Viruses with an RNA genome are often the cause of zoonotic infections. In order to identify novel pro-viral host cell factors, we screened a haploid insertion–mutagenized mouse embryonic cell library for clones that are resistant to Rift Valley fever virus (RVFV). This screen returned the low-density lipoprotein receptor–related protein 1 (LRP1) as a top hit, a plasma membrane protein involved in a wide variety of cell activities. Inactivation ofLRP1in human cells reduced RVFV RNA levels already at the attachment and entry stages of infection. Moreover, the role of LRP1 in promoting RVFV infection was dependent on physiological levels of cholesterol and on endocytosis. In the human cell line HuH-7, LRP1 also promoted early infection stages of sandfly fever Sicilian virus and La Crosse virus, but had a minor effect on late infection by vesicular stomatitis virus, whereas encephalomyocarditis virus was entirely LRP1-independent. Moreover, siRNA experiments in human Calu-3 cells demonstrated that also SARS-CoV-2 infection benefitted from LRP1. Thus, we identified LRP1 as a host factor that supports infection by a spectrum of RNA viruses.

Background: The non-structural protein 1 (NS1) of influenza A virus (IAV) is a key player in inhibiting antiviral response in host cells, thereby facilitating its replication. However, other roles of NS1, which are independent of antagonising host cells' antiviral response, are less characterised. Methods: To investigate these unidentified roles, we used a recombinant virus, which lacks NS1 expression, and observed its phenotypes during the infection of antiviral defective cells (RIG-I KO cells) in the presence or absence of exogeneous NS1. Moreover, we used virus-like particle (VLP) production system to further support our findings. Results: Our experiments demonstrated that IAV deficient in NS1 replicates less efficiently than wild-type IAV in RIG-I KO cells and this replication defect was complemented by ectopic expression of NS1. As suggested previously, NS1 is incorporated in the virion and participates in the regulation of viral transcription and translation. Using the VLP production system, in which minigenome transcription or viral protein production was unaffected by NS1, we demonstrated that NS1 facilitates viral genome packaging into VLP, leading to efficient minigenome transfer by VLP. Furthermore, the incorporation of NS1 and the minigenome into VLP were impaired by introducing a point mutation (R38A) in the double stranded RNA-binding domain of NS1. Conclusion: These results suggest a novel function of NS1 in improving genome packaging in a dsRNA bindingdependent manner. Taken together, NS1 acts as an essential pro-viral regulator, not only by antagonizing host immunity but also by facilitating viral replication and genome packaging.

Viruses with an RNA genome are the main causes of zoonotic infections. In order to identify novel pro-viral host cell factors, we screened a haploid insertion-mutagenized mouse embryonic cell library for clones that rendered them resistant to the zoonotic Rift Valley fever virus (RVFV; family Phleboviridae, order Bunyavirales). This screen returned the Low Density Lipoprotein Receptor-Related protein 1 (LRP1, or CD91) as top hit, a 600 kDa plasma membrane protein known to be involved in a wide variety of cell activities. Inactivation of LRP1 expression in human cells reduced RVFV infection at the early stages of infection, including the particle attachment to the cell. In the highly LRP1-positive human HuH-7 cell line, LRP1 was required for the early infection stages also of Sandfly fever Sicilian virus (SFSV; family Phleboviridae, order Bunyavirales), vesicular stomatitis (VSV; family Rhabdoviridae, order Mononegavirales), Encephalomyocarditis virus (EMCV, family Picornaviridae), and the coronaviruses MERS-CoV, SARS-CoV-1, and SARS-CoV-2. While for RVFV, EMCV, and MERS-CoV the replication cycle could eventually catch up, LRP1 requirement for the late infection stage in HuH-7 cells was observed for SFSV, La Crosse virus (LACV; family Peribunyaviridae, order Bunyavirales), VSV, SARS-CoV-1, and SARS-CoV-2. For SARS-CoV-2, the absence of LRP1 stably reduced viral RNA levels in human lung Calu-3 cells, and both RNA levels and particle production in the hepatic HuH-7 cells. Thus, we identified LRP1 as a host factor that supports various infection cycle stages of a broad spectrum of RNA viruses.