AFRICAN SWINE FEVER VIRUS IS ENVELOPED BY A 2-MEMBRANED COLLAPSED CISTERNA DERIVED FROM THE ENDOPLASMIC-RETICULUM

During the cytoplasmic maturation of African swine fever virus (ASFV) within the,viral factories, the DNA-containing core becomes wrapped by two shells, an inner lipid envelope and an outer icosahedral capsid, We have previously shown that the inner envelope is derived from precu rsor membrane-like structures on which the capsid layer is progressive ly assembled. In the present work we analyzed the origin of these vira l membranes and the mechanism of envelopment of ASFV. Electron microsc opy studies on permeabilized infected cells revealed the presence of t wo tightly apposed membranes within the precursor membranous structure s as well as polyhedral assembling particles. Both membranes could he detached after digestion of intracellular virions with proteinase K, I mportantly, membrane loop structures were observed at the ends of open intermediates, which suggests that the inner envelope is derived from a membrane cisterna, Ultraestructural and immunocytochemical analyses showed a close association and even direct continuities between the e ndoplasmic reticulum (ER) and assembling virus particles at the border ing areas of the viral factories. Such interactions become evident wit h an ASFV recombinant that inducibly expresses the major capsid protei n p72, In the absence of the inducer, viral morphogenesis was arrested at a stage at which partially and fully collapsed ER cisternae enwrap ped the core material. Together, these results indicate that ASFV, lik e the poxviruses, becomes engulfed by a two-membraned collapsed cister na derived from the ER.