Arg-Gly-Asp recognition by a cell adhesion receptor requires its 130-kDa alpha subunit

A major Arg-Gly-Asp-directed receptor on M21 human melanoma cells is a heterodimer of alpha and beta chains which under reducing conditions have molecular masses of 130 and 105 kDa, respectively. This receptor is one member of a large family of cell adhesion receptors that shares antigenic determinants with the vitronectin receptor of fibroblasts and the platelet IIb X IIIa complex. Both subunits of the M21 cell adhesion receptor acquire high mannose-type oligosaccharides that are processed before transport to the cell surface. In addition, the alpha chain undergoes a proteolytic cleavage step. Pulse-chase immunoprecipitation analysis demonstrates that, following its synthesis, the beta chain remains unbound to alpha chain for 1-2 h and in this free form is unable to bind an Arg-Gly-Asp containing heptapeptide. Conversely, the biosynthetic precursor of the alpha chain is fully capable of binding the Arg-Gly-Asp-containing peptide immediately after its synthesis. Thus, Arg-Gly-Asp recognition by one member of this cell adhesion receptor family requires its 130-kDa alpha chain which appears to be functional prior to post-translational modifications.