During the assembly of vaccinia virus, the intracellular mature virus becomes enwrapped by a cellular cisterna to form the intracellular enveloped virus (IEV), the precursor of the extracellular enveloped virus (EEV). In this study, we have characterized the origin of this wrapping cisterna by electron microscopic immunocytochemistry using lectins, antibodies against endocytic organelles, and recombinant vaccinia viruses expressing proteins which behave as Golgi resident proteins. No labelling for endocytic marker proteins could be detected on the wrapping membrane. However, the wrapping membrane labelled significantly for a trans Golgi network (TGN) marker protein. The recycling pathway from endosomes to the TGN appears to be greatly increased following vaccinia virus infection, since significant amounts of endocytic fluid-phase tracers were found in the lumen of the TGN, Golgi complex, and the wrapping cisternae. Using immunoelectron microscopy, we localized the vaccinia virus membrane proteins W-p37, W-p42, W-p2l, and W-hemagglutinin (VV-HA) in large amounts in the wrapping cisternae, in the outer membranes of the IEV, and in the outermost membrane of the EEV. The bulk of the cellular W-p37, W-p2l, and W-p42 were in the TGN, whereas W-HA was also found in large amounts on the plasma membrane and in endosomes. Collectively, these data argue that the TGN becomes enriched in vaccinia virus membrane proteins that facilitate the wrapping event responsible for the formation of the IEV.

We identified a protein component of the intracellular mature vaccinia virion membrane that is a target of a potent neutralizing monoclonal antibody, 7D11, obtained from Alan L Schmaljohn. By immunofluorescent and electron microscopic analysis, MAb 7D11 was found to stain intracytoplasmic viral factories, virion membranes in cell sections, and the surface of negatively stained preparations of purified virions. The MAb 7D11 antigen, which is synthesized at late times in infection, has apparent molecular masses of 25 and 29 kDa under nonreducing and reducing conditions, respectively. The membrane antigen was most efficiently extracted from virions by NP40 detergent in combination with a reducing agent; in addition, the protein partitioned exclusively into the detergent phase when extracted with Triton X-114. Although the N-terminus of the immunoaffinity-purified protein was blocked, sequence analysis of trypic peptides revealed that the MAb 7D11 antigen was identical to the myristylated protein encoded by the L1R open reading frame previously described by C.A. Franke, E.M. Wilson, and D.E. Hruby (1990, J. Virol. 64, 5988-5996). Validation of this genetic assignment was provided by the ability of MAb 7D11 to immunoprecipitate a [3H]myristic acid-labeled product of the expected molecular weight from infected cells. In addition, we discovered that the previously described neutralizing monoclonal antibody 2D5 (Y. Ichihashi, T. Takahashi, and M. Oie, 1994, Virology 202, 834-843) also recognizes the L1R protein.

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