Transport of Ribonucleoproteins into the Host Nucleus

Transport of Ribonucleoproteins into the Host Nucleus An unusual characteristic of the influenza virus life cycle is its dependence on the nucleus. Trafficking of the viral genome into and out of the nucleus is a tightly regulated process with all viral RNA synthesis occurring in the nucleus. The eight influenza virus genome segments never exist as naked RNA but are associated with four viral proteins to form viral ribonucleoprotein complexes (vRNPs). The major viral protein in the RNP complex is the nucleocapsid protein (NP), which coats the RNA. The remaining proteins PB1, PB2 and PA bind to the partially complementary ends of the viral RNA, creating the distinctive panhandle structure. These RNPs (10-20nm wide) are too large to passively diffuse into the nucleus and therefore, once released from an incoming particle must rely on the active import mechanism of the host cell nuclear pore complex. All proteins in the RNP complex can independently localize to the nucleus due to the presence of nuclear localization signals (NLSs) which mediate their interaction with the nuclear import machinery, including the RanGTPase (Fodor, 2004; Deng et al., 2006). However the signals on NP have been shown to be both sufficient and necessary for the import of viral RNA.