THE INFLUENZA-VIRUS NS1 PROTEIN IS A POLY(A)-BINDING PROTEIN THAT INHIBITS NUCLEAR EXPORT OF MESSENGER-RNAS CONTAINING POLY(A)

The influenza virus NS1 protein inhibits the nuclear export of a splic ed viral mRNA, NS2 mRNA (F. V. Alonso-Caplen, M. E. Nemeroff, Y. Qiu, and R. M. Krug, Genes Dev. 6:255-267, 1992). To identify the sequence in NS2 mRNA that is recognized by the NS1 protein, eve developed a gel shift assay for the formation of specific RNA-protein complexes. With this assay, it was established that the NS1 protein binds to the poly (A) sequence at the 3' end of NS2 mRNA and of other mRNAs. In addition , the NS1 protein was shown to bind to poly(A) itself. This specificit y was also observed in vivo. The NS1 protein inhibited the nuclear exp ort of every poly(A)-containing mRNA that was tested. In contrast, the NS1 protein failed to inhibit the nuclear export of an mRNA whose 3' end was generated by cleavage without subsequent addition of poly(A). Addition of poly(A) to this mRNA enabled the NS1 protein to inhibit mR NA export. The implications of these results for the role of the NS1 p rotein during virus infection are discussed.