Vaccinia virus utilizes microtubules for movement to the cell surface

Vaccinia virus (VV) egress has been studied using confocal, video, and elec tron microscopy. Previously, intracellular-enveloped virus (IEV) particles were proposed to induce the polymerization of actin tails, which propel IEV particles to the cell surface. However, data presented support an alternat ive model in which microtubules transport virions to the cell surface and a ctin tails form beneath cell-associated enveloped virus (CEV) particles at the cell surface. Thus, VV is unique in using both microtubules and actin f ilaments for egress. The following data support this proposal. (a) Microsco py detected actin tails at the surface but not the center of cells. (b) VV mutants lacking the A33R, A34R, or A36R proteins are unable to induce actin tail formation but produce CEV and extracellular-enveloped virus. (c) CEV formation is inhibited by nocodazole but not cytochalasin D or 4-amino-5-(4 -methylphenyl)-7-(t-butyl)pyrazolo(3,4-d)pyrimidine (PP1). (d) IEV particle s tagged with the enhanced green fluorescent protein fused to the VV B5R pr otein moved inside cells at 60 mum/min. This movement was stop-start, was a long defined pathways, and was inhibited reversibly by nocodazole. This vel ocity was 20-fold greater than VV movement on actin tails and consonant wit h the rate of movement of organelles along microtubules.