Influenza virus NS1 protein inhibits pre-mRNA splicing and blocks mRNA nucleocytoplasmic transport.

The influenza virus RNA segment 8 encodes two proteins, NS1 and NS2, by differential splicing. The collinear transcript acts as mRNA for NS1 protein, while the spliced mRNA encodes NS2 protein. The splicing of NS1 mRNA was studied in cells transfected with a recombinant plasmid that has the cDNA of RNA segment 8 cloned under the SV40 late promoter and polyadenylation signals. As described for influenza virus-infected cells, NS1 mRNA was poorly spliced to yield NS2 mRNA. However, inactivation of the NS1 gene, but not the NS2 gene, led to a substantial increase in the splicing efficiency, as shown by the relative accumulations of NS1 and NS2 mRNAs. This effect was not specific for NS1 mRNA, since the splicing of the endogenous SV40 early transcript was altered in such a way that t-Ag mRNA was almost eliminated. These changes in the splicing pattern coincided with a strong inhibition of the mRNA nucleocytoplasmic transport. Both NS1 and NS2 mRNAs were retained in the nucleus of cells expressing NS1 protein, but no effect was observed when only NS2 protein was expressed. Furthermore, other mRNAs tested, such as T-Ag mRNA and the non-spliceable nucleoprotein transcript, were also retained in the nucleus upon expression of NS1 protein, suggesting that it induced a generalized block of mRNA export from the nucleus.