Tomato bushy stunt virus protein structure

The remaining three antiparallel /3 structures form a miscellaneous category (see Fig. 84). Lactate dehydrogenase d2 and gene 5 protein each has several two-stranded antiparallel j8 ribbons, but they do not coalesce into any readily described overall pattern. The N-terminal domain of tomato bushy stunt virus protein has a unique /3 structure in which equivalent pieces of chain from three different subunits wrap around a 3-fold axis to form what has been called a /3 annulus (Harrison et ah, 1978). Each of the three chains contributes a short strand segment to each of three three-stranded, interlocking /3 sheets. This domain provides one of the subunit contacts that hold the 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. 

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. 

One of the most intriguing recent examples of disordered structure is in tomato bushy stunt virus (Harrison et ah, 1978), where at least 33 N-terminal residues from subunit types A and B, and probably an additional 50 or 60 N-terminal residues from all three subunit types (as judged from the molecular weight), project into the central cavity of the virus particle and are completely invisible in the electron density map, as is the RNA inside. Neutron scattering (Chauvin et ah, 1978) shows an inner shell of protein separated from the main coat by a 30-A shell containing mainly RNA. The most likely presumption is that the N-terminal arms interact with the RNA, probably in a quite definite local conformation, but that they are flexibly hinged and can take up many different orientations relative to the 180 subunits forming the outer shell of the virus particle. The disorder of the arms is a necessary condition for their specific interaction with the RNA, which cannot pack with the icosahedral symmetry of the protein coat subunits. 

The structure of the capsid (protein shell) for an icosahedral virus such as tomato bushy stunt virus. Pentons (P) are located at the 12 vertices of the icosahedron. Hexons (H), of which there are 20, form the edges and faces of the icosahedron. Each penton is composed of five protein subunits and each hexon is composed of six protein subunits. In all, the structure contains 180 protein subunits. 

A single mechanism will not describe even the "controlled aggregation of proteins in nature. A single viral coat protein, for example, may form several different contacts with itself and other proteins, depending on its final position within the shell structure. Indeed, the original postulate of "quasi-equivalent binding at lattice points in virus capsules was modified to "non-equivalency" when the first structure was solved at atomic dimensions. For example, the coat of Tomato bushy stunt virus consists of 180 identical subunits of a 43 kD protein which self-interact at lattice points in at least three distinct ways k... 

The tertiary fold of three of the rhinovirus proteins (VP1, VP2 and VP3) and their quaternary organisation within the HRV14 capsid are similar to the structures of the plant viruses TBSV (tomato bushy stunt virus) and SBMV (southern bean mosaic virus), agreeing to within approximately 3 A. In HRV14 VP4 is an internal structural protein in contact with VP1 and VP2. Protrusions on VP1, VP2 and VP3 together create a deep cleft or canyon on the viral surface which is 25 A deep and... 

J. D. Bernal and I. Fankuchen, Nature, 139 (1937) 923. See further recent reviews I. Fan-KUCHEN, X- Ray Diffraction and Protein Structure, in Advances in Protein Chemistry, edited by M. L. Anson and J. T. Edsall, Vol, II, 1945, Academic Press Inc. New York and N. W. Pirie, Physical and Chemical Properties of Tomato Bushy Stunt Virus and the Strains of Tobacco Mosaic Virus in Advances in Enzymology, edited by F. F. Nord and C. H. Werkman, Vol. V, 1945, Interscience Publishers New York. See also Volume I of Colloid Science for long range forces. 

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. 

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