Battle between influenza A virus and a newly identified ZAPL antiviral activity

Influenza A virus infection causes a highly contagious annual respiratory disease in humans as well as periodic pandemics with higher mortality rates. The Krug laboratory has shown that one of the major ways that the influenza virus NS1 protein counteracts host antiviral responses is to bind the 30 kDa subunit of the cleavage and polyadenylation specificity factor (CPSF30). As a consequence, 3’ end processing of cellular pre-mRNAis is inhibited, leading to reduced production of cellular mRNAs, including interferon mRNAs. I showed that NS1-CPSF30 complexes contain an array of cellular proteins. I purified the NS1-CPSF30 complexes from virus infected cells by affinity selection of CPSF30 and the NS1 protein. I identified the associated cellular proteins by mass spectrometry. Two cellular RNA helicases, DDX21 and DHX30, were identified. SiRNA knockdown of either RNA helicase enhanced virus replication, indicating that DDX21 and DHX30 inhibit influenza A virus replication. Further study demonstrated that DDX21 RNA helicase inhibits viral RNA synthesis, and is countered by the NS1 protein. The cellular ZAPL antiviral protein was also identified in the NS1-CPSF30 complexes. Previous studies have shown that ZAPL antiviral activity is mediated by its N-terminal zinc-fingers, which targets viral mRNA of several viruses for degradation. Little is known about the antiviral role of the ZAPL C-terminal PARP domain. Here I discovered the antiviral role of ZAPL C-terminal PARP domain against influenza A virus. I showed that the ZAPL PARP domain targets the viral polymerase PA and PB2 proteins. These two viral polymerases are poly(ADP-ribosylated), presumably by other PARP protein(s). The ZAPL-associated, poly(ADP-ribosylated) PA and PB2 are then ubiquitinated and proteasomally degraded. This ZAPL antiviral activity is counteracted by the binding of polymerase PB1 protein to the WWE region adjacent to the PARP domain, and causes PA and PB2 to dissociate from ZAPL and thus escape degradation. Because PB1 displaces PA and PB2 and protects them from ZAPL-mediated degradation, endogenous ZAPL only moderately inhibits influenza A virus replication (20-30-fold), as determined by siRNA knockdown experiment. These results suggest that influenza A virus has partially won the battle against the newly identified ZAPL antiviral activity.