The vaccinia virus A38L gene product is a 33-kDa integral membrane glycoprotein

The vaccinia virus gene A38L encodes a highly hydrophobic protein with amino acid similarity to mammalian integrin-associated protein (IAP). In this report we have identified the A38L protein of strain Western Reserve (WR), defined its membrane topology, and analyzed its role in virus production and virulence. An antiserum raised against an A38L peptide identified the A38L gene product as a 33-kDa protein which is expressed at low levels during virus infection. A serum from a rabbit previously infected with WR virus recognized the A38L protein, thus confirming that the A38L gene is expressed in vivo. Using a coupled in vitro-translation/membrane-translocation system the 33-kDa protein was shown to be a membrane-associated and glycosylated form of a 29-kDa polypeptide precursor. The membrane topology of the A38L protein was defined by its glycosylation and protease sensitivity when associated with microsomal membranes. The N-terminal immunoglobulin-like variable domain was protected from exogenous protease and was therefore in the lumen of the vesicle, whereas the C-terminus was sensitive and therefore cytoplasmic. A38L deletion and revertant viruses were constructed and were used to study the involvement of A38L in virus assembly, release, and virulence. Deletion of the A38L gene caused a slight reduction in virus plaque size but did not affect the production of intracellular mature virus or extracellular enveloped virus particles in tissue culture cells nor the virulence of the virus in the murine intranasal model. The A38L protein therefore possesses similar sequence and membrane topology to the mammalian IAP protein but is not required for virus particle production or virulence.