Atomic Structure of T3 Viruses

Because the simple viruses are regular structures, they will often crystallize, and such crystals may be suitable for X-ray diffraction, Several viruses formed using icosa-hedral symmetry principles have now been solved to atomic resolution, Among T=3 viruses, the structures of

Virus And Atom Photos

FIGURE 2.5 Gallery of three-dimensional reconstructions of icosahedral viruses from cryoelectron micrographs. All virus structures are surface shaded and are viewed along a twofold axis of symmetry. All of the images are of intact virus particles except for the herpes simplex structure, which is of the nucleocapsid of the virus. [Most of the images are taken from Baker et al. (1999), except the images of Ross River virus and of dengue virus, which were kindly provided by Drs. R. J. Kuhn and T. S. Baker.]

FIGURE 2.5 Gallery of three-dimensional reconstructions of icosahedral viruses from cryoelectron micrographs. All virus structures are surface shaded and are viewed along a twofold axis of symmetry. All of the images are of intact virus particles except for the herpes simplex structure, which is of the nucleocapsid of the virus. [Most of the images are taken from Baker et al. (1999), except the images of Ross River virus and of dengue virus, which were kindly provided by Drs. R. J. Kuhn and T. S. Baker.]

several plant viruses, including tomato bushy stunt virus (TBSV), turnip crinkle virus (TCV), and Southern bean mosaic virus (SBMV), which represent more than one family, have been solved. All three of these viruses have capsid proteins possessing the eight-stranded antiparallel P sandwich. T=3 means that 180 identical molecules of capsid protein are utilized to construct the shell. The structures of two insect viruses have also been solved. The T=3 capsid of the insect nodavirus, flock house virus, is illustrated in Fig. 2.6.

The 180 subunits in these T=3 structures interact with one another in one of two different ways, such that the

Nodavirus Structure

FIGURE 2.6 Diagrammatic representation of a T = 3 virus, flock house virus. The positions of the three identical proteins that make up a triangular face are only quasi-equivalent. The angle between the A and B5 units (shown with an red oval and in diagram C) is more acute than that along the C-C2 edge, shown with a blue oval, and diagram B. This difference in the angles is due to the presence of an RNA molecule located under the C-C2 edge. [From Johnson (1996, Fig. 4).]

FIGURE 2.6 Diagrammatic representation of a T = 3 virus, flock house virus. The positions of the three identical proteins that make up a triangular face are only quasi-equivalent. The angle between the A and B5 units (shown with an red oval and in diagram C) is more acute than that along the C-C2 edge, shown with a blue oval, and diagram B. This difference in the angles is due to the presence of an RNA molecule located under the C-C2 edge. [From Johnson (1996, Fig. 4).]

protein shell can be thought of as being composed of an assembly of 60 AB dimers and 30 CC dimers (Fig. 2.6A). The bond angle between the two subunits of the dimer is more acute in the AB dimers than in the CC dimers (Figs. 2.6B and C). For the plant viruses, there are N-terminal and C-terminal extensions from the capsid proteins that are involved in interactions between the subunits and with the RNA. The N-terminal extensions have a positively charged, disordered domain for interacting with and neutralizing the charge on the RNA and a connecting arm that interacts with other subunits. In the case of the CC dimers, the connecting arms interdigitate with two others around the icosahedral threefold axis to form an interconnected internal framework. In the case of the AB conformational dimer, the arms are disordered, allowing sharper curvature. For flock house virus, the RNA plays a role in controlling the curvature of the CC dimers, as illustrated in Fig. 2.6.

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