Virus satellite tobacco necrosis

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...

The size of this viral particle is of course larger than that of a virus with only 60 subunits. The diameter of tomato bushy stunt virus is 330 A compared with 180 A for satellite tobacco necrosis virus. The increase in volume of the capsid means that a roughly four times larger RNA molecule can be accommodated. 

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.

Satellite tobacco necrosis virus is an example of a T = 1 virus structure. The 60 identical subunits interact tightly around the fivefold axes on the surface of the shell and around the threefold axes on the inside. These interactions form a scaffold that links all subunits together to complete the shell. 

Jones, T.A., Liljas, L. Structure of satellite tobacco necrosis virus after crystallographic refinement at 2.5 A resolution. /. Mol. Biol. 177 735-768, 1984. 

Most of the EF-hand motifs have one water (—X). On the other hand, in Rhizopus chinensis aspartic proteinase (Suguna et al., 1987) there is one main-chain carbonyl oxygen bound to calcium and six water molecules to complete the pentagonal bipyramidal coordination. Calcium coordination has been measured in several viruses, such as Southern bean mosaic virus (Silva and Rossmann, 1985), satellite tobacco necrosis virus (Jones and Liljas, 1984), and tomato bushy stunt virus IV (Olson et al., 1983). 

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... 

Satellite tobacco necrosis virus 247 Saturation in binding 326 Saturationcurve(s) 326 - 331,458 - 461,475-477... 

Satellite tobacco necrosis virus 1,300,000 60 Virus coat... 

The lanthanides form a series of ions of closely related size and bonding characteristics and in many respects resemble Ca +, for which they often substitute isomorphously in biological systems. Since different Ln ions can be probed with particular spectroscopic techniques (e.g. Eu + and Tb +, fluorescence Gd +, ESR Nd +, electronic spectra), in favourable circumstances it should be possible to obtain information about the binding site of spectroscopically inactive Ca + in several ways. Systems smdied include the calciumbinding sites in calmodulin, trypsin, parvalbumin, and the Satellite tobacco necrosis virus. 

Satellite viruses are those that are dependent for their own replication on some (catalytic) activity encoded in another helper virus that coinfects the host cell. The structures of three plant ssRNA satellite viruses represent some of the highest resolutions known and have been comparatively reviewed (Ban et al., 1995). The structures of satellite tobacco mosaic virus (STMV) (Larson et al., 1993a,b), satellite tobacco necrosis virus (STNV) (Jones and LUjas, 1984 Liljas et al., 1982), and satellite panicum mosaic virus (SPMV) (Ban and McPherson, 1995) have T=1 capsids composed of 60 identical copies of unembellished jelly-roll j3 barrels constructed of only 155 to 195 amino acids (Fig. la see Color Insert). What is remarkable is how little the assembly context of these domains is conserved. The same end always points toward the 5-fold axis, but the domains are rotated to different extents around the 5-fold axis. Furthermore, between STNV and the others, there is a 70° rotation of the barrel about its long axis. Contacts across the dimer interface are... 

The satellite ssRNA viruses are a group of ssRNA icosahedral viruses, which are the satellites to certain plant viruses (Pritsch and Mayo, 1989). Satellite tobacco necrosis virus (STNV), a satellite virus to tobacco necrosis virus, was in fact one of the first icosahedral virus structures to be determined by X-ray crystallography (Liljas et al, 1982). In more recent years, two other satellite plant viruses, satellite tobacco mosaic virus (STMV) (Larson et al, 1993, 1998) and satellite panicum mosaic virus (SPMV) (Ban and McPherson, 1995), have been determined. These are perhaps the simplest and smallest icosahedral viruses whose structures have been determined by X-ray crystallography. Of relevance to our discussion is the structure of STMV determined to 1.8-A resolution, which shows extraordinary details about genome organization (Larson et al, 1998 Larson and McPherson, 2001). 

Bendey, G. A., Lewit-Bentley, A., Liljas, L., Skoglund, U., Roth, M., and Unge, T. (1987). Structure of RNA in satellite tobacco necrosis virus A low resolution neutron diffraction study using H20/ H20 solvent contrast variation. J. Mol. Biol. 194, 129-141. 

RNA structure and, 49-50 satellite tobacco necrosis virus (STNV) and, 49-50... 

Fig. 14.16. Nanoscaled systems of the biosphere and of chemistry show the same topology schematic representation of the icosahedron spanned by the 12 centres of the (Mo)IVIo5 units of the M0132 type cluster (a) and of the satellite tobacco necrosis virus (STNV) with the triangulation number F = 1...

Several plant viruses have been studied using X-ray crystallography and conventional X-ray sources. These are, in no particular order, TBSV (tomato bushy stunt virus), SBMV (southern bean mosaic virus) and STN V (satellite tobacco necrosis virus) - all spherical viruses - and TMV (tobacco mosaic virus) - a cylindrical virus. These virus crystals diffract relatively well and are reasonably stable to radiation. 

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)...

Fig. 15.28. Schematic hydrogen-bonding diagram of the jeiiy-roil sheet topology of satellite tobacco necrosis virus (2STV) drawn using our HERA program [43]...

A25. Reichmann, M.E. The satellite tobacco necrosis virus ... 

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