Modulation of nuclear localization of the influenza virus nucleoprotein through interaction with actin filaments

The influenza virus genome is transcribed in the nuclei of infected cells b ut assembled into progeny virions in the cytoplasm. This is reflected in th e cellular distribution of the virus nucleoprotein (NP), a protein which en capsidates genomic RNA to form ribonucleoprotein structures. At early times postinfection NP is found in the nucleus, but at later times it is found p redominantly in the cytoplasm. NP contains several sequences proposed to ac t as nuclear localization signals (NLSs), and it is not clear how these are overridden to allow cytoplasmic accumulation of the protein. We find that NP binds tightly to filamentous actin in vitro and have identified a duster of residues in NP essential for the interaction. Complexes containing RNA, NP, and actin could be formed, suggesting that viral ribonucleoproteins al so bind actin. In cells, exogenously expressed NP when expressed at a high level partitioned to the cytoplasm, where it associated with F-actin stress fibers. In contrast, mutants unable to bind F-actin efficiently were impor ted into the nucleus even under conditions of high-level expression. Simila rly, nuclear import of NLS-deficient NP molecules was restored by concomita nt disruption of F-actin binding. We propose that the interaction of NP wit h F-actin causes the cytoplasmic retention of influenza virus ribonucleopro teins.