A peptide isolated from a random phage peptide library is a structural mimic to the P1, P4-diadenosine 5'-tetraphosphate binding site on its receptor.

We have previously demonstrated that a monoclonal antibody (mAb TL4), which inhibits P1, P4-diadenosine 5'-tetraphosphate (Ap4A) binding to its receptor, selected a consensus RGS tripeptide from a random phage hexapeptide library. RGS interfered with Ap4A binding to its membrane receptor [Liu, G., Bryant, R. T. & Hilderman, R. H. (1996) Biochemistry 35, 197-201]. However the mechanism by which RGS interfered with Ap4A binding to its receptor was not determined. In this communication, we demonstrate that RGS interacts with Ap4A to prevent [3H]Ap4A binding to the Ap4A membrane receptor. To further characterize the mechanism by which RGS inhibits Ap4A binding to its receptor, we used mAb TL4 to screen a 15-residue random peptide phage library and DNAs from 24 clones were sequenced. 20 clones contain a RGSSS sequence while 17 of these clones contained an identical 15-amino-acid insert (clone A). Gel-filtration studies of previously equilibrated clone A phage and [3H]Ap4A support the idea that [3H]Ap4A interacts specifically with clone A phage while RGS effectively competes for [3H]Ap4A interaction on clone A phage. These data are consistent with RGS mimicking a sequence on the receptor essential for Ap4A binding.