cartilage-inducing factor-B is a unique protein structurally and functionally related to transforming growth factor-beta.

Cartilage-inducing factors-A (CIF-A) and -B (CIF-B), purified from bovine bone on the basis of their ability to induce the cartilage phenotype in vitro, are proteins with molecular weights of 26,000 composed of two apparently identical disulfide-linked chains. CIF-A is apparently identical to TGF-beta from human platelets (Seyedin S. M., Thompson, A. Y., Bentz, H., Rosen, D. M., McPherson, J. M., Conti, A., Siegel, N. R., Galluppi, G. R., and Piez, K. A. (1986) J. Biol. Chem. 261, 5693-5695). We have now found that, like CIF-A and TGF-beta, CIF-B induces anchorage-independent proliferation of NRK-49F cells when these cells are simultaneously treated with epidermal growth factor. Furthermore, CIF-B competes with CIF-A for the same cell membrane receptors in NRK-49F cells. Partial amino acid sequencing reveals that CIF-B is a distinct molecule with extensive homology to CIF-A/TGF-beta. These results show that CIF-B and TGF-beta are structurally and functionally similar molecules, but differ more from each other than does TGF-beta from different species.