The RGD Sequence in the Cytomegalovirus DNA Polymerase Accessory Protein Can Mediate Cell Adhesion

Abstract The murine cytomegalovirus (MCMV) polymerase processivity factor ppM44 (also referred to as pp50) is an abundant phosphoprotein found in MCMV-infected cells. Sequence analysis of the MCMV M44 open reading frame revealed an “RGD” motif that is also present in the human cytomegalovirus (HCMV) UL44 open reading frame. In this report, histidine-tagged M44 protein produced in Escherichia coli or the vaccinia/T7 expression system was purified to near homogeneity by metal chelation affinity chromatography using His*Bind resins. We demonstrated that recombinant M44 protein could mediate cell adhesion via its conserved “RGD” motif, because a single amino acid change (RGD to RGE) abolished cell attachment. In addition, cell adhesion was abolished in the presence of EDTA. We next showed that recombinant HCMV UL44, but not human herpesvirus type 6 p41, which lacks the RGD motif, could mediate cell adhesion in a similar manner. We also provided evidence that ppM44 was present in the culture medium during virus infection. Thus these results suggested that in addition to its primary role as the polymerase processivity factor, MCMV ppM44 may serve as a substrate for integrin-binding via its conserved RGD motif, with the potential for a novel role in the MCMV replication cycle.