The vaccinia virus A9L gene encodes a membrane protein required for an early step in virion morphogenesis

The A9L open reading frame of vaccinia virus was predicted to encode a memb rane-associated protein. A transcriptional analysis of the A9L gene indicat ed that it was expressed at late times in vaccinia virus-infected cells. La te expression, as well as virion membrane association, was demonstrated by the construction and use of a recombinant vaccinia virus encoding an A9L pr otein with a C-terminal epitope tag. Immunoelectron microscopy revealed tha t the A9L protein was associated with both immature and mature virus partic les and was oriented in the membrane with its C terminus exposed on the vir ion surface. To determine whether the A9L protein functions in viral assemb ly or infectivity, we made a conditional-lethal inducible recombinant vacci nia virus. In the absence of inducer, A9L expression and virus replication were undetectable. Under nonpermissive conditions, viral late protein synth esis occurred, but maturational proteolytic processing was inhibited, and t here was an accumulation of membrane-coated electron-dense bodies, crescent s, and immature virus particles, many of which appeared abnormal. We conclu ded that the product of the A9L gene is a viral membrane-associated protein and functions at an early stage in virion morphogenesis.