Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 202

Characteristic features of members of the Mimiviridae, which infect single-cell eukaryotes, are their very large, double-stranded DNA genomes and correspondingly huge particles. Initial examinations of these viruses by electron microscopy established that they comprise multiple layers, include a lipid membrane within an external capsid, and in some cases include a dense layer of surface fibers (Fig. 4.1). Despite their large size, mimivirus particles exhibit some familiar structural features, notably icosahedral symmetry and a capsid built from a major capsid protein with the double β-barrel jelly roll topology. To date, cryo-EM reconstructions of these viruses have achieved only a relatively low resolution, because of the need for much computational power (e.g., ~3 × 10⁶ CPU hours for a 21-Å view of Cafeteria roenbergensis virus) and/or removal of dense surface fibers (as for a 65-Å reconstruction of Acanthamoeba polyphaga mimivirus). Such studies have revealed the icosahedral organization of major capsid proteins (Fig. 4.28) and in some cases their arrangement into assemblies that interact around the five- or threefold axes of symmetry. The most unique feature, first observed in Acanthamoeba polyphaga mimivirus, is the presence of a star-shaped structure at one vertex (Fig 4.28B). This stargate, which allows release of internal contents of virus particles into the cytoplasm of infected cells, is an exceptionally large vertex structure, with arms extending almost to neighboring vertices. As yet, little is known about how such large capsids (up to ~5,000 Å in diameter) are stabilized, or the organization of their internal components.

Figure 4.27 Structural features of herpesvirus particles. (A) Two slices through a cryo-electron tomogram of a single herpes simplex virus type 1 particle, showing the eccentric tegument cap (arrowheads). Reprinted from Grunewald K et al. 2003. Science 302:1396–1398, with permission. (B) Diagram of the structure of the herpesviral capsid based on high resolution (3.5 Å or better) cryo-EM structures of herpes simplex virus 1 and 2 and illustrated within a virus particle. Shown below is an enlarged view down a 5-fold axis of icosahedral symmetry. (C) The single portal of herpes simplex virus type 1 nucleocapsids visualized by staining with an antibody specific for the viral UL6 protein conjugated to gold beads is shown to the left. The gold beads are electron dense and appear as dark spots in the electron micrograph. They are present at a single vertex in each nucleocapsid, which therefore contains one portal. A 16-Å reconstruction of the UL6 protein portal based on cryo-EM is shown on the right. Adapted from Trus BL et al. 2004. J Virol 78:12668–12671, with permission. (D) Central slice through a cryo-electron tomographic reconstruction based on symmetryfree averaging is shown radially colored as indicated. Portal vertex-associated density is shown in cyan and purple. Adapted from Schmid MF et al. 2012. PLoS Pathog 8:e1002961, under license CC BY 4.0. © Schmid et al. Courtesy of A.C. Steven, National Institutes of Health (A and C), W. Chiu, Baylor College of Medicine (B), and F.J. Rixon, MRC-University of Glasgow Center for Virus Research, Glasgow, United Kingdom (D).