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Icosahedral Symmetry

The cubic symmetry found in viruses is based on that of an icosahedron, one of the five classical "Platonic solids" of geometry An ¡cosahedron has 12 vertices (corners), 30 edges, and 20 faces, each an equilateral triangle. It has axes of two-, three-, and fivefold rotalional symmetry, passing through its edges, faces, and vertices, respectively (Fig. 1-3A-C) The icosahedron is the optimum solution to the problem of constructing, from repeating subunils, a strong structure to enclose a maximum volume Before icosahedrons were discovered in viruses the same principles were applied by the architect Buck-minster Fuller to the construction of icosahedral buildings ("geodesic domes"). An object with icosahedral symmetry need not appear angular in outline; the virions of many animal viruses with icosahedral symmetry appear spherical with a bumpy surface.

Only certain arrangements ol the capsomers can fit into the faces, edges, and vertices of the viral icosahedron. The capsomers on the faces and edges of adenovirus particles, for example, bond to six neighboring capsomers and are called hexamers; those at the vertices bond to five neighbors and are called paitamers (Figs. 1-1A and 1-2A). In virions of some viruses both hexamers and pentamers consist of the same polypeptide(s); in those of other viruses they are formed from different polypeptides. The arrangements of capsomers on the capsids of virions of three small icosahedral viruses are shown in Fig.l-3D-F.

High-Resolution Structure

The recent demonstration by X-ray crystallography of the structure of the capsids of several picornaviruses (small RNA viruses), as well as a parvovirus and a papovavirus (small DNA viruses), at near atomic resolution has provided a remarkable insight into their organization and assembly, the location of antigenic sites, and aspects of their attachment and penetration into cells. In several picornaviruses examined, the amino acids of each of the three larger structural proteins are packaged so as to have a wedge-shaped eight-stranded antiparallel (3-barrel domain (Fig. 1-4). The outer contour of the virion depends on the packing of these domains and on the way the loops project from the framework. The capsomers of the parvovirus consist of an unusually large wedge-shaped protein with a fl-barrel core, hence the ability to form a 250 A shell from only 60 subunits of a single protein. High-resolution studies with picornaviruses and polyomavirusos have revealed that the cap-sid proteins have flexible "arms" which interlock with arms of an adjacent structural unit to mediate assembly and stability of the virion. Cations may also stabilize the interface between subunits, and arms extending from internal proteins may interact with proteins of the outei capsid. In virions of telramer), each of which consists of fin interna! domain, n hvduiphobit transmembrane domain, and a hydrophilic external domain Some 50 molecules of .1 small membrane-associated protein, M2 (not slioivn), form a small number of "pores" in the lipid bilayrr |/\, By John Mack, from R M Burnett, 111 "Biological Macromolerulrs and Assemblies Virus Structures" (F Jurnak and A McPhrrsun, eds ), Vol I, p 337 Wiley, New York, l<)84, B, 1'iom t I' I" Mattem, m "MoIlxu-lar Biology of Animal Viruses" (D P Nayak, ed ), p 5 Dekkei, New York, 1977 |

Chapter 1 Structure and Composition of Viruses loG molecule

Chapter 1 Structure and Composition of Viruses loG molecule

canyon containing llgand

Fig. 1-5 Model of the interaction between receptor an host cells and ligand on a rhinovuus I ho ligands are silunted within surface depressions ("canyons") near axes of fivefold symmetry, j location which serves to prevent access of antibody to those crucial sites However. anybodies specific for antigenic sites on the rim of the canyon can block vinon-cell interaction by steric fundi nnce [Modified from M. G Rossmann and R R Rueckerl, Microbiol Set 4, 206 (1987).]

canyon containing llgand families (Fig 1-2C,D); matrix protein provides added rigidity to the virion. For example, the envelope of rhabdoviruses with its projecting peplomers is closely applied to a layer of matrix protein which in turn interfaces with a helical nucleocapsid within Some enveloped viruses, including arenaviruses, bunyaviruses, and coronaviruses, have no matrix protein.

Envelopes are not restricted to viruses ol helical symmetry; icosahedral viruses belonging to several families (herpesviruses, togaviruses, flaviviruses, and retroviruses) have envelopes. The infectivity of most enveloped viruses depends on the integrity of the envelope, but some poxviruses have an envelope which is not necessary for infectivity.

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