Interaction of the influenza virus nucleoprotein with F-actin

Abstract

Introduction: The influenza virus genome is transcribed in the nucleus of infected cells but assembled into progeny virions in the cytoplasm. This is reflected in the cellular distribution of the virus nucleoprotein (NP), a protein which encapsidates genomic RNA to form ribonucleoprotein (RNP) structures: at the early stage post-infection, NP is found in the nucleus, but later it is found predominantly in the cytoplasm. The purpose of this study was to examine the possibility that cytoplasmic NP interacts with actin microfilaments. Methods: Bacterially expressed NP was tested for the ability to interact with filamentous (F)-actin in a variety of in vitro assays, and the localisation of actin and exogenously expressed NP in mammalian cells was examined microscopically. Results: Purified NP bound actin filaments in vitro and showed partial colocalisation with β-actin in vivo. Electron microscopy showed that NP induced bundling of actin fibres in vitro. In confirmation of this, NP caused a dramatic increase in the low-shear viscosity and light-scattering properties of F-actin suspensions. Conclusions: NP binds F-actin in vitro and can alter the mechanical properties of actin filaments. This raises the possibility that influenza virus may use the host-cell cytoskeleton during virus replication.