Correspondence

Vaccinia virus (VACV) has two infectious forms called intracellular mature virus and extracellular enveloped virus (EEV). Two of the seven viral proteins in the EEV outer envelope, A33 and A34, are type II membrane glycoproteins that each interact with another EEV protein called B5; however, evidence for direct A33–A34 interaction is lacking. The localization and stability of A34 is affected by B5 and here data are presented showing that A34 is also affected by A33. In the absence of A33, just as without B5, the level, localization and glycosylation profile of A34 was altered. However, the glycosylation profile of A34 without A33 is different to that observed in the absence of B5, and A34 accumulates in the Golgi apparatus rather than in the endoplasmic reticulum. Thus, A34 requires more than one other EEV protein for its processing and cellular transport. Vaccinia virus (VACV) is the prototypical member of the genus Orthopoxvirus of the family Poxviridae. It replicates in the cytosol and produces multiple types of infectious virions (Smith et al., 2002; Condit et al., 2006; Roberts & Smith, 2008). The first infectious progeny is the intracel-lular mature virus (IMV), which is surrounded by a single lipid envelope (Dales & Siminovitch, 1961; Hollinshead et al., 1999) and remains in the cell until cell lysis. However, some IMV are transported via microtubules to the early endosomes or trans-Golgi network where they are wrapped by two cellular membranes containing several VACV proteins. The resulting intracellular enveloped virus (IEV) is then transported on microtubules to the cell surface where the outer membrane fuses with the plasma membrane to externalise a double-enveloped virus by exocytosis. This virion is called cell-associated enveloped virus (CEV) if it remains on the cell surface, or extracellular enveloped virus (EEV) if it is released from the cell. The CEV/EEV oute