Crystallization of brome mosaic virus and T=1 brome mosaic virus particlesfollowing a structural transition

Brome mosaic virus (BMV), a T = 3 icosahedral plant virus, can be dissociat ed into coat protein subunits and subunit oligomers at pH 75 in the presenc e of concentrated salts. We have found that during the course of this treat ment the coat protein subunits are cleaved, presumably by plant cell protea ses still present in the preparation, between amino acids 35 and 36. The tr uncated protein subunits will then reorganize into T = 1 icosahedral partic les and can be crystallized from sodium malonate. Quasi elastic light scatt ering and atomic force microscopy results suggest that the transition from T = 3 to T = 1 particles can occur by separate pathways, dissociation into coat protein subunits and oligomers and reassembly into T = 1 particles, or direct condensation of the T = 3 virions to T = 1 particles with the shedd ing of hexameric capsomeres. The latter process has been directly visualize d using atomic force microscopy. Native T = 3 virions have been crystallize d in several different crystal forms, but neither a rhombohedral form nor e ither of two orthorhombic forms diffract beyond about 3.4 Angstrom. Tetrago nal crystals of the T = 1 particles, however, diffract to at least 256 Angs trom resolution. Evidence suggests that the T = 1 particles are more struct urally uniform and ordered than are native T = 3 virions. A variety of anom alous virus particles having diverse sizes have been visualized In preparat ions of BMV used for crystallization. In some cases these aberrant particle s are incorporated into growing crystals where they are frequently responsi ble for defect formation. (C) 2001 Academic Press.