The RGD (Arg-Gly-Asp) motif functions as a recognition site for adhesive proteins responsible for a number of cell-cell interactions. Certain viruses use this sequence as a receptor-binding site by interaction with cellular integrins. To elucidate the role of the RGD sequence of the phi29 terminal protein (TP), seven modified TPs were generated by site-directed mutagenesis. Most of the TP mutants were not efficiently used as primers, leading to a reduction of the TP-dAMP complex formation in the presence of the phi29 TP-DNA template. Moreover, these mutant TPs were poorly deoxyadenylylated by phi29 DNA polymerase in the absence of template. Analysis of primer TP/DNA polymerase complex formation showed that the modified TPs were affected in the formation of the heterodimeric complex. These results indicate that the RGD sequence present in phi29 TP is primarily involved in interaction with the viral DNA polymerase.