Structural transitions of satellite tobacco mosaic virus particles

Satellite tobacco mosaic virus (STMV) can undergo at least two physical tra nsitions that significantly alter its mechanical and structural characteris tics. At high pH the 17-nm STMV particles expand radially by about 5 Angstr om to yield particles having diameters of about 18 nm. This pH-induced tran sition is further promoted by aging of the virions and degradation of the R NA, so that swollen particles ultimately appear even at neutral pH. While t he native 17-nm particles crystallize as orthorhombic or monoclinic crystal s which diffract to high resolution (1.8 Angstrom), the enlarged 18-nm pa a rticles crystallize in a cubic form which diffracts to no better than 5 A. In the transition, not only do the capsid protein subunits move radially ou tward, but the helical RNA segments with which they interact do as well. Th is is noteworthy because it demonstrates that the RNA and the protein shell are capable of coordinated movement, and that neither structure is rigidly defined or independent of the other. Using atomic force microscopy, it can be shown that STMV particles, upon drying, lose their mechanical rigidity and undergo deformation. Virions initially 17 nm in diameter shrink to more uniform final sizes than do 18 nm, initially swollen particles. This trans ition appears to be irreversible, as the particles do not reassume their fo rmer size nor structural rigidity upon rehydration. Evidence is also presen ted that preparations of native virus and their crystals are naturally some what heterogeneous and contain a variety of particles of anomalous size. (C ) 2001 Academic Press.