Icosahedral capsid

Plenary 4. George J Thomas Jr et at, e-mail address thomasgj ,cctr.mnkc.edu (RS). Protein folding and assembly into superstructures. (Slow) time resolved RS probing of virus construction via protein assembly into an icosahedral (capsid) shell. 

The number of helical turns in these structures is larger than those found so far in two-sheet p helices. The pectate lyase p helix consists of seven complete turns and is 34 A long and 17-27 A in diameter (Figure 5.30) while the p-helix part of the bacteriophage P22 tailspike protein has 13 complete turns. Both these proteins have other stmctural elements in addition to the P-helix moiety. The complete tailspike protein contains three intertwined, identical subunits each with the three-sheet p helix and is about 200 A long and 60 A wide. Six of these trimers are attached to each phage at the base of the icosahedral capsid. 

Togaviruses Rubella Spherical particles 70 nm in diameter, a tightly adherent envelope surrounds an icosahedral capsid Causes German measles in children. An infection contracted in the early stages of pregnancy can induce severe multiple congenital abnormalities, e.g. deafness, blindness, heart disease and mental retardation... 

The non-enveloped human viruses all have icosahedral capsids. The structural proteins undergo a self-assembly process to form capsids into which the viral nucleic acid is packaged. Most non-enveloped viruses accumulate within the cytoplasm or nucleus and are only released when the cell lyses. 

Icosahedral capsid viruses and clathrins are examples of coat proteins of which there are many. Another example that has been extensively studied is coat protein II, or COPII, which is composed of an inner cage and outer coat [5], The inner cage is a cuboctahedron approximately 60 nm across. It has square and triangular faces which can only be constructed if four protein strands emanate from the structure s hub, rather than the three seen in clathrins. It also transpires that the proteins interact with each other at the vertices without any of the extensive interdigitation seen in clathrin cages. 

An excellent example where a capsid virus has been given a new supramolecular application can be found in the work of Nolte who took an icosahedral capsid virus, cowpea chloritic mottle virus (CCMV) and used it as a nanoreactor for polymer synthesis [30], Natural CCMV spontaneously assembles in acidic aqueous solution and disassembles in basic solution. The capsid contains pores open at pH 5 to release RNA into the host. Once the RNA leaves, the empty capsule is left. The Nolte group was able to assemble the subunits around polystyrene sulfonate with a mass of 9.9 kDa but the resulting structure had a different morphology to the natural system. Indeed, capsules formed around polymers with masses between 2 and 85 kDa but not around those with masses above 100 kDa. This raised the question of the potential for polymers to form within a capsid but to test the possibility a mixture of botanical, biological and chemical approaches was needed. 

Luteoviridae—180 copies of subunit in 320- to 360-A-diameter T=3 icosahedral capsid... 

Bromoviridae—Icosahedral capsids, details vary (see below). Structures for three of five genera Fold Capsid protein, Jelly-roll (3 sandwich... 

Bromovirus—180 subunits in a 290-A-diameter T=3 icosahedral capsid Cozvpea chlorotic mottle virus Plants... 

The adenovirus capsid is an icosahedral capsid that can be described as pseudo-T=25. The 12 pentameric vertices are each composed of 5 molecules of the penton bases, carrying a trimeric fiber. The 20 faces are composed of the 4 molecules of hexon protein, which itself is a trimer of the 967-residue-long polypeptide II. The crystal structure of the hexon has been solved and it reveals that the trimeric subunits are intimately intertwined, thus accounting for their observed stability. The capsid also... 

All herpesvirus virions have a characteristic appearance (Fig. ID) with an icosahedral capsid surrounded by a thick ( 500 A) layer of protein designated the tegument (Rixon, 1993 Steven and Spear, 1997). The entire particle is enclosed by a spherical lipid envelope (Szilagyi and Berriman, 1994). The core of infectious virions is believed to consist exclusively of the double-stranded DNA genome without any associated protein (Booy et al, 1991 Zhou et al, 1999). The structures of the tegument and envelope are poorly understood as they are indeterminate in both size and composition and have a limited symmetrical relationship to the capsid (Chen et al, 1999 Zhou et al, 1999). 

Icosahedral capsid particles, 390 Icosahedral single-stranded RNA (ssRNA)... 

An icosahedral capsid has 12 fivefold, 20 threefold and 30 twofold symmetry axes. This situations yields a second question, whether clusters of coat proteins having one of these axial symmetries share a form lattice of the same type as in the axially symmetric biomolecules discussed above. 

Up to now, this 1 MDa protein has been observed only in Bacillaceae. Computer modeling has confirmed that the central cavity of the icosahedral capsid is large enough to accommodate the riboflavin synthase trimer (Figure 16). However, there are no corresponding symmetry properties between the two modules that could suggest a potential orientation. The molecular structure is not known in closer detail. The import and export of substrates and products is also an open problem. It is hard to see how the channels in the capsid wall could allow the passage of 6,7-dimethyl-8-ribityllumazine, and, even less, of riboflavin. Dynamic fluctuations of the capsid structure have been proposed but remain speculative. 

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