Intracellular localization of vaccinia virus extracellular enveloped virusenvelope proteins individually expressed using a Semliki Forest virus replicon

The extracellular enveloped virus (EEV) form of vaccinia virus is bound by an envelope which is acquired by wrapping of intracellular virus particles with cytoplasmic vesicles containing trans-Golgi network markers. Six virus -encoded proteins have been reported as components of the EEV envelope. Of these, four proteins (A33R, A34R, A56R, and B5R) are glycoproteins, one (A3 6R) is a nonglycosylated transmembrane protein, and one (F13L) is a palmity lated peripheral membrane protein. During infection, these proteins localiz e to the Golgi complex, where they are incorporated into infectious virus t hat is then transported and released into the extracellular medium. We have investigated the fates of these proteins after expressing them individuall y in the absence of vaccinia infection, using a Semliki Forest virus expres sion system. Significant amounts of proteins A33R and A56R efficiently reac hed the cell surface, suggesting that they do not contain retention signals for intracellular compartments. In contrast, proteins A34R and F13L were r etained intracellularly but showed distributions different from that of the normal infection. Protein A36R was partially retained intracellularly, dec orating both the Golgi complex and structures associated with actin fibers. A36R aas also transported to the plasma membrane, where it accumulated at the tips of cell projections. Protein B5R was efficiently targeted to the G olgi region. A green fluorescent protein fusion with the last 42 C-terminal amino acids of B5R was sufficient to target the chimeric protein to the Go lgi region. However, B5R-deficient vaccinia virus showed a normal localizat ion pattern for other EEV envelope proteins. These results point to the tra nsmembrane or cytosolic domain of B5R protein as one, but not the only, det erminant of the retention of EEV proteins in the wrapping compartment.