Engagement of the cellular receptor for glycoprotein B of human cytomegalovirus activates the interferon-responsive pathway.

Transmembrane receptors enable cells to respond to ligands by altering cellular metabolism. A consequence of binding of extracellular signaling proteins, such as growth factors, hormones, and cytokines, to their cognate receptors is transcriptional activation of previously quiescent genes. Interferons (IFNs) are cytokines that induce a well-characterized signal transduction pathway. The IFN-mediated signaling response begins with a ligand-receptor interaction and concludes with activated gene expression of family of genes known as the IFN-stimulated genes (ISGs) (reviewed in references 24 and 32). Type 1 IFN (IFN-α/β) activates gene expression by engaging a common cellular receptor complex, whereas type II IFN (IFN-γ) engages a distinct receptor (27, 29). Signaling requires tyrosine phosphorylation, but the receptor subunits themselves lack intrinsic tyrosine kinase activity. IFN receptors become rapidly phosphorylated by members of the Janus family of kinases (Jaks) which are associated with their cytoplasmic domains (24). Subsequently latent cytoplasmic transcription factors (STATs) are recruited, phosphorylated, and multimerized into functionally active transcriptional transactivators which traffic to the nucleus and induce gene expression by binding to particular cis-acting regulatory regions. In general, genes that respond to IFN-α contain a highly conserved upstream element termed the IFN-stimulated response element, whereas IFN-γ-responsive genes contain a IFN-γ activation site (24, 32). Unique and distinguishing complements of the Jak/STAT members along with a DNA binding protein known as p48 are responsible for the induction specificity (30, 42, 43). Although the IFN response pathway is the best characterized, it is now clear that a variety of membrane receptors respond to extracellular signaling molecules by activating combinations of Jak and STAT proteins, resulting in gene induction. Infection of permissive cells with human cytomegalovirus (HCMV), a herpesvirus, results in the progression of a sequentially ordered set of physiological responses similar to growth factor- or cytokine-induced cellular activation. Cellular activation involves a rapid and measurable induction of a number of immediate-early signaling mediators that begins with membrane-associated events such as activation of phospholipase C and phospholipase A2 (2, 3, 72). Transient accumulations of activated cellular transcription factors such as NF-κB (9, 40, 58, 76), Sp1 (77), AP-1, and CRE/B (9, 58) are also detectable with rapid kinetics. Recently, differential display analysis was used to investigate the cellular changes that occur in response to HCMV infection (78). This study demonstrated that HCMV infection had a profound effect on host cell gene expression, as evidenced by the accumulation of a number of in HCMV-infected fibroblasts cytomegalovirus-inducible gene mRNAs compared to mock-infected cells. Interestingly, all of the cellular cytomegalovirus-inducible genes (15 independent mRNAs) were determined to be known or novel ISGs that were also induced by IFN-α. Activation of transcription factors and induction of the ISGs was independent of viral gene expression in that the response was as robust with UV-inactivated virus as with replication-competent virus. This fact indicates that a constituent of the virus particle mediates the induction. Virion components that could potentially mediate the gene induction are envelope proteins or tegument proteins that when delivered to cells after successful virus entry are known to activate transcription (44, 75). Envelope proteins mediate the early events in infection such as attachment and penetration. HCMV entry into host cells is a complex process involving sequential viral glycoprotein-host cell receptor interactions (21, 23). Virus attachment is initiated by binding to cell surface heparan sulfate proteoglycans (HSPGs) (23, 52). This initial binding is easily dissociable but rapidly converts to a more stable binding state thought to be mediated by engagement of a second receptor (21, 23). After virus-cell contact, fusion is initiated presumably involving rearrangements or conformational changes of fusogenic glycoproteins. The virion envelope fuses with the plasma membrane, resulting in localized deposits of envelope glycoproteins and delivery of virion contents including the tegument proteins, an amorphous, protein-dense material that includes transcriptional transactivators, as well as the DNA-containing capsid (22, 44). Glycoprotein B (gB) of HCMV is the most abundant component of the envelope, a target of neutralizing antibodies (14, 28, 35, 53) and an essential replication component (11a). Protein binding experiments with a purified, soluble form of gB (gB-S) revealed that gB serves as the primary viral ligand capable of interacting with two independent binding sites (11). A proportion of the binding activity was attributable to HSPGs, but the majority of gB’s binding was to an independent saturable binding site. Here we report that a consequence of the interaction of gB with the non-heparin receptor is the initiation of intracellular signaling and activation of the IFN-responsive pathway.