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For highly pathogenic viruses, reporter assays that can be rapidly performed are critically needed to identify potentially functional mutations for further study under maximal containment (e.g., biosafety level 4 [BSL-4]). The Ebola virus nucleoprotein (NP) plays multiple essential roles during the viral life cycle, yet few tools exist to study the protein under BSL-2 or equivalent containment. Therefore, we adapted reporter assays to measure NP oligomerization and virion-like particle (VLP) production in live cells and further measured transcription and replication using established minigenome assays. As a proof-of-concept, we examined the NP-R111C substitution, which emerged during the 2013‒2016 Western African Ebola virus disease epidemic and rose to high frequency. NP-R111C slightly increased NP oligomerization and VLP budding but slightly decreased transcription and replication. By contrast, a synthetic charge-reversal mutant, NP-R111E, greatly increased oligomerization but abrogated transcription and replication. These results are intriguing in light of recent structures of NP oligomers, which reveal that the neighboring residue, K110, forms a salt bridge with E349 on adjacent NP molecules. By developing and utilizing multiple reporter assays, we find that the NP-111 position mediates a complex interplay between NP's roles in protein structure, virion budding, and transcription and replication., Competing Interests: The authors declare no conflict of interest.

Mason-Pfizer monkey virus (M-PMV) capsids that have assembled in the cytoplasm must be transported to and associate with the plasma membrane prior to being enveloped by a lipid bilayer during viral release. Structural studies have identified a positive-charge density on the membrane-proximal surface of the matrix (MA) protein component of the Gag polyprotein. To investigate if basic amino acids in MA play a role in intracellular transport and capsid-membrane interactions, mutants were constructed in which lysine and arginine residues (R10, K16, K20, R22, K25, K27, K33, and K39) potentially exposed on the capsid surface were replaced singly and in pairs by alanine. A majority of the charge substitution mutants were released less efficiently than the wild type. Electron microscopy of mutant Gag-expressing cells revealed four distinct phenotypes: K16A and K20A immature capsids accumulated on and budded into intracellular vesicles; R10A, K27A, and R22A capsid transport was arrested at the cellular cortical actin network, while K25A immature capsids were dispersed throughout the cytoplasm and appeared to be defective at an earlier stage of intracellular transport; and the remaining mutant (K33A and K39A) capsids accumulated at the inner surface of the plasma membrane. All mutants that released virions exhibited near-wild-type infectivity in a single-round assay. Thus, basic amino acids in the M-PMV MA define both cellular location and efficiency of virus release. [ABSTRACT FROM AUTHOR]