African Swine Fever Virus Protein pE199L Mediates Virus Entry by Enabling Membrane Fusion and Core Penetration

African swine fever virus (ASFV) is a complex nucleocytoplasmic large DNA virus (NCLDV) causing a lethal hemorrhagic disease that currently threatens the global pig industry. Despite its relevance in the infectious cycle, very little is known about the internalization of ASFV in the host cell. Here, we report the characterization of ASFV protein pE199L, a cysteine-rich structural polypeptide with similarity to proteins A16, G9, and J5 of the entry fusion complex (EFC) of poxviruses. Using biochemical and immunomicroscopic approaches, we found that, like the corresponding poxviral proteins, pE199L localizes to the inner viral envelope and behaves as an integral transmembrane polypeptide with cytosolic intramolecular disulfide bonds. Using an ASFV recombinant that inducibly expresses the E199L gene, we found that protein pE199L is not required for virus assembly and egress or for virus-cell binding and endocytosis but is required for membrane fusion and core penetration. Interestingly, similar results have been previously reported for ASFV protein pE248R, an inner membrane virion component related to the poxviral L1 and F9 EFC proteins. Taken together, these findings indicate that ASFV entry relies on a form of fusion machinery comprising proteins pE248R and pE199L that displays some similarities to the unconventional fusion apparatus of poxviruses. Also, these results provide novel targets for the development of strategies that block the first stages of ASFV replication.