Viruses icosahedral

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.

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. 

An icosahedral virus particle composed of 252 capsomeres 240 being hexons and 12 being pentons... 

Figure 5.6 A simple icosahedral virus. Each face has three subunits. A single subunit consists of one or more proteins, (a) Whole virus particle, (b) Virus particle opened up nucleic acid released.

Coulibaly, E, ChevaUer, C., Gutsche, I., Pous, J., Navaza, J., BressaneUi, S., Bernard Delmas, B. and Rey F. A. (2005). The birnavirus crystal structure reveals structural relationships among icosahedral viruses. Cell 120, 761-772. 

Chiu, W. and Rixon, F. J. (2002). High resolution structural studies of complex icosahedral viruses a brief overview. Virus Res. 82, 9-17. 

Lee, K. K. and Johnson, J. E. (2003). Complimentary approaches to structure determination of icosahedral viruses. Curr. Opin. Struct. Biol. 13, 558-569. 

Rayment, I. (1983). Molecular replacement method at low resolution optimum strategy and intrinsic limitations as determined by calculations on icosahedral virus models. Acta Crystallogr. A 39,102-116. 

The Hu and Bentley model is the only one that tries to describe VLP production and assembling in baculovirus infected insect cells [105]. Nevertheless, regarding VLP assembly, the formalism presented is completely theoretical and based on the assembly pathway of icosahedral viruses. From a process development point of view, this model does not generate enough output to make it applicable to bioreaction operational parameters definition. However, it can be used as the basis for a more structured approach to the VLP assembling process in baculovirus infected insect cells. 

Quasi-equivalence in virus coats. A large number of icosahedral viruses have coats consisting of 180 identical subunits. For example, the small RNA-containing bacteriophage MS 2 consists of an eicosahedral shell of 180 copies of a 129-residue protein that encloses one molecule of a 3569-residue RNA.89 Tire virus also contains a single molecule of a 44-kDa protein, the A protein, which binds the virus to a bacterial pilus to initiate infection. Related bacteriophages GA, fr, f2, and QP90"1 have a similar... 

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... 

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. 

Larger icosahedral viruses that have been structurally well characterized do not obey the simple quasi-equivalence rule. For example, adenovirus capsids, for which T = 25, are built of 240 hexons (six-coordinated units) that are trimers of the major structural protein, and the 12 pentons consist of a different protein (Burnett 1984). Polyomavirus capsids, for which T = 7, are built of a single major structural protein,... 

An example of an icosahedral virus is Enterobacteria phage PhiX174, PDB ID 2BPA... 

Ochoa WF et al (2006) Generation and structural analysis of reactive empty particles derived from an icosahedral virus. Chem Biol 13 771-778 PDBID 2BFU... 

DETERMINATION OF ICOSAHEDRAL VIRUS STRUCTURES BY ELECTRON CRYOMICROSCOPY AT SUBNANOMETER RESOLUTION... 

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