Influenza virus transcription occurs in the nuclei of infected cells, where the viral genomic RNAs are complexed with a nucleoprotein (NP) to form ribonucleoprotein (RNP) structures. Prior to assembly into progeny virions, these RNPs exit the nucleus and accumulate in the cytoplasm. The mechanisms responsible for RNP export are only partially understood but have been proposed to involve the viral M1 and NS2 polypeptides. We found that the drug leptomycin B (LMB), which specifically inactivates the cellular CRM1 polypeptide, caused nuclear retention of NP in virus-infected cells, indicating a role for the CRM1 nuclear export pathway in RNP egress. However, no alteration was seen in the cellular distribution of M1 or NS2, even in the case of a mutant virus which synthesizes greatly reduced amounts of NS2. Furthermore, NP was distributed throughout the nuclei of infected cells at early times postinfection but, when retained in the nucleus at late times by LMB treatment, was redistributed to the periphery of the nucleoplasm. No such change was seen in the nuclear distribution of M1 or NS2 after drug treatment. Similar to the behavior of NP, M1 and NS2 in infected cells, LMB treatment of cells expressing each polypeptide in isolation caused nuclear retention of NP but not M1 or NS2. Conversely, overexpression of CRM1 caused increased cytoplasmic accumulation of NP but had little effect on M1 or NS2 distribution. Consistent with this, NP bound CRM1 in vitro. Overall, these data raise the possibility that RNP export is mediated by a direct interaction between NP and the cellular CRM1 export pathway.The influenza virus genome consists of eight segments of single-stranded RNA that encode a total of 10 identified polypeptides. The genomic RNA segments are of negative sense and are always found in association with viral polypeptides: the three subunits of an RNA-dependent RNA polymerase (PB1, PB2, and PA) and, in stoichiometric quantities, a single-strand RNA-binding nucleoprotein (NP) (28). In virions, these ribonucleoprotein (RNP) structures are packaged within a shell of the viral M1 polypeptide underlying the lipid bilayer, along with the hemagglutinin (HA) and neuraminidase integral membrane glycoproteins. Minor virion components include M2, a small transmembrane ion channel, and the NS2 polypeptide (28). Influenza virus particles enter the cell by receptor-mediated endocytosis. Following acidification of the endosome, the M1 polypeptide dissociates from the RNP segments and virion RNPs (vRNPs) are released into the cytoplasm (30, 31). Unusually for a virus with no DNA coding stage, influenza virus transcription occurs in the nucleus (20,22). Accordingly, after release of the RNPs into the cytoplasm, they migrate into the nucleus, in an active process that is thought to be mediated by the cellular importin ␣/␤ pathway (39). Once in the nucleus, vRNPs act as the template for synthesis of mRNAs, which are exported into the cytoplasm for translation. The vRNPs also act as the template for synthesis of full-length cRNA co...

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A 67-year-old woman with a long history of back pain began a walking program for exercise. Six weeks later, she experienced pain in the posterior aspect of her left thigh that became increasingly severe. She was referred to an orthopedic surgeon, who ordered a plain radiograph, the findings of which were reported as normal. He found localized tenderness in the upper third of the posterior thigh. Bone scintigraphy revealed a localized area of increased uptake at the site, and computed tomography (CT) showed calcification in the tendon of the gluteus maximus at its insertion into the posterior cortex of the left femur. This was thought to be consistent with gluteal tendinopathy, because it is similar to previously reported cases.