The vaccinia virus B5 protein requires A34 for efficient intracellular trafficking from the endoplasmic reticulum to the site of wrapping and incorporation into progeny virions.

The glycoproteins encoded by the vaccinia virus A34R and B5R genes are involved in intracellular envelope virus formation and are highly conserved among orthopoxviruses. A recombinant virus that has the A34R gene deleted and the B5R gene replaced with a B5R gene fused to the enhanced green fluorescent protein (B5R-GFP) gene was created (vB5R-GFP/DeltaA34R) to investigate the role of A34 during virion morphogenesis. Cells infected with vB5R-GFP/DeltaA34R displayed GFP fluorescence throughout the cytoplasm, which differed markedly from that seen in cells infected with a normal B5R-GFP-expressing virus (vB5R-GFP). Immunofluorescence and subcellular fractionation demonstrated that B5-GFP localizes with the endoplasmic reticulum in the absence of A34. Expression of either full-length A34 or a construct consisting of the lumenal and transmembrane domains restored normal trafficking of B5-GFP to the site of wrapping in the juxtanuclear region. Coimmunoprecipitation studies confirmed that B5 and A34 interact through their luminal domains, and further analysis revealed that in the absence of A34, B5 is not efficiently incorporated into virions released from the cell.