Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 175

The nucleocapsids of some enveloped animal viruses, as well as certain plant viruses and bacteriophages, are rod-like or filamentous structures with helical symmetry. Helical symmetry is described by the number of structural units per turn of the helix, the axial rise per unit, and the pitch of the helix (Fig. 4.6A). A characteristic feature of a helical structure is that any volume can be enclosed simply by varying the length of the helix. Such a structure is said to be open. In contrast, capsids with icosahedral symmetry (described below) are closed structures with fixed internal volume.

From a structural point of view, the best-understood helical nucleocapsid is that of tobacco mosaic virus, the very first virus to be identified. The virus particle comprises a single molecule of (+) strand RNA, about 6.4 kb in length, enclosed within a helical protein coat (Fig. 4.6B; see also Fig. 1.9). The coat is built from a single protein with an extended shape. Repetitive interactions among coat protein subunits form disks, which in turn assemble as a long, rod-like, right-handed helix. In the interior of the helix, each coat protein molecule binds three nucleotides of the RNA genome. The coat protein subunits therefore engage in identical interactions with one another and with the genome, allowing the construction of a large, stable structure from multiple copies of a single protein.

The particles of several families of animal viruses with (−) strand RNA genomes, including filoviruses, paramyxoviruses, rhabdoviruses, and orthomyxoviruses, contain internal structures with helical symmetry that are encased within an envelope. In all cases, these structures contain an RNA molecule, many copies of an RNA-packaging protein (designated NP or N), and the viral RNA polymerase and associated enzymes responsible for synthesis of mRNA and viral genomes. Despite common helical symmetry and similar composition, the internal components of these (−) strand RNA viruses exhibit considerable diversity in morphology and organization. For example, the nucleocapsids of the filovirus Zaire ebolavirus and the paramyxovirus Sendai virus are long, filamentous structures in which the NP proteins, like the tobacco mosaic virus coat protein, make regular interactions with the RNA genome. In contrast, the nucleocapsids of rhabdoviruses such as vesicular stomatitis virus are bullet shaped (Fig. 4.6C). Furthermore, an additional viral protein is essential to maintain their organization: vesicular stomatitis virus nucleocapsids released from within the envelope retain the dimensions and morphology observed in intact particles but become highly extended and filamentous once the matrix (M) protein is also removed (Fig. 12.21). X-ray crystallography of a ring-like N protein-RNA complex containing 10 molecules of the protein bound to RNA has revealed that each N protein molecule binds to 9 nucleotides of the RNA that is largely sequestered within cavities formed by the protein (Fig. 4.7). Furthermore, each N subunit makes extensive and regular contacts with neighboring N molecules, as predicted from first principles by Crick and Watson.