Summary Influenza viruses exploit host cell machinery to replicate, resulting in epidemics of respiratory illness. In turn, the host expresses anti-viral restriction factors to defend against infection. To find host-cell modifiers of influenza A H1N1 viral infection, we used a functional genomic screen and identified over 120 influenza A virus-dependency factors with roles in endosomal acidification, vesicular trafficking, mitochondrial metabolism and RNA splicing. We discovered that the interferon-inducible trans-membrane proteins, IFITM1, 2 and 3, restrict an early step in influenza A viral replication. The IFITM proteins confer basal resistance to influenza A virus, but are also inducible by interferons type I and II, and are critical for interferon's virustatic actions. Further characterization revealed that the IFITM proteins inhibit the early replication of flaviviruses, including dengue virus and West Nile virus. Collectively this work identifies a new family of anti-viral restriction factors that mediate cellular resistance to at least three major human pathogens.

Human angiotensin‐converting enzyme 2 (ACE2) is a functional receptor for SARS coronavirus (SARS‐CoV). Here we identify the SARS‐CoV spike (S)‐protein‐binding site on ACE2. We also compare S proteins of SARS‐CoV isolated during the 2002–2003 SARS outbreak and during the much less severe 2003–2004 outbreak, and from palm civets, a possible source of SARS‐CoV found in humans. All three S proteins bound to and utilized palm‐civet ACE2 efficiently, but the latter two S proteins utilized human ACE2 markedly less efficiently than did the S protein obtained during the earlier human outbreak. The lower affinity of these S proteins could be complemented by altering specific residues within the S‐protein‐binding site of human ACE2 to those of civet ACE2, or by altering S‐protein residues 479 and 487 to residues conserved during the 2002–2003 outbreak. Collectively, these data describe molecular interactions important to the adaptation of SARS‐CoV to human cells, and provide insight into the severity of the 2002–2003 SARS epidemic.

SUMMARY TRIM25 mediates Lys 63-linked ubiquitination of the N-terminal CARDs of the viral RNA sensor RIG-I, leading to type I interferon (IFN) production. Here, we report that the influenza A virus non-structural protein 1 (NS1) specifically inhibits TRIM25-mediated RIG-I CARD ubiquitination, thereby suppressing RIG-I signal transduction. A novel domain in NS1 comprising E96/E97 residues mediates its interaction with the coiled-coil domain of TRIM25, thus blocking TRIM25 multimerization and RIG-I CARD ubiquitination. Furthermore, a recombinant influenza A virus expressing an E96A/E97A NS1 mutant is defective in blocking TRIM25-mediated anti-viral IFN response and loses virulence in mice. Our findings reveal a novel mechanism of influenza virus to inhibit host IFN response and also emphasize the vital role of TRIM25 in modulating viral infections.