No abstract

The influenza virus NS1 protein is the only known example of a protein that inhibits the nuclear export of mRNA. To identify the functional domains of this protein, we introduced 18 2-or 3-amino-acid substitutions at approximately equally spaced locations along the entire length of the protein. Two functional domains were identified. The domain near the amino end (amino acids 19 through 38) was shown to be the RNA-binding domain, by using a gel shift assay with purified NS1 protein and spliced viral NS2 mRNA as the RNA target. The second domain, which is in the carboxy half of the molecule, was presumed to be the effector domain that interacts with host nuclear proteins to carry out the nuclear RNA export function, by analogy with the eflector domain of the Rev proteins of human immunodeficiency virus (HIY) and other lentiviruses which facilitate rather than inhibit nuclear RNA export. The NS1 protein has a 10-amino-acid sequence that is similar to the consensus sequence in the eflector domains of lentivirus Rev proteins, specifically including two crucial leucines at positions 7 and 9 of this sequence. However, the effector domains of the NS1 and Rev (HIV type 1 [HIV-1]) proteins differed in several significant ways including the following: (i) unlike the HIV-1 Rev protein, NS1 elector domain mutants were negative recessive rather than negative dominant, (ii) the NS1 effector domain is about three times larger than the effector domain of the HIV-1 Rev protein, and (iii) unlike the HIV-1 protein, NS1 effector domain mutants exhibited a surprising property, a changed intracellular/intranuclear distribution, compared with the wild-type protein. These differences strongly suggest that the effector domains of the NS1 and Rev proteins interact with different nuclear protein targets, which likely explains the opposite effects of these two proteins on nuclear mRNA export.

No abstract

Analysis of viral glycoprotein expression on surfaces of monensin-treated cells using a fluorescence-activated cell sorter (FACS) demonstrated that the sodium ionophore completely inhibited the appearance of the vesicular stomatitis virus (VSV) G protein on (Madin-Darby canine kidney) MDCK cell surfaces. In contrast, the expression of the influenza virus hemagglutinin (HA) glycoprotein on the surfaces of MDCK cells was observed to occur at high levels, and the time course of its appearance was not altered by the ionophore. Viral protein synthesis was not inhibited by monensin in either VSV-or influenza virus-infected cells. However, the electrophoretic mobilities of viral glycoproteins were altered, and analysis of pronase-derived glycopeptides by gel filtration indicated that the addition of sialic acid residues to the VSV G protein was impaired in monensin-treated cells. Reduced incorporation of fucose and galactose into influenza virus HA was observed in the presence of the ionophore, but the incompletely processed HA protein was cleaved, transported to the cell surface, and incorporated into budding virus particles. In contrast to the differential effects of monensin on VSV and influenza virus replication previously observed in monolayer cultures of MDCK cells, yields of both viruses were found to be significantly reduced by high concentrations of monensin in suspension cultures, indicating that cellular architecture may play a role in determining the sensitivity of virus replication to the drug. Nigericin, an ionophore that facilitates transport of potassium ions across membranes, blocked the replication of both influenza virus and VSV in MDCK cell monolayers, indicating that the ion specificity of ionophores influences their effect on the replication of enveloped viruses.Cells infected with enveloped RNA viruses provide valuable systems for elucidating the pathways involved in subcellular transport of membrane glycoproteins (8,20,44). As a result of viral inhibition of host protein synthesis, only one or a few viral membrane glycoproteins are synthesized in infected cells. These viruses possess limited genetic capacity, and the glycosylation and transport of their membrane glycoproteins to the cell surface are probably carried out by the same systems used by the host cell for biogenesis of its own membrane glycoproteins.Monovalent ionophores such as monensin, which is reported to interfere with the translocation of secretory as well as most membrane glycoproteins, have been used to characterize the pathways of intracellular glycoprotein transport in eucaryotic cells (18,(37)(38)(39)(40)(41)43). In addition, the effects of monensin on IgM and H2 glycosylation in lymphoid cells and fibronectin glycosylation in cultured human fibroblasts, as well as its influence on oligosaccharide maturation of

Influenza virus NS1 mRNA is spliced by host nuclear enzymes to form NS2 mRNA, and this splicing is regulated in infected cells such that the steady-state amount of spliced NS2 mRNA is only about 10% of that of unspliced NS1 mRNA. This regulation would be expected to result from a suppression in the rate of splicing coupled with the efficient transport of unspliced NS1 mRNA from the nucleus. To determine whether the rate of splicing of NS1 mRNA was controlled by trans factors in influenza virus-infected cells, the NS1 gene was inserted into an adenovirus vector. The rates of splicing of NS1 mRNA in cells infected with this vector and in influenza virus-infected cells were measured by pulse-labeling with [3H]uridine. The rates of splicing of NS1 mRNA in the two systems were not significantly different, strongly suggesting that the rate of splicing of NS1 mRNA in influenza virus-infected cells is controlled solely by cis-acting sequences in NS1 mRNA itself. In contrast to the rate of splicing, the extent of splicing of NS1 mRNA in the cells infected by the adenovirus recombinant was dramatically increased relative to that occurring in influenza virus-infected cells. This could be attributed largely, if not totally, to a block in the nucleocytoplasmic transport of unspliced NS1 mRNA in the recombinant-infected cells. Most of the unspliced NS1 mRNA was in the nuclear fraction, and no detectable NS1 protein was synthesized. When the 3' splice site of NS1 mRNA was inactivated by mutation, NS1 mRNA was transported and translated, indicating that the transport block occurred because NS1 mRNA was committed to the splicing pathway. This transport block is apparently obviated in influenza virus-infected cells. These experiments demonstrate the important role of the nucleocytoplasmic transport of unspliced NS1 mRNA in regulating the extent of splicing of NS1 mRNA.Splicing of eucaryotic pre-mRNAs occurs in the nucleus, and in most cases only the spliced mRNA products are detected in the cytoplasm. Retrovirus and influenza virus splicing deviates from this pattern (3,14,16,17,20). In these cases, both the spliced mRNA product(s) and the unspliced pre-mRNA(s) code for proteins, and with retroviruses, the completely unspliced pre-mRNA also serves as genomic RNA for progeny virus. Consequently, only a portion rather than all of the pre-mRNA(s) is spliced, and the unspliced pre-mRNA(s) is transported from the nucleus.Two influenza virus mRNAs, the Ml (matrix) and NS1 (nonstructural protein 1) mRNAs, are spliced by host nuclear enzymes to form smaller mRNAs, M2 mRNA and mRNA3 from Ml mRNA and NS2 mRNA from NS1 mRNA (11,12,(15)(16)(17)20). In infected cells, the extent of splicing is regulated such that the steady-state amounts of the spliced mRNAs are only about 10% of those of the unspliced mRNAs (16,20). This regulation could occur at various levels. However, if the unspliced and spliced viral mRNAs are relatively stable, then the steady-state kinetic model for the generation of spliced mRNA would predict that the 1:10 ratio of s...