Orthopoxviruses produce two, antigenically distinct, infectious virions, intracellular mature virions and extracellular virions (EV). A33 and B5 are found on EV but not intracellular mature virions. To investigate the function of A33, a recombinant virus that has A33R deleted and expresses B5R-GFP (vB5R-GFP/ΔA33R) was generated. A comparison of vB5R-GFP/ΔA33R to an analogous virus (vΔA33R) revealed an additional defect in infectious EV production that was not apparent when A33R was present. Characterization of these recombinants revealed that EV produced in the absence of A33 had undetectable levels of B5-GFP. Both recombinants released similar amounts of EV but there were differences in their infectivity. Approximately equal numbers of virions produced by these recombinants were able to bind cells even though EV produced by vB5R-GFP/ΔA33R do not contain B5. These results suggest that in the absence of A33, the cytoplasmic tail of B5 contributes to its incorporation into the envelope of progeny virions.
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