Viruses helical symmetry

Helical symmetry was thought at one time to exist only in plant viruses. It is now known, however, to occur in a number of animal virus particles. The influenza and mumps viruses, for example, which were first seen in early electron micrographs as roughly spherical particles, have now been observed as enveloped particles within the envelope, the capsids themselves are helically symmetrical and appear similar to the rods of TMV, except that they are more flexible and are wound like coils of rope in the centre of the particle. 

Virus symmetry The nucleocapsids of viruses are constructed in highly symmetrical ways. Symmetry refers to the way in which the protein morphological units are arranged in the virus shell. When a symmetrical structure is rotated around an axis, the same form is seen again after a certain number of degrees of rotation. Two kinds of symmetry are recognized in viruses which correspond to the two primary shapes, rod and spherical. Rod-shaped viruses have helical symmetry and spherical viruses have icosahedral 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). 

At the present time, there is no accepted chelating agent which can be used against common influenza viruses in humans. A virus has a core of either DNA or RNA and a protective coat of many identical protein units. All viruses are either rods or spheres, that is the protein coats are cylindrical shells having helical symmetry or spherical shells having icosahedral symmetry. Viruses reproduce inside living cells, where each viral nucleic acid directs the synthesis of about 1000 fresh viruses. These are then released and the host cell may die. 

Helical symmetry The polymeric proteins of filamentous viruses and the cytoskel-ton possess helical symmetry, in which subunits are related by a translation, as well as a rotational component. Actin, myosin, tubulin and various other fibrous proteins all interact with helical symmetry, which is often called screw symmetry. Screw symmetry, which relates the positions of adjacent subunits, combines a translation along the helix axis with the rotation. Actin forms a two-stranded helix of globular actin subunits. However, important variations in the helix parameters occur (Egehnan et al, 1982). The rise per subunit is relatively constant, but the twist or relative rotation around the helix axis is highly variable. This polymorphic tendency is probably important for the smooth functioning of muscle contraction, which involves considerable force generation. 

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... 

Using totally abiotic dendron subunits, Percec et al. [200,228] built structures that adapted the shape of either a rod-like virus with helical symmetry or an icosahedral virus with cone-shaped symmetry. Figure 30 outlines each of these viruses and their respective abiotic mimics. 

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...

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). 

Complex viruses Some virions are even more complex, being composed of several separate parts, with separate shapes and symmetries. The most complicated viruses in terms of structure are some of the bacterial viruses, which possess not only icosahedral heads but helical tails. In some bacterial viruses, such as the T4 virus of Escherichia coli, the tail itself is a complex structure. For instance, T4 has almost 20 separate proteins in the tail, and the T4 head has several more proteins. In such complex viruses, assembly is also complex. For instance, in T4 the complete tail is formed as a subassembly, and then the tail is added to the DNA-containing head. Finally, tail fibers formed from another protein are added to make the mature, infectious virus particle. 

Mackay called attention to yet another limitation of the 230 space-group system. It covers only those helices that are compatible with the three-dimensional lattices. All other helices that are finite in one or two dimensions are excluded. Some important virus structures with icosahedral symmetry are among them. Also, there are very small... 

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