Viruses structure units

Can any number of identical subunits be accommodated in the asymmetric unit while preserving specificity of interactions within an icosahedral arrangement This question was answered by Don Caspar then at Children s Hospital, Boston, and Aaron Klug in Cambridge, England, who showed in a classical paper in 1962 that only certain multiples (1, 3, 4, 7...) of 60 subunits are likely to occur. They called these multiples triangulation numbers, T. Icosahedral virus structures are frequently referred to in terms of their trian-gulation numbers a T = 3 virus structure therefore implies that the number of subunits in the icosahedral shell is 3 x 60 = 180. 

Figure 16.6 A T = 3 icosahedral virus structure contains 180 subunits in its protein shell. Each asymmetric unit (one such unit is shown in thick lines) contains three protein subunits A, B, and C. The icosahedral structure is viewed along a threefold axis, the same view as in Figure 16.5. One asymmetric unit is shown in dark colors.

The refined coordinates will correspond to either the icosahedral asymmetric unit or the crystallographic asymmetric unit, hence symmetry operations must be applied to generate the whole capsid. A useful repository of virus structure information is the website http // viperdb.scripps.edu/ where portions of the viral capsid can be generated. 

Desmosomes and Filaments. The cohesiveness and internal structure of SC cells are dependent on the structures that are called desmosomes (25, 26, 27). These structures unite SC cells of different layers (Figure 3) and of the same layer (Figures 4 and 5). The sequence of embryonic development is a thickening of opposed cell membranes followed by intercellular disc formation and intracellular formation of the attachment plate (25, 29). This sequence is blocked if normal protein production in rough endoplasmic reticulum is altered by virus activity (28). When complete, the desmosomes of basal cells sprout filaments from their attachment plates (25, 27, 30). These filaments become longer, thicker, and more numerous as cells differentiate and migrate toward the skin surface (31, 32). Filaments arise either from the attachment plate (27) or they form loops that are anchored there (26). The smallest filaments... 

The genetic material of a virus, then, contains the code for one of these structure units, which is read many times to make the number of proteins needed to assemble via... 

We have demonstrated that Ag nanorod-based SERS is not only sensitive to purified virus, but also is able to sense the presence of virus after infection in biological media 49). To demonstrate this, we compared the SERS spectra of uninfected Vero cell lysate, RSV-infected cell lysate and purified RSV. The results show that major Raman bands can be assigned to different constituents of the cell lysate and the virus, such as nucleic acids, proteins, protein secondary structure units and amino acid residues present in the side chains and the backbone. However, our most significant result was that vibrational modes due to the virus could be unambiguously identified in the SERS spectrum of the Vero cell lysate after infection 49). 

It may therefore be concluded that the asymmetric structure unit of the picornaviral capsid is composed of three polypeptides and conforms to the prediction of Finch and Klug. It has a molec]jlar mass of approximately 86,000 daltons, a diameter of about 68 A (35) and is repeated 60 times in the virus capsid. This corresponds to the simplest icosahedral lattice, with a triangulation number of 1 (57) Considering the I4S pentamers as capsomeres, there would be 12 capsomeres per virion. The location of the 6 (7P4) polypeptides in the capsid has not yet been established, but it is possible that they are distributed over the internal surface and in direct contact with the virion RNA (5Q, 39I see below). 

An amphipathic helix is defined as a helix in which the distribution of amino acid residues forms opposing polar and nonpolar faces. It is an important structural unit included in proteins and peptides and is responsible for interaction with biological membranes to elicit their biological functions such as membrane fusion. Influenza virus hemagglutinin [1], fertilin [1], and meltrin-a [2] contain amphipathic fusion peptides, which are likely to adopt a helical conformation during the fusion reaction. To clarify the role of amphipathic peptides in membrane fusion reactions, we synthesized five types of amphipathic model peptides and examined their helix formation, membrane binding and membrane fusion activities. 

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. 

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... 

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