Intracellular warfare between human influenza viruses and human cells: the roles of the viral NS1 protein.

Virus-infected mammalian cells are a battleground: cells mount antiviral defenses against the virus, and these defenses are countered by the infecting virus. The eventual outcome of this war is a major determinant of the pathogenesis of the virus infection. In this minireview we will discuss the intracellular warfare between human influenza viruses and human cells. Influenza A and B viruses, two of the four influenza virus genera, cause widespread epidemics in humans (Lamb and Krug, 2001; Wright and Webster, 2001). Influenza A viruses, which have been isolated from a wide variety of avian and mammalian species, are responsible for the human pandemics that have caused high mortality rates, and the highly pathogenic virus that was transmitted from chickens to humans in Hong Kong in 1997 is an influenza A virus (Wright and Webster, 2001). Influenza B virus appears to infect only humans (Wright and Webster, 2001), although influenza B virus has recently been isolated from seals (Osterhaus et al., 2000). Both influenza A and B viruses contain eight genomic RNA segments, and most of the proteins encoded by the corresponding genomic RNA segments serve similar functions (Lamb and Krug, 2001). We will focus solely on the role of one virus-encoded protein in the virus–cell war: the NS1 protein, which is encoded by the smallest genomic RNA segment of both influenza A and B viruses. This protein is designated as nonstructural (NS) because it is synthesized in infected cells, but is not incorporated into virions (Lamb and Krug, 2001). The NS1 proteins of influenza A virus (NS1A protein) and influenza B virus (NS1B protein) share one function: the binding of double-stranded RNA (dsRNA) via their N-terminal RNA-binding domains (Hatada and Fukuda, 1992; Lu et al., 1995; Wang et al., 1999; Yuan et al., 2002) (Fig. 1). The three-dimensional structure of the RNA-binding domain of the NS1A protein exhibits a novel dimeric six-helical chain fold (Chien et al., 1997; Liu et al., 1997) that differs from that of any other known RNA-binding proteins in the structural database, including the dsRNAbinding domain (dsRBD) found in many cellular proteins (Ryter and Schultz, 1998; Ramos et al., 2000). Computer modeling and mutagenesis experiments indicate that the RNA-binding domain of the NS1B protein exhibits a dimeric six-helical chain fold similar to that of the NS1A RNA-binding domain except that there are large loops between the three -helices in each monomer (Wang et al., 1999; Yuan et al., 2002). The RNA-binding domains of the NS1A and NS1B bind dsRNA with low affinity, approximately 5000-fold lower than the dsRBDs found in cellular proteins (McCormack and Samuel, 1995; Schmedt et al., 1995; Wang et al., 1999; Ohman et al., 2000; Tian and Mathews, 2001; Yuan et al., 2002). All other functions of the NS1A and NS1B proteins differ markedly, as will be discussed later, resulting in major differences in the mechanisms by which these two proteins mediate viral countermeasures against cellular antiviral responses.