The extracellular form of vaccinia virus acquires its outer envelope by wrapping with cytoplasmic membranes that contain at least seven virus-encoded proteins, of which four are glycoproteins. We searched for interactions between the vaccinia virus A33 glycoprotein and proteins A34, A36, B5, F12, and F13. First, when myc epitope-tagged A33 was expressed in combination with other envelope proteins, A33 colocalized with B5 and A36, suggesting that direct A33-B5 and A33-A36 interactions occur in the absence of infection. A recombinant vaccinia virus (vA33Rmyc) was constructed by introduction of the myc-tagged A33 version (A33myc) into A33-deficient vaccinia virus. A33myc partially restored plaque formation and colocalized with enveloped virions in infected cells. Coimmunoprecipitation experiments with extracts of vA33Rmyc-infected cells confirmed the existence of a physical association of A33 with A36 and B5. Of these, the A33-B5 interaction is a novel finding, whereas the interaction between A33 and A36 has been previously characterized. Vaccinia virus is the most studied member of the family Poxviridae, which includes structurally large and genetically complex DNA viruses that replicate and assemble in the cytoplasm of infected cells. Vaccinia virus particles are assembled and released through different stages, including those of the noninfectious immature virus (IV), the intracellular mature virus (IMV), the intracellular enveloped virus (IEV), the cellassociated enveloped virus (CEV), and the extracellular enveloped virus (EEV) (24,36,37). IMV particles are cytosolic and remain intracellular until the cells are lysed. Some IMVs leave the assembly areas in a microtubule-dependent movement and become wrapped by a double layer of intracellular membranes derived from the early endosomes (38, 39) or trans-Golgi network (34) to form IEV. Subsequently, IEVs move to the cell surface (again requiring microtubules), where the outer membrane fuses with the plasma membrane, thus generating enveloped virions, with an extra membrane with respect to IMV, at the cell surface. Particles that remain attached to the plasma membrane are termed CEV, while those released are called EEV (2). In cell culture, enveloped virions (CEV plus EEV) are responsible for virus transmission, since mutants that form normal amounts of IMV but are blocked in virus wrapping are impaired in transmission (1). Specific roles in virus transmission have been assigned to CEV and EEV. CEV particles can induce the formation of actin tails, which are important for efficient cell-to-cell spread. Conversely, EEVs mediate longrange dissemination of virus (2, 6, 28).Seven virus-encoded IEV envelope proteins have been described and characterized. The A33 (29), A34 (7), B5 (9, 16), F13 (14), and A56 (35) proteins are present in IEV, CEV, and EEV, although approximately one-third of EEV particles lack A56 (20). Proteins A36 and F12 are present in IEV but absent from CEV and EEV envelopes (26,39,40,46). The proteins present in the IEV and EEV envelopes have been sho...