The Ebola virus interferon antagonist VP24 undergoes active nucleocytoplasmic trafficking

Abstract

Viral interferon (IFN) antagonist proteins mediate evasion of IFN-mediated innate immunity and are often multifunctional, having distinct roles in viral replication processes. Functions of the Ebola virus (EBOV) IFN antagonist VP24 include nucleocapsid assembly during cytoplasmic replication and inhibition of IFN-activated signalling by STAT1. For the latter, VP24 prevents STAT1 nuclear import via competitive binding to nuclear import receptors (karyopherins). Many viral proteins, including proteins from viruses with cytoplasmic replication cycles, interact with the trafficking machinery to undergo nucleocytoplasmic transport, with key roles in pathogenesis. Despite established karyopherin interaction, the nuclear trafficking profile of VP24 has not been investigated. We find that VP24 becomes strongly nuclear following overexpression of karyopherin or inhibition of nuclear export pathways. Molecular mapping indicates that cytoplasmic localisation of VP24 depends on a CRM1-dependent nuclear export sequence at the VP24 C-terminus. Nuclear export is not required for STAT1 antagonism, consistent with competitive karyopherin binding being the principal antagonistic mechanism while export mediates return of nuclear VP24 to the cytoplasm for replication functions. Thus, nuclear export of VP24 might provide novel targets for antiviral approaches. ImportanceEbola virus (EBOV) is the causative agent of ongoing outbreaks of severe haemorrhagic fever with case-fatality rates between 40 and 60%. Proteins of many viruses with cytoplasmic replication cycles similar to EBOV interact with the nuclear trafficking machinery, resulting in active nucleocytoplasmic shuttling important to immune evasion and other intranuclear functions. However, exploitation of host trafficking machinery for nucleocytoplasmic transport by EBOV has not been directly examined. We find that the EBOV protein VP24 is actively trafficked between the nucleus and cytoplasm, and identify the specific pathways and sequences involved. The data indicate that nucleocytoplasmic trafficking is important for the multifunctional nature of VP24, which has critical roles in immune evasion and viral replication, identifying a new mechanism in infection by this highly lethal pathogen, and potential target for antivirals.