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

Osteopontin (OPN) is a secreted glycoprotein implicated in cell adhesion. I t contains the arginine-glycine-asparatic acid (RGD) cell adhesive domain a nd the thrombin cleavage sequence. Although thrombin cleavage of OPN has be en shown to be of physiological importance, the function of C-terminal OFN fragment cleaved by thrombin remains unknown. To determine its role, we per formed cell adhesion assays using glutathione S-transferase-OPN fusion prot ein fragments and full-length OPN fusion protein. The N-terminal fragment c ontaining RGD motif promoted enhanced adhesion of mouse and human fibroblas ts by 2.9 and 2.8 folds in comparison with full-length OPN, respectively. T he enhanced adhesion of both cells mediated by N-terminal fragment was sign ificantly suppressed by addition of C-terminal fragment lacking RGD motif t hat has less cell adhesive property than full-length OPN. These results sug gest that the C-terminal domain may play a pivotal role in regulating OPN f unctions by suppressing the RGD-dependent cell adhesion.