The carboxyl-terminal fragment of osteopontin suppresses arginine-glycine-asparatic acid-dependent cell adhesion.

Osteopontin (OPN) is a secreted glycoprotein implicated in cell adhesion. It contains the arginine-glycine-asparatic acid (RGD) cell adhesive domain and the thrombin cleavage sequence. Although thrombin cleavage of OPN has been shown to be of physiological importance, the function of C-terminal OPN fragment cleaved by thrombin remains unknown. To determine its role, we performed cell adhesion assays using glutathione S-transferase-OPN fusion protein fragments and full-length OPN fusion protein. The N-terminal fragment containing RGD motif promoted enhanced adhesion of mouse and human fibroblasts by 2.9 and 2.8 folds in comparison with full-length OPN, respectively. The enhanced adhesion of both cells mediated by N-terminal fragment was significantly suppressed by addition of C-terminal fragment lacking RGD motif that has less cell adhesive property than full-length OPN. These results suggest that the C-terminal domain may play a pivotal role in regulating OPN functions by suppressing the RGD-dependent cell adhesion.