Vaccinia virus (VV) produces two antigenically distinct infectious virions, intracellular mature virus (IMV) and extracellular enveloped virus (EEV). Structurally, EEV consists of an IMV with an additional outer membrane containing proteins that are absent from IMV. EEV is important for virus dissemination both in vitro and in vivo. Studies of EEV entry have been hampered by having two infectious virions and by the rupture of the EEV outer membrane in the majority of EEV virions during their purification. To overcome these problems, we have developed a novel approach to study VV entry that is based on confocal microscopy and does not require EEV purification. This assay relies on immunofluorescent staining and detection of individual, intracellular, uncoated virus cores. By this method, we show that EEV entry, in contrast to IMV, is dependent on a low-pH pathway and that the IMV enwrapped inside the EEV exhibits a low-pH fusogenic activity. Together with neutralization data demonstrating that exposure to low pH disrupts the EEV outer membrane, this study strongly supports a model for EEV entry which consists of binding, endocytosis, low-pH-induced disruption of the EEV outer membrane and fusion of the exposed IMV with the endosomal membrane releasing the core into the cytosol. The roles of the EEV outer membrane in virus dissemination and virus entry are discussed in relation to this model.


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