Introduction
Influenza A virus (IAVs) ,which contains eight negative single-strand RNA segments, could facilitate new virus through gene reassortment or mutation, such as the 2009 pandemic H1N1 in human that caused serious deaths and economic losses(Vincent A,2014). Between 2011 and 2018, a predominant emergent EA reassortant genotype 4 (G4) virus in pigs had pdm/09 and TR-derived internal genes and showed increasing human infectivity(Sun, 2020). The virial RNA (vRNA) is bound to RNA-dependent RNA polymerase complex (RdRp) and encapsidated by the nucleoprotein (NP), forming the so-called viral ribonucleoprotein (vRNP) complex(Lamb,1983). The vRNP is the key to IAV life cycle and important for viral pathogenicity and host range determinants(Davis ,2017). NP is essential for the translocation of vRNP into the nucleus of host cells and could be used as a target to block the replication of influenza virus specifically (Ozawa, 2007 ; Cros, 2005). A series of host proteins have been reported to regulate nuclear entry through interacting with NP, such as α-actinin-4(Sharma, 2014), CRM1(Pickens, 2018), UAP56(Chiba, 2018), Hsp40(Batra, 2016) and MOV10(Zhang, 2016), hence played important roles in promoting or inhibiting virus replication. So, identification of new NP-binding host proteins will help to provide new targets for prevention and/or control of influenza viruses.
Phospholipid scramblase 1 (PLSCR1) was first identified as a calcium-binding type II membrane protein that could be induced by (Li ,2016). It is involved in multiple biological processes, such as gene transcription regulation, cell proliferation, differentiation and apoptosis (Gui ,2020; Huang ,2020; Li , 2006; Stray , 2005). Recently, the antiviral activity of PLSCR1 received extensive attentions. PLSCR1 inhibits HBV infection and replication through mediating ubiquitin-dependent degradation of HBx protein (Yuan, 2015), and mediates resistance of HCV infection through interfering with the viral entry into host cells (Gong, 2011). PLSCR1 also interacts with the CD4 receptor on the T lymphocyte membrane to inhibit HIV infection and interacts with the ANG in the nucleus to regulate rRNA transcription (Zhu, 2013). Moreover, PLSCR1 inhibits vesicular stomatitis virus (VSV) and encephalomyocarditis virus through promoting the secretion of IFN (Dong , 2004).
Recently, it was reported that PLSCR1 impairs nuclear import of IVA through interacting with NP, thereby inhibiting IVA replication (Luo, 2018). Overexpression of PLSCR1 in infected cells suppresses the nuclear import of NP and significantly inhibits virus replication. In contrast, siRNA knockdown or CRISPR/Cas9 knockout of PLSCR1 in cultured cells increases virus propagation (Luo , 2018). Further investigation showed that PLSCR1 forms a trimeric complex with NP and importin α, which inhibits the incorporation of importin β and suppress nuclear importation of NP (Luo , 2018).
Immunoglobulin-like domain-containing receptor 1 (ILDR1) is an evolutionally conserved type I transmembrane protein that contains immunoglobulin (Ig)-like domain. ILDR1 and its two paralogs, ILDR2 and LSR, have been identified as components of tricellular tight junctions (tTJs), specialized structures where the corners of three epithelial cells meet to form a barrier of the cellular sheet. Mutations of ILDR1 gene are associated with autosomal recessive hearing impairment DFNB42, and knockout of Ildr1 gene in mice leads to degeneration of mechanosensory cochlear hair cells, resulting in hearing loss (Higashi ,2015). Our previous data showed that ILDR1 interacts with a series of splicing factors and regulates alternative splicing (Liu, 2017). In the present work, we demonstrate that ILDR1 promotes IAV replication through interacting with PLSCR1. Our data suggest the existence of a previously unknown pathway in regulating IAV infection, which sheds light on IAV prevention and/or treatment.