A collagen-binding protein involved in the differentiation of myoblasts recognizes the Arg-Gly-Asp sequence

We had earlier demonstrated that a 46-kDa glycoprotein is involved in the differentiation of rat skeletal myoblasts. We now show that the binding of this glycoprotein to collagen and gelatin is disrupted by Arg-Gly-Asp (RGD) containing peptide but not by Arg-Gly-Glu (RGE). The former peptide also selectively elutes the 46-kDa glycoprotein bound to gelatin-Sepharose. Since all other proteins which bind RGD sequences have been found at the cell surface, we attempted to localize the 46-kDa glycoprotein by means of immuno fluorescent staining and radioiodine labeling. Surprisingly, the majority of the protein was found to be localized in the endoplasmic reticulum. Protease treatment of a microsomal fraction revealed that the protein is in the interior of the reticulum. Immunoprecipitation experiments, using a polyclonal antibody against the 46-kDa protein, demonstrated that no closely related proteins exist in myoblasts and also confirmed that the protein was not a fragment of a cell-surface localized protein. These findings suggest that the RGD sequence is also used in protein recognition within the cell.