Membrane remodelling during vaccinia virus morphogenesis

Abstract

Background information. VACV (vaccinia virus) is one of the most complex viruses, with a size exceeding 300 nm and more than 100 structural proteins. Its assembly involves sequential interactions and important rearrangements of its structural components.Results. We have used electron tomography of sections of VACV-infected cells to follow, in three dimensions, the remodelling of the membrane components of the virus during envelope maturation. The tomograms obtained suggest that a number of independent 'crescents' interact with each other to enclose the volume of an incomplete ellipsoid in the viral factory area, attaining the overall shape and size characteristic of the first immature form of the virus [IV (immature virus)]. The incorporation of the DNA into these forms leads to particles with a nucleoid [IVN (IV with nucleoid)] that results in local disorganization of the envelope in regions near the condensed DNA. These particles suffer the progressive disappearance of the membrane outer spikes with a change in the shape of the membrane, becoming locally curled. The transformation of the IVN into the mature virus involves an extreme rearrangement of the particle envelope, which becomes fragmented and undulated. During this process, we also observed connections between the outer membranes with internal ones, suggesting that the latter originate from internalization of the IV envelope.Conclusions. The main features observed for VACV membrane maturation during morphogenesis resemble the breakdown and reassembly of cellular endomembranes.

Fine structure of the vaccinia virion determined by controlled degradation and immunolocalization

“…The first change in volume of the viral particle occurs during virion morphogenesis during the IV to MV transition. In this stage, the spherical IV particles are transformed into the brick/barrel shape of mature virions and their dimensions are significantly be reduced (Chichon et al, 2009; Condit et al, 2006). It is possible that during this contraction step, water would be excluded from the internal milieu of the cores, and consequently the internal structures of the virion would contract including nucleocapsid that would condense forming the tubular-like structure previously described (Condit et al, 2006; Heuser, 2005; McFadden et al, 2012; Peters and Muller, 1963).…”

Poxvirus membrane biogenesis

“…However, careful measurements of the thickness of the IV membrane (Hollinshead et al, 1999) and freeze-fracture (Heuser, 2005) fully support the prior single lipid membrane model and deep-etch and immunoelectron microscopy demonstrate that the “spicule layer” is a honeycomb lattice comprised of VACV D13 protein trimers external to the single membrane (Heuser, 2005; Szajner et al, 2005). The controversy ended when proponents of the double-membrane and others reported electron tomography images confirming a single membrane (Chichon et al, 2009; Chlanda et al, 2009). …”

Viral Miniproteins

“…Poxviruses replicate in the cytoplasm, where they generate a variety of membrane structures (19) (Figure 6). For vaccinia virus, the prototype mammalian poxvirus, numerous mutants interfere with the biogenesis or function of viral membranes.…”