Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 204

Particles of poxviruses such as vaccinia virus also comprise multiple, distinct structural elements, but none of these exhibit obvious icosahedral or helical symmetry. A second distinctive feature is that two forms of infectious particles are produced in vaccinia virus-infected cells (see Chapter 13), termed mature virions and enveloped extracellular virions, which differ in the number and origin of membranes. Mature virions are large, enveloped structures (~350 to 370 × 250 × 270 nm) comprising at least 75 proteins that appear in the electron microscope as brick or barrel shaped (depending on the orientation) (Fig. 4.29A). A number of internal structures have been observed by examination of thin sections through purified particles or by cryo-electron tomography (Fig. 4.29B). These features include the core wall, which surrounds the central core that contains the ~200-kbp DNA genome, and lateral bodies. Remarkably, the core contains some 20 enzymes with many different activities. Although viral proteins that contribute to these various structures have been identified, our understanding of vaccinia virus architecture remains at low resolution.

Figure 4.28 Features of mimivirus capsids. (A) Cryo-EM reconstruction of Cafeteria roenbergensis mimivirus at 21-Å resolution. Although not decorated with fibers, the surface of this capsid is characterized by high protrusions formed by surface loops of the double β-barrel jelly roll major capsid protein. These pseudohexagonal structure units are organized in discrete arrays, termed pentasymmetrons (purple) and trisymmetrons (blue, red, green, cyan, and orange), which form the vertices and interact at the threefold axes of symmetry, respectively. One of the 30 edges of the icosahedral particle is indicated in cyan, and the position of five-, three-, and twofold axes of symmetry by red symbols. Adapted from Xiao C et al. 2017. Sci Rep 7:5484, under license CC BY 4.0. Courtesy of C. Xiao, University of Texas at El Paso. (B) Cryo-EM reconstruction of Acanthamoeba polyphaga mimivirus following sequential digestion with lysozyme and the protease bromelain. This treatment was applied to remove (or reduce) the dense array of surface fibers (Fig. 4.1), which increase ice thickness around the particles and hence signal-to-noise because of random scattering of electrons. The reconstruction (61 Å), which is based on only fivefold averaging, is viewed down the stargate (blue). The size of this assembly with extensions of the arms some 200 Å almost to neighboring vertices is clearly evident. Adapted from Xiao C et al. 2009. PLoS Biol 7:e92 under license CC BY 4.0. © 2009 Xiao et al. Courtesy of M. Rossman, Purdue University.