African swine fever virus structural protein pE120R is essential for virustransport from assembly sites to plasma membrane but not for infectivity

This report examines the role of African swine fever virus (ASFV) structura l protein pE120R in virus replication. Immunoelectron microscopy revealed t hat protein pE120R localizes at the surface of the intracellular virions. C onsistent with this, coimmunoprecipitation assays showed that protein pE120 R binds to the major capsid protein p72. Moreover, it was found that, in ce lls infected with an ASFV recombinant that inducibly expresses protein p72, the incorporation of pE120R into the virus particle is dependent on p72 ex pression. Protein pE120R was also studied using an ASFV recombinant in whic h E120R gene expression is regulated by the Escherichia coli lac repressor- operator system. In the absence of inducer, pE120R expression was reduced a bout 100-fold compared to that obtained with the parental virus or the reco mbinant virus grown under permissive conditions. One-step virus growth curv es showed that, under conditions that repress pE120R expression, the titer of intracellular progeny was similar to the total virus yield obtained unde r permissive conditions, whereas the extracellular virus yield was about 10 0-fold lower than in control infections. Immunofluorescence and electron mi croscopy demonstrated that, under restrictive conditions, intracellular mat ure virions are properly assembled but remain confined to the replication a reas. Altogether, these results indicate that pE120R is necessary for virus dissemination but not for virus infectivity. The data also suggest that pr otein pE120R might be involved in the microtubule-mediated transport of ASF V particles from the viral factories to the plasma membrane.