Tobacco mosaic virus, symmetry

A typical virus with helical symmetry is the tobacco mosaic virus (TMV). This is an RNA virus in which the 2130 identical protein subunits (each 158 amino acids in length) are arranged in a helix. In TMV, the helix has 16 1/2 subunits per turn and the overall dimensions of the virus particle are 18 X 300 nm. The lengths of helical viruses are determined by the length of the nucleic acid, but the width of the helical virus particle is determined by the size and packing of the protein subunits. 

Completely different mechanisms are involved in the self-assembly of the tobacco mosaic virus (TMV). This virus consists of single-strand RNA, which is surrounded by 2,130 identical protein units, each of which consists of 158 amino acid residues. A virus particle, which requires the tobacco plant as a host, has a rodlike structure with helical symmetry ( Stanley needles ). It is 300 nm long, with a diameter of 18nm. The protein and RNA fractions can be separated, and the viral... 

The other major type of symmetry found in oligomers, helical symmetry, also occurs in capsids. Tobacco mosaic virus is a right-handed helical filament made up of 2,130 identical subunits (Fig. 4-25b). This cylindrical structure encloses the viral RNA. Proteins with subunits arranged in helical filaments can also form long, fibrous structures such as the actin filaments of muscle (see Fig. 5-30). 

Finding rotation axes for some three-dimensional figures is more difficult, but the same in principle. Remember that nature is not always simple when it comes to symmetry—the protein disk of the tobacco mosaic virus has a 17-fold rotation axis ... 

Viruses with helical symmetry, or "linear" viral capsids, have their genetic material encased in a helix of identical protein subunits, the length of whieh is determined by the length of the encased nucleic acid. There are three main classes of simple helical viruses the rigid rod viruses, the flexuous plant viruses, and the filamentous bacteriophage-.-TTi e most studied of these viruses is the rigid rod Tobacco Mosaic Virus (TMV). 

In most V, the nucleic acid is protected (e.g. from the action of nucleases) by a protein coat. The only exception to this structural feature is provided by the Viroids (see), which lack any proteins of their own. The coat protein consists of many identical subunits, e.g. the coat protein of Tobacco mosaic virus (see) contains 158 amino acid residues of known primary sequence. In the mature vims, between 2,100 and 2,700 of these subunits are arranged like the steps of a spiral staircase, and the spirally wound nueleic aeid lies in a groove in eaeh subunit. A cavity remains in the interior of the particle, which therefore has the appearance of a tube, is rod-shaped and displays helical symmetry (Figs. 1 2). In other V, 2 or 4 subunits combine to form capsomers, which in turn associate... 

The same concepts are present in nature that has evolved examples of natural supramolecular capsules deputed to the transport and/or the storage of important chemical species. Examples are the tobacco mosaic virus characterized by a rodlike appearance and composed of 2130 molecules of coat protein that surround one molecule of genomic RNA 6400 bases long. Another important representative is apoferritin, which is a capsular aggregate with octahedral symmetry formed by 24 identical peptides that present a cavity of over 230 that can store up to 4500 iron atoms as ferric hydrous oxides. 

Fig. 34 Tobacco mosaic virus (TMV) an example of a well-defined nanocompound [S-6 (S-4)213o] consisting of an ss-RNA (core) and protein subunits (shell), with nanoscale dimensions of 18 nm diameter and 300 nm length, and a helical symmetry [195, 206]. Reproduced with...

Viruses consist of nucleic acid molecules (RNA or DNA) encased in a protein coating. Virns capsids (protein shells) can be near spherical or rod-like (helical). Spherical viruses often have an icosahedral structure (a polyhedron with 20 triangular faces, Fig. 6.25a). Within each face, the snbunits of the viral capsid have different symmetries. Many common viruses including rhi-novirus (responsible for the common cold) and herpes simplex virus have icosahedral structures. In contrast, the first virus to be discovered, tobacco mosaic virus, has a helical structure (Fig. 6.25b), leading to a rod-shaped particle 300 nm long and 18 nm in diameter. Here the proteins are wrapped around RNA. 

Viruses represent particularly evolved forms of macro-molecular assemblies. Mature virions are encoded by a protective coat that is formed in part by virally encoded proteins. Viruses come in many shapes and sizes, but they all share the property of using multiple copies of coat proteins to protect their genomic material. In many cases the proteins assemble to form a symmetric shell, where the symmetries are either helical as found in tobacco mosaic virus or icosahedral as seen in the spherical viruses. The use of multiple copies of a protein to form a viral coat is enormously efficient from a genomic point of view however, it introduces several interesting structural problems. 

This virus helical structure is commonly seen in ssRNA virus. It has a capsid with a central cavity or hollow tube that is made by proteins self-assanbled in a circular fashion, creating a disc-like shape. The disc shapes are attached helically creating a tube comprising the nucleic acid in the middle. An example of a virus with a helical symmetry is the tobacco mosaic virus (TMV) (Fig. 15.1A). 

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