Purification of two smooth muscle glycoproteins related to integrin. Distribution in cultured chicken embryo fibroblasts.

We have purified two membrane glycoproteins from chicken gizzard smooth muscle. In the presence of reducing agents, these proteins have molecular weights on sodium dodecyl sulfate-polyacrylamide gel electrophoresis of 165,000 and 130,000, but they migrate at 165,000 and 110,000 without reduction. The two proteins can also be isolated as a complex in buffers containing physiologic salt concentrations. This complex has physical properties similar to two proteins of the integrin family of receptors for extracellular matrix proteins, the cell substratum attachment antigen from chicken embryos, and the glycoprotein IIb IIIa complex from mammalian platelets. When the smooth muscle complex is visualized by electron microscopy, it has a striking resemblance to both avian integrin and the glycoprotein IIb IIIa complex. Smooth muscle is a good source of the 165,000 and 130,000 proteins, and purification of both the individual subunits and the complex is achieved using conventional biochemical techniques. Antibodies directed against the 130,000 protein cross-react with integrin but do not cross-react with the 165,000 protein. Immunofluorescence microscopy using these antibodies reveals staining of fibroblast focal contacts and fibrillar streaks which coalign with fibronectin. Whereas monoclonal antibodies against integrin label the periphery of the focal contact more intensely than the center, the anti-130,000-protein serum stains the entire focal contact. Antibodies directed against the 165,000 protein also stain focal contacts and fibrillar streaks of fibroblasts in tissue culture. On the basis of similar physical properties, biochemical characteristics, and immunological cross-reactivity we conclude that the 165,000/130,000 complex is a smooth muscle integrin.