Protein satellite tobacco necrosis virus

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. 

Very few self-sufficient viruses have only 60 protein chains in their shells. The satellite viruses do not themselves encode all of the functions required for their replication and are therefore not self-sufficient. The first satellite virus to be discovered, satellite tobacco necrosis virus, which is also one of the smallest known with a diameter of 180 A, has a protein shell of 60 subunits. This virus cannot replicate on its own inside a tobacco cell but needs a helper virus, tobacco necrosis virus, to supply the functions it does not encode. The RNA genome of the satellite virus has only 1120 nucleotides, which code for the viral coat protein of 195 amino acids but no other protein. With this minimal genome the satellite viruses are obligate parasites of the viruses that parasitize cells. 

Figure 16.S Schematic illustration of the way the 60 protein subunits are arranged around the shell of safellite tobacco necrosis virus. Each subunit is shown as an asymmetric A. The view is along one of the threefold axes, as in Figure 16.3a. (a) Three subunifs are positioned on one triangular tile of an Icosahedron, in a similar way to that shown in 16.4a. The red lines represent a different way to divide the surface of the icosahedron into 60 asymmetric units. This representation will be used in the following diagrams because it is easier to see the symmetry relations when there are more than 60 subunits in the shells, (b) All subunits are shown on the surface of the virus, seen in the same orientation as 16.4a. The shell has been subdivided into 60 asymmetric units by the red lines. When the corners are joined to the center of the virus, the particle is divided into 60 triangular wedges, each comprising an asymmetric unit of the virus. In satellite tobacco necrosis virus each such unit contains one polypeptide chain...

One of the most striking results that has emerged from the high-resolution crystallographic studies of these icosahedral viruses is that their coat proteins have the same basic core structure, that of a jelly roll barrel, which was discussed in Chapter 5. This is true of plant, insect, and mammalian viruses. In the case of the picornaviruses, VPl, VP2, and VP3 all have the same jelly roll structure as the subunits of satellite tobacco necrosis virus, tomato bushy stunt virus, and the other T = 3 plant viruses. Not every spherical virus has subunit structures of the jelly roll type. As we will see, the subunits of the RNA bacteriophage, MS2, and those of alphavirus cores have quite different structures, although they do form regular icosahedral shells. 

Figure 16.14 Schematic diagrams of three different viral coat proteins, viewed in approximately the same direction. Beta strands I through 8 form the common jelly roll barrel core, (a) Satellite tobacco necrosis virus coat protein, (b) Subunit VPl from poliovirus.

One of the smallest of the encapsulated RNA-containing viruses is the satellite tobacco necrosis virus. It replicates only when the plant is also infected with the larger tobacco necrosis virus. The satellite virus, whose three-dimensional structure is known from X-ray diffraction studies,485 contains a 1200-nucleotide strand of RNA which encodes a 195-residue protein. 

An example is the tiny satellite tobacco necrosis virus,72 diameter 18 nm, whose coat contains just 60 subunits of a 195-residue protein. Its genome is a... 

Fig. 15.21. Ribbon diagrams [54] showing an example of each of the three classes of four-stranded Greek-key motif, a (4,0) Greek key from p-hydroxybenzoate hydroxylase (1PHH) b (3,1) Greek key from satellite tobacco necrosis virus coat protein (2STV) c (2,2) Greek key from o-amylase inhibitor (tendamistat) (1 HOE)...

Fig. 15.27. Ribbon diagram [54] of the jelly-roll topology of satellite tobacco necrosis virus coat protein (2STV). This is a special case of the Greek-key structure, involving a five-stranded double Greek key (shaded dark) and three extra strands (see Figure 15.28)...

FIGURE 12 Ribbon representations of typical all-,e proteins, (a) Retinol binding protein, (b) immunoglobulin fold as seen in the Cd8 Surface glycoprotein N-terminal domain, (c) pectate lyase, and (d) viral coat protein found in satellite tobacco necrosis virus. 

Of all the viruses at present known and which have been studied biochemically, the simplest is a special virus described by Reichman (1964) as satellite of the tobacco necrosis virus. This virus is a ribonucleoprotein with the smallest RNA molecule of any virus (mol. wt. 395,000). This RNA is composed of 1200 nucleotides. Given the triplet nucleotide character of the genetic code, such an RNA cannot code more than 400 amino acid residues. Since each protein molecule of the virus coat contains about 370-380 amino acid residues, clearly the RNA of this virus can code only one protein. 

However, the volume of information inscribed in 1200 nucleotides is insufficient for spontaneous propagation of the virus in plant cells, and it can reproduce only as the satellite of another virus—virus of tobacco necrosis, possessing RNA with a molecular wei t of 2 X 10. Proteins of these viruses are serologically different, but the basic virus evidently enables certain factors to be synthesized which are equally essential for the formation of both viruses. RNA synthesis in the host cells takes place on DNA templates and is catalyzed by DNA-dependent RNA-polymerase. It may be assumed that a special factor—the enzyme RNA-replicase— which is not used for reproduction of the satellite virus is essential for synthesis of virus RNA on RNA templates (autoreplication of RNA). 

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