Phosphorylation dynamically controls the replication of influenza virus

Abstract

It is well known that phosphorylation regulates the functions of viral proteins and the life cycle of influenza A virus (IAV). As nucleoprotein (NP) and Matrix protein 1 (M1) are the most abundant proteins in the viral ribonucleoprotein (vRNP) complex and matrix layer of influenza A virus, and NS1 exerts most functions and interactions with host factor within IAV infection period as reports, these three proteins were widely developed for their hyper-phosphorylation state during IAV infectious course. The NP is a major component of the vRNP complex. During the replication of influenza virus, and each viral RNA segment is encapsulated by multiple copies of the NP. The vRNP complex undergoes a course of nuclear import, viral RNA replication and transcription, and nuclear export, during which NP serves as one of the determinants. Dozens of phosphorylation sites have been identified and determined to regulate the viral polymerase activity and nuclear-cytoplasmic shuttling of NP. Residue S165 have been demonstrated to mediate self-oligomerization of NP through phosphorylation by PKC, while Y10 and Y296 dynamically controlled the nuclear import and export respectively. M1, the most abundant protein in virions, also has multiple functions in the influenza A virus life cycle, including uncoating, transcription, nuclear export of vRNPs, assembly, and budding. Early studies indicated that M1 contains phosphoserine and phosphothreonine residues and has the potential to be phosphorylated by PKC and ERKs. Recently, several phosphorylation sites on M1 (including a phosphotyrosine) were reported. However, a majority of phosphorylated residues of M1 has not yet been identified, except for phosphorylated Y132 on M1 which has been confirmed to mediate the nuclear import of M1 via modulation of M1-importin interaction. The multi-functional NS1, which is encoded by the eighth RNA segment, is one of the IAV virulence factors. The multi-functional NS1, which is encoded by the eighth RNA segment, is one of the IAV virulence factors. Through binding to double-strand RNA and retinoic acid-induced gene 1 protein (RIG-I), NS1 antagonizes host innate/adaptive immune responses and inhibits interferon (IFN) production, and NS1 also binds to the NP of the vRNP complex to facilitate efficient virus replication. Several reports demonstrated that cellular host kinase such as PKC α , CDK, and ERK involved into the NS1 phosphorylation, causing phosphorylation occurred on multiple sites on NS1. These sites were proved to control IAV replication through mediating the association of NS1 with dsDNA, TRIM25, or RIG-I, being known as major sensors in RIG-like receptor signal pathway to production antiviral IFN. In addition, there exist phosphorylation occurring in other viral proteins, included PA, PB1, PB1-F2, NEP, and M2. In the present report, we majorly retrospectively reviewed the phosphorylation sites and their functions on these three proteins, and briefly summarized the phosphorylation researches on other viral proteins. We believed that this report provided a novel aspect of virus-host interaction study, and a theoretical basis for searching anti-flu target works in drug research and development area.