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Brome mosaic virus (BMV), a T = 3 icosahedral plant virus, can be dissociated
into coat protein subunits and subunit oligomers at pH 7.5 in the presence of
concentrated salts. We have found that during the course of this treatment the
coat protein subunits are cleaved, presumably by plant cell proteases still
present in the preparation, between amino acids 35 and 36. The truncated protein
subunits will then reorganize into T = 1 icosahedral particles and can be
crystallized from sodium malonate. Quasi elastic light scattering 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 shedding of
hexameric capsomeres. The latter process has been directly visualized using
atomic force microscopy. Native T = 3 virions have been crystallized in several
different crystal forms, but neither a rhombohedral form nor either of two
orthorhombic forms diffract beyond about 3.4 A. Tetragonal crystals of the T = 1
particles, however, diffract to at least 2.5 A resolution. Evidence suggests
that the T = 1 particles are more structurally uniform and ordered than are
native T = 3 virions. A variety of anomalous virus particles having diverse
sizes have been visualized in preparations of BMV used for crystallization. In
some cases these aberrant particles are incorporated into growing crystals where
they are frequently responsible for defect formation.
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