Adhesive interactions of cells with extracellular materials are critically important events during embryogenesis and invasion by cancer cells. Neither the molecular composition of adhesion sites nor the function of molecules associated with cell adhesion is presently understood. We intend to obtain information on the molecular anatomy and function of the membrane attachment sites in normal and virally transformed cells and to test the hypothesis that cell adhesion is a result of specific interactions between transmembrane glycoproteins and their peripheral intra- and extracellular components. A novel in vitro model involving cells cultured on planar, crosslinked gelatin substrata will be used for morphological, chemical, and functional assays of adhesions. The goals of this work are to define these transmembrane interactions in normal and virally transformed cells, to evaluate their functional significance, and to ferret out new molecules involved in adhesion. To accomplish these goals, we have employed antibodies directed to purified cell surface proteins as double-immunolabeled probes to elucidate the molecular anatomy and pathology of adhesion sites by the ultra-thin frozen sectioning method and then use these purified proteins to determine their binding specificity in vitro. Other more function-oriented studies will evaluate the significance of these molecular associations. We have showed a 140 kilodalton membrane adhesion complex co-localizing with fibronectin and alpha-actinin in normal and transformed cells. The monoclonal antibody JG22E, directed against the 140 kilodalton complex, inhibits attachment and spreading of both normal and transformed cells to fibronectin substratum. However, we found that there is an increased expression of transformation-associated proteases that degrade fibronectin at cell contact sites. We examined the ultrastructural localization, substrate, and inhibitor specificity of the proteases for their roles in the invasion of transformed cells into the extracellular matrix. We will also continue to identify other transmembrane proteins using new monoclonal antibodies and immunoelectron microscopy. (A)
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