The vaccinia virus A4OR gene product is a nonstructural, type II membrane glycoprotein that is expressed at the cell surface.

Gene A40R from vaccinia virus (VV) strain Western Reserve has been characterized. The open reading frame (ORF) was predicted to encode a 159 amino acid, 18152 Da protein with amino acid similarity to C-type animal lectins and to the VV A34R protein, a component of extracellular enveloped virus (EEV). Northern blotting and S1 nuclease mapping showed that gene A40R is transcribed early during infection from a position 12 nucleotides upstream of the ORF, producing a transcript of approximately 600 nucleotides. Rabbit anti-sera were raised against bacterial fusion proteins containing parts of the A40R protein. These were used to identify an 18 kDa primary translation product and N- and O-glycosylated forms of 28, 35 and 38 kDa. The A40R proteins were detected early during infection, formed higher molecular mass complexes under non-reducing conditions and were present on the cell surface but absent from virions. The proteins partitioned with integral membrane proteins in Triton X-114. Canine pancreatic microsomal membranes protected in vitro-translated A40R from proteinase K digestion, suggesting the A40R protein has type II membrane topology. A mutant virus with the A40R gene disrupted after amino acid 50, so as to remove the entire lectin-like domain, and a revertant virus were constructed. Disruption of the A40R gene did not affect virus plaque size, in vitro growth rate and titre, EEV formation, or virus virulence in a murine intranasal model.