AFRICAN SWINE FEVER VIRUS IS WRAPPED BY THE ENDOPLASMIC-RETICULUM

African swine fever (ASF) virus is a large DNA virus that shares the s triking icosahedral symmetry of iridoviruses and the genomic organizat ion of poxviruses. Both groups of viruses have a complex envelope stru cture. In this study, the mechanism of formation of the inner envelope of ASP virus was investigated. Examination of thin cryosections by el ectron microscopy showed two internal membranes in mature intracellula r virions and all structural intermediates, These membranes were in co ntinuity with intracellular membrane compartments, suggesting that the virus gained two membranes from intracellular membrane cisternae. Imm unogold electron microscopy showed the viral structural protein p17 an d resident membrane proteins of the endoplasmic reticulum (ER) within virus assembly sites, virus assembly intermediates, and mature virions . Resident ER proteins were also detected by Western blotting of isola ted virions. The data suggested the ASF virus was wrapped by the ER. A nalysis of the published sequence of ASF virus (R. J. Yanez et al., Vi rology 208:249-278, 1995) revealed a reading frame, XP124L, that encod ed a protein predicted to translocate into the lumen of the ER, Pulse- chase immunoprecipitation and glycosylation analysis of pXP124L, the p roduct of the XP124L gene, showed that pXP124L was retained in the ER lumen after synthesis. When analyzed by immunogold electron microscopy , pXP124L localized to virus assembly intermediates and fully assemble d virions. Western blot analysis detected pXP124L in virions isolated from Percoll gradients. The packaging of pXP124L from the lumen of the ER into the virion is consistent with ASF virus being wrapped by ER c isternae: a mechanism which explains the presence of two membranes in the viral envelope.