Plant viral capsids

Plant Viral Capsids as Programmable Nanobuilding Blocks... 

Viruses with helical symmetry, or "linear" viral capsids, have their genetic material encased in a helix of identical protein subunits, the length of whieh is determined by the length of the encased nucleic acid. There are three main classes of simple helical viruses the rigid rod viruses, the flexuous plant viruses, and the filamentous bacteriophage-.-TTi e most studied of these viruses is the rigid rod Tobacco Mosaic Virus (TMV). 

Volumes 5 and 6 represent the first in a series that focuses primarily on the structure and assembly of virus particles. Volume 5 is devoted to general structural principles involving the relationship and specificity of interaction of viral capsid proteins and their nucleic acids, or host nucleic acids. It deals primarily with helical and the simpler isometric viruses, as well as with the relationship of nucleic acid to protein shell in the T-even phages. Volume 6 is concerned with the structure of the picornaviruses, and with the reconstitution of plant and bacterial RNA viruses. 

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. 

FIGURE 4.3 Schematic representation comparing the full and deconstructed virus vector strategies, (a) TMV expression vector, (b) Provector system for rapid assembly of viral amplicons in planta. P promoter, TMV Pol TMV polymerase, MP movement protein, GOI gene of interest, CP capsid protein, T terminator, RS recombination site. (Adapted from Gleba et al. (2004). Curr. Opin. Plant Biol., 7, 182-188.)... 

The tertiary fold of three of the rhinovirus proteins (VP1, VP2 and VP3) and their quaternary organisation within the HRV14 capsid are similar to the structures of the plant viruses TBSV (tomato bushy stunt virus) and SBMV (southern bean mosaic virus), agreeing to within approximately 3 A. In HRV14 VP4 is an internal structural protein in contact with VP1 and VP2. Protrusions on VP1, VP2 and VP3 together create a deep cleft or canyon on the viral surface which is 25 A deep and... 

Another icosahedral plant virus used as a viral template is CCMV, a member of the Bromoviridae family. CCMV has a diameter of 28.6 nm and is comprised of a coat protein shell encapsulating a single strand of positive-sense RNA. Similar to CPMV, the genome of CCMV is comprised of multiple strands of RNA, with the three unique strands of RNA packaged individually into virus particles. The individual coat protein capsids of CCMV are composed of 190 amino acid residues and have a total mass of 19.8 kDa. Each coat protein subunit folds into the canonical virus -barrel structure. The protein shell of the virus is composed of 180 identical coat protein subunits. In contrast to CPMV, CCMV particles are stable at pH 5 but swell up to 10% in size when the pH is increased to 7. As illustrated in Figure 7, when the virus is swollen, pores are created within the protein shell, which permit the interior of the virns particles to be penetrated by small molecnles. The transition... 

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