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Accession information: (99)00066-6h.htm (shortcode: fig004cso); 4 May 1999
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Assembly of vaccinia virus in a human cell

Christopher M. Sanderson and Geoffrey L. Smith

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Figure 4. Assembly of vaccinia virus in a human cell. SER = smooth endoplasmic reticulum; RER = rough endoplasmic reticulum. The assembly of vaccinia virus is a complex process that results in the formation of two infectious forms of virus: intracellular mature virus (IMV) and extracellular enveloped virus (EEV). (a) Vaccinia-virus morphogenesis starts in cytoplasmic ‘factories’ with the formation of membrane crescents, which extend to form spherical immature virus (IV) particles. (b) IV particles are non-infectious until they undergo morphological condensation into ‘brick-shaped’ IMV particles. IMV particles recruit the virus A27L protein (p14 kDa) onto their surface. (c) During infection, most IMV particles remain within the cell until they are released by cell lysis. However, some IMV particles become enveloped by membranes derived from either the trans-Golgi network (TGN) or tubular endosomes (d) to form intracellular enveloped virus (IEV) (e). (f) Proteins within the outer membrane of IEV particles induce the polarised, unidirectional polymerisation of actin, which propels the particle towards the plasma membrane and assists the infection of neighbouring cells. (g) During this process, the outer membrane of the IEV particle fuses with the plasma membrane of the cell, exposing an infectious particle on the cell surface and leaving proteins that induce actin polymerisation within the plasma membrane. (h) If these particles remain attached to the cell they are called cell-associated enveloped virus (CEV); (i) however, if they are released from the cell they are called extracellular enveloped virus (EEV). (j) In addition to the classical mechanism of vaccinia-virus assembly described above, EEV particles can also form via direct budding of IMV particles through the plasma membrane. This alternative mechanism of EEV formation might be facilitated by the transport of viral glycoproteins from the TGN to the plasma membrane (k) (fig004cso).

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