VACCINIA VIRUS A17L OPEN READING FRAME ENCODES AN ESSENTIAL COMPONENTOF NASCENT VIRAL MEMBRANES THAT IS REQUIRED TO INITIATE MORPHOGENESIS

We generated an antiserum to the predicted C-terminal peptide of the A 17L open reading frame (ORF), which encodes a 23-kDa polypeptide with hydrophobic regions characteristic of membrane proteins. Immuno-electr on microscopy of infected cells indicated that the A17L protein is int imately associated with the earliest characteristic viral membranes, e ven those formed in the presence of the drug rifampin. To study the ro le of the A17L protein in morphogenesis, we constructed recombinant va ccinia viruses in which the endogenous A17L ORF was deleted and a copy of the ORF under the control of the bacteriophage T7 RNA polymerase a nd the Escherichia call lac repressor was inserted into an alternative site in the vaccinia virus genome. Growth of these recombinant viruse s was entirely dependent on the induction of A17L expression by isopro pyl-beta-D-thiogalactopyranoside. Electron microscopic examination of cells infected in the absence of inducer revealed the accumulation of large, well-demarcated electron-dense aggregates but no characteristic membrane-associated viral structures. Viral late protein synthesis oc curred under these conditions, although the maturational proteolytic p rocessing of structural proteins was inhibited. We conclude that the p roduct of the A17L gene is an essential component of the immature vira l membrane and has an early function in viral morphogenesis.