Differential distribution of influenza virus P proteins in nuclei of infected cells

The genome of avian influenza type A viruses comprises eight segments of vRNA [1-4] which are complementary to viral mRNA [5,6]. The latter are monocistronic and code for the following proteins in descending order ofMW: P1, P2, P3, HA, NP, NA, M and NSI [7-10]. NSa and an additional virus specified protein NS2 are found only in infected cells [1113]. The other proteins are constituents of the virion. P1, P2 and P3 are located together with NP and vRNA inside the virion in an RNP complex which will synthesise complementary RNA in vitro [8,14,15 ]. Genetic evidence confirms that the P proteins are concerned with transcription and further that they have a role in replication (see reviews [ 16,23]). The precise functions of P1-3 are unknown although a mutant, defective in the P2 gene (coding for the P1 protein [21]) failed to transport viral RNA from the nucleus [16]. Influenza virus multiplication is absolutely dependent upon the cell nucleus, requiring initially DNA-dependent RNA polymerase II activity [17,18] to synthesise functional viral message [19]. Later in infection, newly synthesised v and mRNA [ 19-21 ] and NP,M and NS1 proteins are found in the nucleus [22,23]. The various functions of these components in the nucleus is unknown, although it is suggested that vRNA synthesis occurs in that location [21,24] and that assembly of vRNA and proteins destined for progeny virions takes place there [23]. Identification of the location of P proteins in infected cells is difficult because they are synthesised in low amounts. However, their involvement in viral RNA synthesis stimulates investigation and we report here on the transfer of newly synthesised P1, P2 and P3 proteins to the nuclei of infected cells.