Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 205

Pithovirus particles are the largest described to date, indeed are visible in the light microscope. They are ovoid or amphora-like in shape and variable in length (most commonly 1.35 to 1.65 nm). The internal nucleoid, which contains the double-stranded DNA genome of some 0.6 Mbp, is enclosed within what is thought to be a lipid membrane, in turn encased in a thick protein layer (the tegument). The apex of pithovirus is closed by a protruding “cork” with a hexagonal, grid-like appearance (Fig. 4.29C and D). Following uptake of virus particles into host cells by phagocytosis, this unusual cork structure is expelled to allow fusion of the viral nucleoid membrane with that of the cellular vacuole. Unprecedented assemblies specialized for release of the viral genome in host cells may prove to be a characteristic property of the very large viruses.

Figure 4.29 Virus particles with alternative architectures. Structural features of the poxvirus vaccinia virus. (A) Surface rendering of intracellular mature particles of vaccinia virus reconstructed from cryo-electron tomograms showing the brick shape and irregular protrusions from the surface. (B) Translucent visualization of the reconstructed particle volume showing the dumbbell-shaped core and external membrane. A and B reprinted from Cyrklaff M et al. 2005. Proc Natl Acad Sci U S A 102:2772–2777, with permission. Courtesy of J.L. Carrascosa, Universida Autonoma de Madrid. The virus Pithovirus sibericum was isolated following culture of a suspension of soil from a sample of permafrost collected in 2000 in Siberia. (C) Morphology of this virus particle, viewed by energy-filtered cryo-EM to facilitate visualization of thick, ice-embedded particles. The internal nucleoid, which appears rather featureless, is likely covered by a membrane, a thick protein layer termed the tegument, and a low-density outer layer. (D) The apical cork, which is made up of vertical fibers and hence appears striated, resides at one end of the particle. C and D reprinted from Okamoto K et al. 2017. Sci Rep 7:13291, under license CC BY 4.0. Courtesy of K. Okamoto, Uppsala University, Sweden.