Many viruses are composed of complex particles with specific functions associated with different structural elements. Complex bacteriophages, for example, have been a subject of study for decades, and the details of their morphogenesis and low resolution structure have been determined. A variety of viruses contain multiple copies of different subunits in their capsids, and the structural roles of each subunit type are still being determined. In most cases these particles are too large to be analyzed by crystallography, but some have been successfully examined by high resolution electron microscopy. Many viruses of medical importance are enveloped by a membrane that contains functionally important proteins. Usually a quasi-symmetric nucleoprotein particle assembles in the cytoplasm and the membrane and associated proteins are acquired when the particle buds through the plasma membrane of the host cell. In the paramyxoviruses, two proteins in the membrane (neuraminidase and hemagglutinin) have been purified, crystallized and analyzed at high resolution. There are a number of examples in which envelopes have been removed and nucleoprotein cores have been analyzed by cryoelectronmicroscopy and image analysis. Their structures display a variety of T numbers, and they are not significantly different from none-nveloped protein capsids. The largest structure ever determined by crystallography is the core of the nonenveloped, double-stranded RNA bluetongue virus that is delivered to the cytoplasm of infected cells. The particle is over 600 A in diameter, has a T = 13 outer capsid and an inner capsid with T = 1 symmetry formed of 120 copies of the same gene product. The subunit that forms the outer T = 13 shell is shown in Fig. 2.
The continued rapid development of single crystal X-ray diffraction, cryo-electron microscopy and associated image processing techniques virtually guarantees rapid progress in understanding the structure and function of large complex viruses. These complex structures will certainly provide insights for broader biological mechanisms as well.
See also: Virus structure: Atomic structure; Como-viruses (Comoviridae); Human immunodeficiency viruses (Retroviridae): Molecular biology, Anti-retroviral agents, General features; Picornaviruses - insect (Picornaviridae); Poiioviruses (Picorna-viridae): General features, Molecular biology; Sind-bis and Semliki Forest viruses (Togaviridae); Single-stranded RNA phages (Leviviridae).
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