ASSEMBLY OF AFRICAN SWINE FEVER VIRUS - ROLE OF POLYPROTEIN PP220

Polyprotein processing is a common strategy of gene expression in many positive-strand RNA viruses and retroviruses but not in DNA viruses, African swine fever virus (ASFV) is an exception because it encodes a polyprotein, named pp220, to produce several major components of the v irus particle, proteins p150, p37, p34, and p14. Tn this study, we ana lyzed the assembly pathway of ASFV and the contribution of the polypro tein products to the virus structure, Electron microscopic studies rev ealed that virions assemble from membranous structures present in the viral factories. Viral membranes became polyhedral immature virions af ter capsid formation on their convex surface. Beneath the lipid envelo pe, two distinct domains appeared to assemble consecutively: first a t hick protein layer that we refer to as core shell and then an electron -dense nucleoid, which was identified as the DNA-containing domain, Im munofluorescence studies showed that polyprotein pp220 is localized in the viral factories. At the electron microscopic level, antibodies to pp220 labeled all identifiable forms of the virus from the precursor viral membranes onward, thus indicating an early role of the polyprote in pp220 in ASFV assembly. The subviral localization of the polyprotei n products, examined on purified virions, was found to be the core she ll, In addition, quantitative studies showed that the polyprotein prod ucts are present in equimolar amounts in the virus particle and accoun t for about one-fourth of its total protein content. Taken together, t hese results suggest that polyprotein pp220 may function as an interna l protein scaffold which would mediate the interaction between the nuc leoid and the outer layers similarly to the matrix proteins of other v iruses.