TRIM25 inhibits Influenza A infection by destabilising its mRNA and is redundant for the RIG-I pathway

The E3 ubiquitin ligase TRIM25 is a key factor in the innate immune response to RNA viruses. TRIM25 has been shown to play a role in the retinoic-acid-inducible gene-1 (RIG-I) pathway, which triggers expression of type 1 interferons upon viral infection. We and others have recently shown that TRIM25 is an RNA-binding protein, however not much is known about the RNA-binding roles of TRIM25 in the innate immune response to RNA viruses. Here, we demonstrate that influenza A virus (IAV A/PR/8/34_NS1(R38K41A)) infection is inhibited by TRIM25. Surprisingly, host RNA-binding deficient mutant TRIM25{Delta}RBD and TRIM25{Delta}RING, which lack E3 ubiquitin ligase activity rescued IAV inhibition in TRIM25 knock-out cells. Furthermore, we show that in human cultured cells activation of the RIG-I/interferon type 1 pathway mediated by an IAV-derived 5-triphosphate RNA does not require TRIM25 activity. Additionally, knocking out TRIM25 does not affect the activity of the IAV polymerase. We present new evidence that TRIM25 restricts IAV by directly binding to and destabilising its mRNAs. Finally, we show that direct tethering of TRIM25 to RNA is sufficient to downregulate the targeted RNA. In summary, our results uncover a novel mechanism that TRIM25 uses to inhibit IAV infection and regulate RNA metabolism.