Proteins coats of viruses

We might also note another important difference between animal and bacterial cells. Bacterial cells have rigid cell walls containing peptidoglycan and associated substances. Animal cells, on the other hand, lack cell walls. This difference is important for the way by which the virus genome enters and exits the cell. In bacteria, the protein coat of the virus remains on the outside of the cell and only the nucleic acid enters. In animal viruses, on the other hand, uptake of the virus often occurs by endocytosis (pinocytosis or phagocytosis), processes which are characteristic of animal cells, so that the whole virus particle enters the cell. The separation of animal virus genomes from their protein coats then occurs inside the cell. 

Fig. 5.21 Cryoelectron micrograph of a single virus-like particle showing the well-defined protein coating of the 12 nm diameter Au nanoparticle (black disk). (Reprinted with permission from [98]. Copyright (2006) American Chemical Society).

The question of the criteria of autopoiesis is formalized at length, but not always clearly, in the primary literature on autopoiesis. Varela, in his latest book (2000), has simplified these criteria into three basic ones, which can he expressed as follows Verify (1) whether the system has a semi-permeable boundary that (2) is produced from within the system and (3) that encompasses reactions that regenerate the components of the system. Thus, a virus is not an autopoietic system, as it does not produce the protein coat of its boundary or the nucleic acids (the host cell does this, and it is living). A computer virus is also not autopoietic, as it needs a computer system that is not produced hy the virus itself. A growing crystal is not autopoietic, as the components are not generated from an internalized network of reactions. 

The enzyme complex that catalyses steps d to/of Fig. 25-20 has an unusual composition. An a3 trimer of 23.5-kDa subunits is contained within an icosahe-dral shell of 60 16-kDa (3 subunits, similar to the protein coats of the icosahedral viruses (Chapter 7). The (3 subunits catalyze the formation of dimethylribityllu-mazine (steps d, e), while the a3 trimer catalyzes the dismutation reaction of step/, the final step in riboflavin formation.365 A separate bifunctional bacterial ATP-dependent synthetase phosphorylates riboflavin and adds the adenylyl group to form FAD.366 Two separate mammalian enzymes are required.367 Synthesis of deazaflavins of methanogens (Fig. 15-22) follows pathways similar to those of riboflavin. However, the phenolic ring of the deazaflavin originates from the shikimate pathway.368... 

At the far left, we can see the nucleic acid and protein structures shown in frame 1. In addition, we show a much larger protein, the immunoglobulin G antibody molecule. Four separate polypeptide chains join to make up an antibody molecule two heavy chains (blue) of about 400 amino acids and two light chains (purple) of about 200 amino acids. The antibody is about 16 nm in width. Finally, at the far right, we show the core particle from a small plant virus, the reovirus. Only the icosahedral protein coat of the virus can be seen. The reovirus particle is about 60 nm across. The nucleic acids of the virus are sequestered inside the virus core. The reovirus family is unusual in that its nucleic acids are all double-stranded RNA molecules. 

Viruses are small infective agents consisting essentially of nucleic acid (either RNA or DNA enclosed in a protein coat). Some viruses contain additional lipoproteins, which may contain antigenic viral glycoproteins. Viruses are intracellular parasites with no metabolic machinery of their own. To replicate, they must attach to and enter the living host cell animal, plant, or bacteria and use its metabolic process. 

Multiple interactions in the same plane can lead to the formation of sheets where, for example, each monomer can interact with six neighbors in a hexagonal close-packing arrangement (Fig. 5-8). Sheets can, with a slight readjustment, be converted into cylindrical tubes (Fig. 5-8) or even into spheres. These closed structures can provide even greater stability since they maximize the number of interactions that can be made. The protein coats of certain viruses are excellent examples of this. Microtubules, which consist of the protein tubulin, can be converted readily between sheet and tubular forms, at least in the purified form. 

Genes in all cellular organisms are made of DNA. The same is true for some viruses, but for others the genetic material is RNA. Viruses are genetic elements enclosed in protein coats that can move from one cell to another but are not capable of independent growth. One well-studied example of an RNA virus is the tobacco mosaic virus, which infects the leaves of tobacco plants. This virus consists of a single strand of RNA (6930 nucleotides) surrounded by a protein coat of 2130 identical subunits. An RNA-directed RNA polymerase catalyzes the replication of this viral RNA. 

Viruses are one of the smallest biological entities (except viroids and prions) that carry all the information necessary for their own reproduction. They are unique, differing from procaryotes and eucaryotes in that they carry only one type of nucleic acid as genetic material, which can be transported by the vims from one cell to another. Viruses are composed of a shell of protein enclosing a core of nucleic acid, either ribonucleic acid (RNA) or deoxyribonucleic acid (DNA), that codes for viral reproduction. The outer shell serves as a protective coat to keep the nucleic acid intact and safe from enzymatic destruction. In addition to their protein coat, some viruses contain an outer covering known as an outer envelope. This outer envelope consists of a lipid or polysaccharide material. 

Capsid The protein coating of a virus, which protects the nucleic acid core from the environment and usually determines the shape of the virus. 

Uncoating Process in which protein coats of animal viruses that have entered ells are removed by proteolytic enzymes. 

Viruses Infectious entities that contain the nucleic acid to code for their own structure but that lack the enzymatic machinery of a cell they replicate by invading a cell and using its machinery to express the viral genome. Most viruses consist of httle but nucleic acid enclosed in a protein coat some viruses also have an outer hpid-bilayer envelope. 

A viral infection begins when an enzyme in the protein coat of the virus makes a hole in the host cell, allowing the viral nucleic acids to enter and mix with the materials in the host cell (see Figure 17.17). If the virus contains DNA, the host cell begins to replicate the viral DNA in the same way it would replicate normal DNA. Viral DNA produces viral RNA, and a protease processes proteins to produce a protein coat to form a viral particle that leaves the cell. The cell synthesizes so many virus particles that it eventually releases new viruses to infect more cells. 

The protein coat of polioviruses consists of four different peptides with molecular weights of 32000, 30000, 28000 and 6000. They can be separated by gel electrophoresis after disruption of the virus particles by heat and SDS (Summers et al., 1965). Each virus particle possesses approximately 60 copies of every peptide. The peptides have been named "viral proteins (VP, ... 

Acid rain may disrupt some of natiue s microbial ecosystems, many of which are now poorly understood. For example, several insect species are in part kept in check by "good" viruses that are found on leaves and elsewhere in the environment. If the protein coat of these viruses were disrupted by acid rain and the viruses were rendered less infective, it could lead to an explosion in some insect populations. 

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.

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