Constitutive signaling of the human cytomegalovirus-encoded receptor UL33 differs from that of its rat cytomegalovirus homolog R33 by promiscuous activation of G proteins of the Gq, Gi, and Gs classes.

The human cytomegalovirus (HCMV) UL33 gene is conserved among all beta-herpesviruses and encodes a protein that shows sequence similarity with chemokine receptors belonging to the family of G protein-coupled receptors. Here, we show that HCMV UL33 is predominantly transcribed as a spliced mRNA of which the 5' terminus is localized 55 bp upstream of the start codon. Like its homolog from rat cytomegalovirus (RCMV), R33, UL33 activates multiple signaling pathways in a ligand-independent manner. Although both receptors constitutively activate phospholipase C via G(q/11), and partially via G(i/o)-mediated pathways, they exhibit profound differences in the modulation of cAMP-responsive element (CRE) activation. R33 constitutively inhibits, whereas UL33 constitutively enhances CRE-mediated transcription. For R33, the inhibition of CRE-driven transcription is entirely G(i/o)-mediated. For UL33, however, CRE-mediated transcription is modulated not only through coupling to Galpha(i/o) but also through coupling to Galphas. In addition, UL33 was found to enhance CRE activation through the Rho/p38 pathway, via Gbetagamma. Interestingly, by studying chimeric UL33/R33 proteins, we found the C-terminal cytoplasmic tail of UL33, but not that of R33, to be responsible for the activation of G(i/o) proteins. A UL33-deficient variant of HCMV was generated to analyze UL33-signaling properties in a physiologically relevant model system. Data obtained with infected cells show that HCMV induces CRE activation, and this effect is, at least in part, dependent on UL33 expression. Taken together, our data indicate that constitutive signaling of UL33 differs from that of R33 by promiscuous activation of G proteins of the Gq, G(i/o), as well as Gs class. Thus, HCMV may effectively use UL33 to orchestrate multiple signaling networks within infected cells.