SWAPNIL SUBHASH GHANWAT

• The innate immune response is the first line of defense against viral infection. Host pattern recognition receptors (PRRs) detect pathogen-associated molecular patterns (PAMPs) and trigger several signaling pathways following viral infections. One such pathway, the cGAS-STING pathway, detects cytosolic DNA to induce the production and secretion of type I interferons. The cGAS-STING pathway is activated by the presence of cytosolic DNA. cGAS (cGMP-AMP synthase), a cytosolic DNA detector, binds double-stranded DNA, dimerizes, and catalyzes the production of the second messenger cyclic GMP-AMP (cGAMP) from GTP and ATP. STING, an ER adaptor protein, binds to cGAMP and becomes activated through dimerization, which results in its trafficking from the ER to the Golgi. At the Golgi, STING recruits the kinase TBK1 and the transcription factor IRF3. TBK1 phosphorylates STING, itself, and IRF3. Phosphorylation activates IRF3, causing its translocation to the nucleus, where it induces the production of type I interferons. To counteract the host immune response, human cytomegalovirus (HCMV) encodes several immunoevasin proteins. HCMV glycoproteins such as US6 inhibit antigen presentation by blocking peptide transport via the transporter associated with antigen processing (TAP). The region spanning amino acids 89–108 of US6 was identified as responsible for TAP inhibition. Our studies have identified a novel interaction and function of US6. For the first time, we show that US6 interacts with the host p24 proteins TMED2 and TMED10. US6 inhibits the cytosolic DNA-triggered cGAS-STING pathway and reduces the production of IFNβ1. We have discovered a correlation between the binding of US6 to TMED2 and TMED10 and its ability to inhibit the production of type I interferons. Using sequential mutants, we show that US6 possesses two distinct and separable regions responsible for its functions. Microscopy reveals that US6 delays STING trafficking from the ER to the Golgi.