Viruses Shapes

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 16.1 Viruses vary in size and shape from the simplest satellite viruses (a) that need another virus for their replication to the T-even bacteriophages (d) that have developed sophisticated mechanisms for injecting DNA into bacteria. Four different virus particles are shown to scale.

Figure 16.2 The icosahedron (top) and dodecahedron (bottom) have identical symmetries but different shapes. Protein subunits of spherical viruses form a coat around the nucleic acid with the same symmetry arrangement as these geometrical objects. Electron micrographs of these viruses have shown that their shapes are often well represented by icosahedra. One each of the twofold, threefold, and fivefold symmetry axes is indicated by an ellipse, triangle, and pentagon, respectively.

The asymmetric unit contains one copy each of the subunits VPl, VP2, VP3, and VP4. VP4 is buried inside the shell and does not reach the surface. The arrangement of VPl, VP2, and VP3 on the surface of the capsid is shown in Figure 16.12a. These three different polypeptide chains build up the virus shell in a way that is analogous to that of the three different conformations A, C, and B of the same polypeptide chain in tomato bushy stunt virus. The viral coat assembles from 12 compact aggregates, or pen tamers, which contain five of each of the coat proteins. The contours of the outward-facing surfaces of the subunits give to each pentamer the shape of a molecular mountain the VPl subunits, which correspond to the A subunits in T = 3 plant viruses, cluster at the peak of the mountain VP2 and VP3 alternate around the foot and VP4 provides the foundation. The amino termini of the five VP3 subunits of the pentamer intertwine around the fivefold axis in the interior of the virion to form a p stmcture that stabilizes the pentamer and in addition interacts with VP4. 

Vibrio (i) Curved, rod-shaped bacterial cell, (ii) Bacterium of the genus Vibrio. Virion Virus particle the virus nucleic acid surrounded by protein coat and in some cases other material. 

Lemon balm isa perennial herb with heart-shaped leaves that has been used for hundreds of years. Its scientific name is Melissa officinalis. Traditionally the herb has been used for Graves disease (see Chap. 51), asa sedative, arrtispasmodic, and an antiviral agent. When used topically, lemon balm has antiviral activity against herpes simplex virus(HSM). No adverse reactions have been reported when lemon balm is used topically. 

The development of high-magnification microscopy made it possible to create images of biological materials at the molecular level. Many of these images show structures that have liquid crystalline aspects. Shown here are aligned mosaic virus molecules and protein molecules in voluntary muscles. In addition, all cell walls are picket fences of rod-shaped molecules in regular yet fluid arra. ... 

They are coccoid-shaped organisms and the feature which at one time consigned them to the virus class was the fact that they would only reproduce in living tissue. 

Human immunodeficiency virus (HIV) Differs from other retroviruses in that the core is cone-shaped rather than icosahedral HIV is transmitted from person to person via blood or genital secretions. The principal target for the virus is the CD4+ T-lymphocyte cells. Depletion of these cells induces immunodeficiency... 

NK cells are a subset of lymphocytes found in blood and lymphoid tissues, especially the spleen. They are about 15 an in diameter, possess a kidney-shaped nucleus and have two or three large granules in the cytoplasm. They are derived from the bone marrow. NK cells have the ability to kill certain tumour lines and normal cells infected by virus. Killing by NK cells is not specific for viral antigenic epitopes, and is not restricted by MHC molecules. They do not possess CD3 but do express CD2, CD 16 and CD56, together with a low-affinity receptor for the Fc portion of IgG. 

The HlV-1 protease is responsible for processing the protein precursors to the enzymes (integrase, protease and reverse transcriptase) and the structural proteins of the HIV-1 virus. Maw and Hall found that topological indices provide rehable QSAR models for the IC50 data of 32 HIV-1 protease inhibitors [29]. The best QSAR model, with r = 0.86, s=0.60 and q = 0.79, was obtained with the shape index Ka, the connechvity index the sum of HE-state indices for ah groups that act as... 

Santos NC and Castanho MARB. 1996. Teaching light scattering spectroscopy The dimension and shape of tobacco mosaic virus. Biophysical Journal 71(3) 1641-1650. 

Virus particles vary widely in size and shape. As we have stated, some viruses contain RNA, others DNA. We have discussed nucleic acids in previous chapters and have noted that the DNA of the cell genome is in the double-stranded form. Some viruses have double-stranded DNA whereas others have single-stranded DNA (Figure 6.3). 

The structures of virions (virus particles) are quite diverse. Viruses vary widely in size, shape, and chemical composition. The... 

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. 

Complex viruses Some virions are even more complex, being composed of several separate parts, with separate shapes and symmetries. The most complicated viruses in terms of structure are some of the bacterial viruses, which possess not only icosahedral heads but helical tails. In some bacterial viruses, such as the T4 virus of Escherichia coli, the tail itself is a complex structure. For instance, T4 has almost 20 separate proteins in the tail, and the T4 head has several more proteins. In such complex viruses, assembly is also complex. For instance, in T4 the complete tail is formed as a subassembly, and then the tail is added to the DNA-containing head. Finally, tail fibers formed from another protein are added to make the mature, infectious virus particle. 

Figure 5.30 The shapes and relative sizes of vertebrate viruses of die major taxonomic groups. Bar = 100 nm.

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