Tobacco mosaic virus protein structure

Antiparallel tt-helix proteins are structures heavily dominated by a-helices. The simplest way to pack helices is in an antiparallel manner, and most of the proteins in this class consist of bundles of antiparallel helices. Many of these exhibit a slight (15°) left-handed twist of the helix bundle. Figure 6.29 shows a representative sample of antiparallel a-helix proteins. Many of these are regular, uniform structures, but in a few cases (uteroglobin, for example) one of the helices is tilted away from the bundle. Tobacco mosaic virus protein has small, highly... 

Many protein molecules are composed of more than one subunit, where each subunit is a separate polypeptide chain and can form a stable folded structure by itself. The amino acid sequences can either be identical for each subunit (as in tobacco mosaic virus protein), or similar (as in the a and )3 chains of hemoglobin), or completely different (as in aspartate transcarbamylase). The assembly of many identical subunits provides a very efficient way of constructing... 

Tobacco mosaic virus protein has a small, highly twisted antiparallel j8 sheet at the base of the helix bundle, with two more helices underneath the sheet (see Fig. 72). Cytochrome bs looks remarkably similar (see Fig. 105), but the helices are much shorter. That structure could have been classified as an up-and-down helix bundle, but we have placed it in the small metal-rich proteins because its helix bundle is very small and distorted and the heme interactions appear more important than the direct helix contacts. 

Fig. 105. Examples of small disulfide-rich or metal-rich proteins (shown on the right side) compared with their more regular counterparts in other structural categories (shown at the left), (a) Tobacco mosaic virus protein, an up-and-down helix bundle (b) cytochrome bs, a distorted up-and-down helix bundle (c) trypsin domain 1, a Greek key antiparallel /3 barrel (d) high-potential iron protein, a distorted Greek key /3 barrel (e) glutathione reductase domain 3, an open-face sandwich fi sheet (f) ferredoxin, a distorted open-face sandwich f) sheet.

A particularly striking feature of the simplified formula of the tobacco mosaic virus (Fig. 14) is the stereotype repetition of several simple structures (combinations UI, LS, AF) which in fact represent the cornerstones of the entire molecule. A phenomenon deserving special interest (apart from a number of symmetrical sequences occurring along the whole chain) is the symmetrical arrangement of the dipeptides of the hydroxyamino acid residues at both terminals of the polypeptide chain. This relative concentration of the serine residues at the ends of the tobacco mosaic virus protein chain has been pointed out by Anderer et al. (1960) as a possible example of nonrandomness. 

A nucleic acid can never code for a single protein molecule that is big enough to enclose and protect it. Therefore, the protein shell of viruses is built up from many copies of one or a few polypeptide chains. The simplest viruses have just one type of capsid polypeptide chain, which forms either a rod-shaped or a roughly spherical shell around the nucleic acid. The simplest such viruses whose three-dimensional structures are known are plant and insect viruses the rod-shaped tobacco mosaic virus, the spherical satellite tobacco necrosis virus, tomato bushy stunt virus, southern bean mosaic vims. 

Figure 5.4 Structure and manner of assembly of a simple virus, tobacco mosaic virus, (a) Electron micrograph at high resolution of a portion of the virus particle, (b) Assembly of the tobacco mosaic virion. The RNA assumes a helical configuration surrounded by the protein capsomeres. The center of the particle is hollow.

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... 

Differences between the spectra of fluorescence and phosphorescence are immediately obvious. For all tryptophans in proteins the phosphorescence spectrum, even at room temperature, is structured, while the fluorescence emission is not. (Even at low temperatures the fluorescence emission spectrum is usually not structured. The notable exceptions include a-amylase and aldolase, 26 protease, azurin 27,28 and ribonuclease 7, staphylococcal endonuclease, elastase, tobacco mosaic virus coat protein, and Drosophila alcohol dehydrogenase 12. )... 

The tobacco mosaic virus (center right), a plant pathogen, has a structure similar to that of MB, but contains ssRNA instead of DNA. The poliovirus, which causes poliomyelitis, is also an RNA virus. In the influenza virus, the pathogen that causes viral flu, the nucleocapsid is additionally surrounded by a coat derived from the plasma membrane of the host cell (C). The coat carries viral proteins that are involved in the infection process. 

True self-assembly is observed in the formation of many oligomeric proteins. Indeed, Friedman and Beychok reviewed efforts to define the subunit assembly and reconstitution pathways in multisubunit proteins, and all of the several dozen examples cited in their review represent true self-assembly. Polymeric species are also formed by true self-assembly, and the G-actin to F-actin transition is an excellent example. By contrast, there are strong indications that ribosomal RNA species play a central role in specifying the pathway to and the structure of ribosome particles. And it is interesting to note that the assembly of the tobacco mosaic virus (TMV) appears to be a two-step hybrid mechanism the coat protein subunits first combine to form 34-subunit disks by true self-assembly from monomeric and trimeric com-... 

Another complex macromolecular aggregate that can reassemble from its components is the bacterial ribosome. These ribosomes are composed of 55 different proteins and by 3 different RNA molecules, and if the individual components are incubated under appropriate conditions in a test tube, they spontaneously form the original structure (Alberts et al., 1989). It is also known that even certain viruses, e.g., tobacco mosaic virus, can reassemble from the components this virus consists of a single RNA molecule contained in a protein coat composed by an array of identical protein subunits. Infective virus particles can self-assemble in a test tube from the purified components. 

Let us consider the structure of the tobacco mosaic virus first. As shown schematically in Figure 5.1, it is composed of a single strand of ribonucleic acid, RNA, covered by a sheath formed from 2130 identical protein units. Thus the whole virus constitutes a rather simple supramolecular assembly. By changing... 

II A radically different type of nucleoprotein is that provided by the smaller RNA viruses of the elongated spiral type like tobacco mosaic, or of the polyhedral type such as tomato bushy stunt, tipula virus or poliomyelitis virus. The only one of these adequately studied has been tobacco mosaic virus, Franklin [19, 20], and here it appears that the protein and not the nucleic acid determines the structure. There is only one RNA chain and this is wound helically so that one protein is in contact with three successive nucleotides. 

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). 

Jaenicke and Lauffer105,145,146 have found changes in the optical rotatory dispersion when the tobacco mosaic virus polymerizes. Lauffer105 interprets this observation as a change of the protein conformation effected by the different structures during exposure on all sides to water or in contact with other protein molecules. 

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