Tobacco mosaic virus, rotational

Satellite viruses are those that are dependent for their own replication on some (catalytic) activity encoded in another helper virus that coinfects the host cell. The structures of three plant ssRNA satellite viruses represent some of the highest resolutions known and have been comparatively reviewed (Ban et al., 1995). The structures of satellite tobacco mosaic virus (STMV) (Larson et al., 1993a,b), satellite tobacco necrosis virus (STNV) (Jones and LUjas, 1984 Liljas et al., 1982), and satellite panicum mosaic virus (SPMV) (Ban and McPherson, 1995) have T=1 capsids composed of 60 identical copies of unembellished jelly-roll j3 barrels constructed of only 155 to 195 amino acids (Fig. la see Color Insert). What is remarkable is how little the assembly context of these domains is conserved. The same end always points toward the 5-fold axis, but the domains are rotated to different extents around the 5-fold axis. Furthermore, between STNV and the others, there is a 70° rotation of the barrel about its long axis. Contacts across the dimer interface are... 

FIGURE 4-2 Rotations. The cross section of the tobacco mosaic virus is a cover diagram from Nature, 1976, 259. 1976, Macmillan Journals Ltd. Reproduced with permission of Aaron Klug. 

Finding rotation axes for some three-dimensional figures is more difficult, but the same in principle. Remember that nature is not always simple when it comes to symmetry—the protein disk of the tobacco mosaic virus has a 17-fold rotation axis ... 

In this section, we treat a simple but important model—the rigid rod (Pecora, 1964, 1968). This model illustrates the conditions under which rotational motions of rigid, nonspherical molecules affect the isotropic spectral distributions. It is also of great practical importance, since it is applicable to a wide variety of real macromolecules such as fibrous proteins, helical polypeptides, and some viruses (e.g., tobacco mosaic virus). 

For the rod-shaped particles of tobacco mosaic virus of length 280 nm at room temperature, for example, rotational diffusion coefficients of 550 s" were found. The rods thus require 0.0018 s in order to return to their equilibrium (randomly oriented) positions. 

The relaxation time t,- can be determined from the slope of experimentally birefringent decay curves by constructing a tangent to the decay curve at the time ti, where i = 1,2,3,4,... and t, is the time required for the birefringence to decay to exp(—i) of its initial steady-state value. Relaxation times (in meters per second) for a tobacco mosaic virus were found to be tq = 0.54, ti = 0.53, T2 = 0.53, T3 = 0.53, and T4 = 0.54. Calculate the rotational diffusion coefficient for this vims (O Konski and Haltner, 1956). 

R. Cush, D. Dorman, and P. S. Russo. Rotational and translational diffusion of tobacco mosaic virus in extended and globular polymer solutions. Macromolecules, 37 (2004), 9577-9584. 

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