The exit of vaccinia virus from infected cells.

Vaccinia virus (VACV) is the prototypic member of the Poxviridae a group of large DNA viruses that replicate in the cell cytoplasm. The entry and exit of VACV are complicated by the existence of two distinct forms of virus, intracellular mature virus (IMV) and extracellular enveloped virus (EEV), that are surrounded by different numbers of lipid membranes and have different surface proteins. Here the mechanisms used by these different forms of VACV to leave the infected cell are reviewed. Whereas some enveloped viruses complete virus assembly by budding through the plasma membrane, infectious poxvirus particles (IMV) are produced within the cytoplasm. These particles are either further enveloped by intracellular membranes to form intracellular enveloped virus (IEV) that are transported to the cell surface on microtubules and exposed on the cell surface by exocytosis, or are released after cell lysis. If the enveloped virion remains attached to the cell surface it is called cell-associated enveloped virus (CEV) and is propelled into surrounding cells by growing actin tails beneath the plasma membrane. Alternatively, the surface virion may be released as EEV.