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Localization of Influenza Virus Proteins to Nuclear Dot 10 Structures in Influenza Virus-Infected Cells

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Localization of Influenza Virus Proteins to Nuclear Dot 10 Structures in Influenza Virus-Infected Cells View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Available online at www.sciencedirect.com R Virology 310 (2003) 29–40 www.elsevier.com/locate/yviro Localization of influenza virus proteins to nuclear dot 10 structures in influenza virus-infected cells Yoshiko Sato,a Kenichi Yoshioka,a Chie Suzuki,a Satoshi Awashima,a Yasuhiro Hosaka,a Jonathan Yewdell,b and Kazumichi Kurodaa,c,* a Department of Virology and Immunology, Osaka University of Pharmaceutical Sciences, 4-20-1 Nasahara, Takatsuki, Osaka 569-1094, Japan b Laboratory of Viral Diseases, National Institute of Allergy and Infectious Disease, National Institutes of Health, Bethesda, MD 20892, USA c Department of Immunology and Microbiology, Nihon University School of Medicine, Oyaguchi-kamicho, Itabashi-ku, Tokyo 173-8610, Japan Received 22 July 2002; returned to author for revision 19 August 2002; accepted 4 December 2002 Abstract We studied influenza virus M1 protein by generating HeLa and MDCK cell lines that express M1 genetically fused to green fluorescent protein (GFP). GFP-M1 was incorporated into virions produced by influenza virus infected MDCK cells expressing the fusion protein indicating that the fusion protein is at least partially functional. Following infection of either HeLa or MDCK cells with influenza A virus (but not influenza B virus), GFP-M1 redistributes from its cytosolic/nuclear location and accumulates in nuclear dots. Immunofluorescence revealed that the nuclear dots represent nuclear dot 10 (ND10) structures. The colocalization of authentic M1, as well as NS1 and NS2 protein, with ND10 was confirmed by immunofluorescence following in situ isolation of ND10. These findings demonstrate a previously unappreciated involvement of influenza virus with ND10, a structure involved in cellular responses to immune cytokines as well as the replication of a rapidly increasing list of viruses. © 2003 Elsevier Science (USA). All rights reserved. Introduction controversial (Enami and Enami, 1996; Kretzschmar et al., 1996; Zhang and Lamb, 1996), it was shown that the cyto- Matrix protein (M1) consists of 252 residues and is the plasmic tails of HA and NA are essential for correct virus most abundant protein in influenza A virions, with about assembly (Mitnaul et al., 1996; Jin et al., 1997). The inter- 3000 molecules per spherical virion (Lamb, 1989). M1 action of M1 with lipids was shown in vitro by the efficient forms a continuous layer at the interior surface of the en- binding of purified M1 protein to liposomes (Bucher et al., velope (Nermut, 1972; Schulze, 1972; Booy et al., 1985; 1980; Gregoriades, 1980; Gregoriades and Frangione, 1981; Fujiyoshi et al., 1994) where it is believed to maintain virion Ruigrok et al., 2000). A minor component of influenza A integrity by bridging the viral envelope and the ribonucle- virion. NS2 [recently renamed nuclear export protein oprotein (RNP) complex. The interaction of M1 with both (NEP)] also associates with M1 (Yasuda et al., 1993; Ward viral integral membrane proteins in the plasma membrane et al., 1995). and newly assembled RNP complex is believed to be crucial M1 regulates RNP transport between cytoplasm and nu- in viral budding. M1 is thought to interact with the lipid cleus. At the penetration step, the exposure of influenza molecules of the envelope and also with the cytoplasmic virions to acidic pH in endosomes activates the proton tails of envelope proteins, hemagglutinin (HA), neuramini- channel activity of M2 protein, which acidifies the viral dase (NA), and M2. Although the latter interaction remains interior (Sugrue and Hay, 1991; Pinto et al., 1992). This induces the dissociation of M1 proteins from RNP (Zhirnov, 1990, 1992; Bui et al., 1996), which is essential for incom- * Corresponding author. Department of Immunology and Microbiol- ogy, Nihon University School of Medicine, Oyaguchi-kamicho, Itabashi- ing RNP to enter the nucleus (Martin and Helenius, 1991). ku, Tokyo 173-8610, Japan. Fax: ϩ81-3-3972-9560. At the late stage of infection, M1 binding to the newly E-mail address: [email protected] (K. Kuroda). formed RNP complexes in the nucleus induces the export of 0042-6822/03/$ – see front matter © 2003 Elsevier Science (USA). All rights reserved. doi:10.1016/S0042-6822(03)00104-1 30 Y. Sato et al. / Virology 310 (2003) 29–40 RNP to the cytoplasm (Martin and Helenius, 1991; Bui et al., 2000). As the binding of M1 inhibits the transport of RNP into nuclei, the exported M1-RNP complex does not reenter the nucleus (Whittaker et al., 1996). NS2 (NEP) probably contributes to the export process (O’Neill et al., 1998). M1 binds RNA and inhibits the initiation of RNA synthesis in vitro (Hankins et al., 1990; Watanabe et al., 1996; Ye et