Cell cultures of chicken embryo and human fibroblasts produce a large extracellular matrix molecule with a six-armed structure that we called a hexabrachion (Erickson, H. P., and J. L. Iglesias, 1984, Nature (Lond.), 311:267-269. In the present work we have determined that the myotendinous (M1) antigen described by M. Chiquet and D. M. Fambrough in chicken tissues (1984, J. Cell Biol., 98:1926-1936), and the glioma mesenchymal extracellular matrix protein described by Bourdon et al. in human tumors (Bourdon, M. A., C. J. Wikstrand, H. Furthmayr, T. J. Matthews, and D. D. Bigner, 1983, Cancer Res. 43:2796-2805) have the structure of hexabrachions. We also demonstrate that the M1 antigen is present in embryonic brain, where it was previously reported absent, and have purified hexabrachions from brain homogenates. The recently described cytotactin (Grumet, M., S. Hoffman, K. L. Crossin, and G. M. Edelman, 1985, Proc. Natl. Acad. Sci. USA, 82:8075-8079) now appears to be identical to the chicken hexabrachion protein. In a search for functional roles, we looked for a possible cell attachment activity. A strong, fibronectin-like attachment activity was present in (NH4)2SO4 precipitates of cell supernatant and sedimented with hexabrachions in glycerol gradients. Hexabrachions purified by antibody adsorption, however, had lost this activity, suggesting that it was due to a separate factor associated with hexabrachions in the gradient fractions. The combined information in the several, previously unrelated studies suggests that hexabrachions may play a role in organizing localized regions of extracellular matrix. The protein is prominently expressed at specific times and locations during embryonic development, is retained in certain adult tissues, and is reexpressed in a variety of tumors.

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