Constitutive serum response factor activation by the viral chemokine receptor homologue pUS28 is differentially regulated by Gαq/11 and Gα16

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 Gi/o and G12/13 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 Gq family. It is well known that activation of Gq 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 Gq 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 Gq and/or G11 as well as RhoA and/or a closely related Rho GTPase. The stimulatory effect of pUS28 and Gαq/11 was independent of phospholipase C-β (PLCβ) activation and was markedly sensitive to inhibition by wild-type, but not by constitutively active Gα16, thus identifying Gα16 as a modulator of Gαq/11 function likely to act by competing with Gαq/11 for and thus uncoupling Gαq/11 from activation by pUS28.