al., 1989; Zvonarjev and Ghendon, 1980) and in vivo (Perez and Donis, 1998). The accumulation of M1 in the nucleus is postulated to mediate the suppression of mRNA synthesis by the polymerase complex of influenza virus late in infection (Shapiro et al., 1987; Zvonarjev and Ghendon, 1980). Other roles of M1 are suggested by its interaction with host proteins. By immunofluorescence, a portion of M1 colocalizes with actin filaments (Bucher et al., 1989), which may account for its resistance to detergent extraction (Ava- los et al., 1997). M1 also associates with histones (Zhirnov and Klenk, 1997) and the activated C-kinase (RACK 1), which may be involved in M1 phosphorylation (Reinhardt and Wolff, 2000). Phosphorylation is the only known post- Fig. 1. Expression of GFP/M1 in HeLa-GFP/M3 and MDCK-GFP/M10 translational modification of M1 (Gregoriades et al., 1984, cells. Panel A shows diffuse pattern of fluorescence from GFP/M1 in 1990). MDCK-GFP/M10 cells and also the image of phase-contrast microscopy in Despite its multiple roles in numerous cellular locales, the same field. (B) Polypeptides in MDCK-GFP (a), MDCK-GFP/M10 (b), the intracellular trafficking of M1 is largely uncharacter- HeLa-GFP (c), and HeLa-GFP/M3 (d) cells were separated by sodium ized. To study M1 in living cells, we constructed a green dodecyl sulfate–polyacrylamide gel electrophoresis and blotted to PVDF membranes. GFP/M1 protein on the membranes was detected by anti-PR8 fluorescent protein (GFP)/M1 fusion gene and established serum. The positions of molecular standards (kDa) are indicated at right. cell lines constitutively expressing the fusion protein. GFP from the jellyfish, Aequorea victoria, is a versatile tool for studying gene expression and trafficking of fusion protein in Immunoblot analysis confirmed that each of the lines vivo or in situ (reviewed in Lippincott-Schwartz et al., expressed a protein of the predicted molecular mass of the 1998; Chelbi-Alix et al., 1995; Tsien, 1998). A number of fusion protein (Fig. 1B). A protein of this mobility could viral proteins have been fused with GFP and their localiza- also be immunoprecipitated from detergent lysates of tion was traced in uninfected cells or cells infected with the MDCK-GFP/M10 cells labeled with [35S]methionine- source virus (Desai and Person, 1998; Elliott and O’Hare, [35S]cysteine (data not shown). Based on these results, we 1997; Presley et al., 1997; Thomas and Maule, 2000). Using concluded that GFP/M1 fusion protein is correctly ex- cells expressing GFP/M1 we found unexpectedly that M1 is pressed in each of the cell lines. localized to ND10 (nuclear dot 10), subnuclear structures that have been implicated in the replication of numerous Incorporation of GFP/M1 into virions viruses and in host cell responses to antiviral cytokines (reviewed in Hodges et al., 1998; Sternsdorf et al., 1997). To examine whether GFP/M1 fusion protein maintains the character of authentic M1 protein, we investigated the incorporation of GFP/M1 fusion protein into virions by Results sucrose density analysis. When the virus preparation from PR8-infected MDCK-GFP/M5-6 cells was fractionated by Expression of GFP/M1 protein in MDCK and HeLa cells velocity sucrose gradient (20 to 50%) centrifugation, we detected GFP/M1 by immunoblot analysis using anti-PR8 We established three cell lines expressing a GFP-M1 serum in the fractions 6 to 8 with the greatest amount in the fusion protein, MDCK-GFP/M10, MDCK-GFP/M5-6, and fraction 6, where the most HA protein was also detected HeLa-GFP/M3. In each of these lines the GFP fluorescence (Fig. 2A). The fractions 6 to 8 contained the bulk of virus was distributed uniformly throughout the cytoplasm and obtained from nontransfected MDCK cells and also from nucleus (Fig. 1A). This pattern of distribution was similar to MDCK-GFP cells (Fig. 2A) Western blotting analysis using that of M1 in previous reports where M1 was solely ex- anti-GFP antibody did not detect GFP protein in the frac- pressed (Bui et al., 1989, 1996). It should be noted, how- tions 6 to 8 of virions from GFP-MDCK cells even after ever, a patchy and reticular distribution of recombinant M1 10-fold concentration of virions in the fractions (Fig. 2B). was also reported (Zhao et al., 1998). The sucrose density analysis demonstrates that the GFP/M1 Y. Sato et al. / Virology 310 (2003) 29–40 31 GFP to M1 does not detectably alter its intracellular distri- bution, ability to associate with other viral structural pro- teins, or contribution to virus structure. Influenza virus infection redistributes GFP/M1 to nuclear dots We next examined the distribution of GFP/M1 in live PR8-infected MDCK-GFP/M5-6 cells (Fig. 3). We noted the first alteration in GFP/M1 distribution at ϳ4 h p.i. after which fluorescence began to concentrate in nuclear dots. By 10 h p.i. nearly all infected cells exhibited this pattern of fluorescence (Fig. 3H). Nuclear dots were distributed throughout nucleoplasm and were excluded from nucleoli (Fig.
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