Expression of the human cytomegalovirus (HCMV)-encoded chemokine receptor homologue pUS28 in mammalian cells results in ligand-dependent and - independent changes in the activity of multiple cellular signal transduction pathways. The ligand-dependent signalling activity of pUS28 has been shown to be predominantly mediated by heterotrimeric G proteins of the G sub(i/o) and G sub(12/13) subfamilies. Ligand-independent constitutive activity of pUS28 causing stimulation of inositol phosphate formation has been correlated with the coupling of pUS28 to G proteins of the G sub(q) family. It is well known that activation of G sub(q) proteins by cell surface receptors is coupled to activation of the Rho GTPase RhoA. Activated RhoA regulates numerous cellular functions, including the activity of the transcription factor serum response factor (SRF). The marked activation of G sub(q) proteins by pUS28 in transfected and HCMV-infected cells prompted us to investigate its effect on SRF activity. The results presented herein demonstrate that expression of pUS28 in COS-7 cells caused a vigorous induction of SRF activity. This effect was observed in the absence of chemokines known to interact with pUS28, and was specifically mediated by endogenous G sub(q) and/or G sub(11) as well as RhoA and/or a closely related Rho GTPase. The stimulatory effect of pUS28 and G alpha sub(q/11) was independent of phospholipase C- beta (PLC beta ) activation and was markedly sensitive to inhibition by wild-type, but not by constitutively active G alpha sub(16), thus identifying G alpha sub(16) as a modulator of G alpha sub(q/11) function likely to act by competing with G alpha sub(q/11) for and thus uncoupling G alpha sub(q/11) from activation by pUS28.