Capsids

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. 

Conway J F, Cheng N, Ziotnick A, Wingfieid P T, Stahi S J and Steven A C 1997 Visuaiization of a 4-heiix bundie in the hepatitis B virus capsid by cryo-eiectron microscopy Nature 386 91-4... 

Further reductions can be achieved by taking symmetry into account, an approach that holds promise for the analysis of large oligomeric proteins such as virus capsids [14]. 

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. 

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. 

The size of this viral particle is of course larger than that of a virus with only 60 subunits. The diameter of tomato bushy stunt virus is 330 A compared with 180 A for satellite tobacco necrosis virus. The increase in volume of the capsid means that a roughly four times larger RNA molecule can be accommodated. 

The protein capsid of picomaviruses contains four polypeptide chains... 

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. 

Capsids can also be formed in the absence of RNA as well as in the presence of small RNA molecules comprising the 19 nucleotides of the... 

Figure 16.18 A dimer is the basic unit that builds up the capsid of bacteriophage MS2. The two subunits (red and biue) are arranged so that the dimer has a p sheet of 10 antiparaliel strands on one side and the hairpins and a heiices on the other side. The heiices from one subunit pack against p strands from the other subunit and vice versa. (Adapted from a diagram provided by L. Liljas.)...

The capsids of polyoma virus and the related SV40 have icosahedral symmetry, with 72 pentameric assemblies of the major capsid protein. The pentamers are linked to their neighbors by flexible arms, with a p strand that augments a p sheet in the invaded pentamer. These flexible arms allow the pentamers to be linked together with both fivefold and sixfold symmetry. 

Arnold, E., et al. Implications of the picornavirus capsid structure for polyprotein processing. Proc. Natl. Acad. ScL USA. 84 21-25, 1987. 

Rayment, I., et al. Polyoma virus capsid structure at 22.5 A resolution. Nature 29S 110-115, 1982. 

Rossmann, M.G. Antiviral agents targeted to interact with viral capsid proteins and a possible application to human immunodeficiency virus. Proc. Natl. Acad. Sci. USA, 85 4625-4627, 1988. 

Valegard, K., et al. The three-dimensional structures of two complexes between recombinant MS2 capsids and RNA operator fragments reveal sequence-specific pro-tein-RNA interactions. /. Mol. Biol. 270 724-738,... 

A capsid is a proteinaceous shell encasing the viral genome. Viral capsids are polymeric, ordered structures composed of one or more virus encoded subunits. 

In some viruses, the capsid is surrounded by a lipid membrane (envelope), which is derived from the host cell membrane at the site of vims budding. The membrane contains viral envelope glycoproteins as well as host cell membrane proteins. 

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