The Role of Ebola Virus VP24 Nuclear Trafficking Signals in Infectious Particle Production
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Ebola virus (EBOV) protein VP24 carries out at least two critical functions. It promotes condensation of viral nucleocapsids, which is crucial for infectious virus production, and it suppresses interferon (IFN) signaling, which requires interaction with the NPI-1 subfamily of importin-α (IMPA) nuclear transport proteins. Interestingly, over-expressed IMPA leads to VP24 nuclear accumulation and a carboxy-terminus nuclear export signal (NES) has been reported, suggesting that VP24 may undergo nuclear trafficking. For the first time, we demonstrate that NPI-1 IMPA overexpression leads to the nuclear accumulation of VP24 during EBOV infection. To assess the functional impact of nuclear trafficking, we generated tetracistronic minigenomes encoding VP24 nuclear import and/or export signal mutants. The minigenomes, which also encode Renilla luciferase and viral proteins VP40 and GP, were used to generate transcription and replication competent virus-like particles (trVLPs) that can be used to assess EBOV RNA synthesis, gene expression, entry and viral particle production. With this system, we confirmed that NES or IMPA binding site mutations altered VP24 nuclear localization, demonstrating functional trafficking signals. While these mutations minimally affected transcription and replication, the trVLPs exhibited impaired infectivity and formation of shortened nucleocapsids for the IMPA binding mutant. For the NES mutants, infectivity was reduced approximately 1000-fold. The NES mutant could still suppress IFN signaling but failed to promote nucleocapsid formation. To determine whether VP24 nuclear export is required for infectivity, the residues surrounding the wildtype NES were mutated to alanine or the VP24 NES was replaced with the Protein Kinase A Inhibitor NES. While nuclear export remained intact for these mutants, infectivity was severely impaired. These data demonstrate that VP24 undergoes nuclear trafficking and illuminates a separate and critical role for the NES and surrounding sequences in infectivity and nucleocapsid assembly.
Author Statement
Ebola virus (EBOV) viral protein 24 (VP24) interacts with importin-α (IMPA) nuclear transport proteins to inhibit interferon signaling. VP24 also contains a nuclear export signal (NES). The capacity to bind IMPA and the presence of a NES suggest that VP24 traffics to the nucleus. However, this had not been demonstrated in the context of EBOV replication. In addition, whether these signals influence virus replication beyond effects on interferon responses has been unclear. Here, we demonstrate that VP24 traffics to the nucleus via IMPA during EBOV infection and in the context of an EBOV transcription and replication competent virus-like particle (trVLP) assay. Using the trVLP system, we also confirmed that VP24 possesses a functional nuclear export signal (NES). Mutations disrupting VP24-IMPA interaction or nuclear export reduced or abrogated trVLP infectivity, respectively. VP24 is known to be necessary for viral nucleocapsid maturation. The VP24 IMPA interface mutants yielded shortened nucleocapsids while VP24 NES mutations led to loss of nucleocapsid formation. Interestingly, VP24 mutants with intact NES function but altered sequences near the NES remain incapable of producing nucleocapsids. Together, these data demonstrate VP24 undergoes nuclear trafficking and reveals additional roles for both the IMPA interaction interface and the NES in nucleocapsid assembly.
