Crystallographic structure of the T=1 particle of brome mosaic virus

T=1 icosahedral particles of amino terminally truncated brome mosaic virus (BMV) protein were created by treatment of the wild-type T = 3 virus with 1 M CaCl2 and crystallized from sodium malonate. Diffraction data were collected from frozen crystals to beyond 2.9 Angstrom resolution and the structure determined by molecular replacement and phase extension. The particles are composed of pentameric capsomeres from the wild-type virions which have reoriented with respect to the original particle pentameric axes by rotations of 37degrees, and formed tenuous interactions with one another, principally through conformationally altered C-terminal polypeptides. Otherwise, the pentamers are virtually superimposable upon those of the original T = 3 BMV particles. The T = 1 particles, in the crystals, are not perfect icosahedra, but deviate slightly from exact symmetry, possibly due to packing interactions. This suggests that the T=1 particles are deformable, which is consistent with the loose arrangement of pentamers and latticework of holes that penetrate the surface. Atomic force microscopy showed that the T = 3 to T = 1 transition could occur by shedding of hexameric capsomeres and restructuring of remaining pentamers accompanied by direct condensation. Knowledge of the structures of the BMV wild-type and T. = 1 particles now permit us to propose a tentative model for that process. A comparison of the BMV T = 1 particles was made with the reassembled T = 1 particles produced from the coat protein of trypsin treated alfalfa mosaic virus (AIMV), another bromovirus. There is little resemblance between the two particles. The BMV particle, with a maximum diameter of 195 Angstrom, is made from distinctive pentameric capsomeres with large holes along the 3-fold axis, while the AIMV particle, of approximate maximum diameter 220 , haAngstroms subunits closely packed around the 3-fold axis, large holes along the 5-fold axis, and few contacts within pentamers. In both particles crucial linkages are made about icosahedral dyads. (C) 2004 Elsevier Ltd. All rights reserved.