TTLL12 Inhibits the Activation of Cellular Antiviral Signaling through Interaction with VISA/MAVS

Upon virus infection, host cells use retinoic-acid-inducible geneI I (RIG-I)-like receptors to recognize viral RNA and activate type I IFN expression. To investigate the role of protein methylation in the antiviral signaling pathway, we screened all the SET domain-containing proteins and identified TTLL12 as a negative regulator of RIG-I signaling. TTLL12 contains SET and TTL domains, which are predicted to have lysine methyltransferase and tubulin tyrosine ligase activities, respectively. Exogenous expression of TTLL12 represses IFN-β expression induced by Sendai virus. TTLL12 deficiency by RNA interference and CRISPR-gRNA techniques increases the induced IFN-β expression and inhibits virus replication in the cell. The global gene expression profiling indicated that TTLL12 specifically inhibits the expression of the downstream genes of innate immunity pathways. Cell fractionation and fluorescent staining indicated that TTLL12 is localized in the cytosol. The mutagenesis study suggested that TTLL12's ability to repress the RIG-I pathway is probably not dependent on protein modifications. Instead, TTLL12 directly interacts with virus-induced signaling adaptor (VISA), TBK1, and IKKε, and inhibits the interactions of VISA with other signaling molecules. Taken together, our findings demonstrate TTLL12 as a negative regulator of RNA-virus-induced type I IFN expression by inhibiting the interaction of VISA with other proteins.