Movements of vaccinia virus intracellular enveloped virions with GFP tagged to the F13L envelope protein

Vaccinia virus produces several forms of infectious virions. Intracellular mature virions (IMV) assemble in areas close to the cell nucleus. Some IMV acquire an envelope from intracellular membranes derived from the trans-Golgi network, producing enveloped forms found in the cytosol (intracellular enveloped virus; IEV), on the cell surface (cell-associated enveloped virus) or free in the medium (extracellular enveloped virus; EEV). Blockage of IMV envelopment inhibits transport of virions to the cell surface, indicating that enveloped virus forms are required for virion movement from the Golgi area. To date, the induction of actin tails that propel IEV is the only well-characterized mechanism for enveloped virus transport. However, enveloped virus transport and release occur under conditions where actin tails are not formed. In order to study these events, recombinant vaccinia viruses were constructed with GFP fused to the most abundant protein in the EEV envelope, P37 (F13L). The P37–GFP fusion, like normal P37, accumulated in the Golgi area and was incorporated efficiently into enveloped virions. These recombinants allowed the monitoring of enveloped virus movements in vivo. In addition to a variety of relatively slow movements (<0·4 μm/s), faster, saltatory movements both towards and away from the Golgi area were observed. These movements were different from those dependent on actin tails and were inhibited by the microtubule-disrupting drug nocodazole, but not by the myosin inhibitor 2,3-butanedione monoxime. Video microscopy (5 frames per s) revealed that saltatory movements had speeds of up to, and occasionally more than, 3 μm/s. These results suggest that a second, microtubule-dependent mechanism exists for intracellular transport of enveloped vaccinia virions.