title: Heavy riboflavin synthase from Bacillus subtilis. Particle dimensions, crystal packing and molecular symmetry.
abstract: Heavy riboflavin synthase from Bacillus subtilis is an enzyme complex consisting of approximately three alpha-subunits (Mr 23.5 X 10(3)) and 60 beta-subunits (Mr 16 X 10(3)). The enzyme has been crystallized from phosphate buffer in a hexagonal crystal modification that belongs to space group P6(3)22. The asymmetric unit of the crystal cell contains ten beta-subunits. The structure of this unusual 10(6) Mr protein has been studied by small-angle X-ray scattering, electron microscopy of three-dimensional crystals, and crystallographic methods. The scattering curves can be interpreted in terms of a hollow sphere model with a ratio of inner and outer radius of 0.3:1. A diameter of 168 A was estimated from the scattering curves, in close agreement with electron microscopic studies. An aggregate with the stoichiometry beta 60, which was obtained by ligand-driven reaggregation of isolated beta-subunits, showed similar shape and dimensions, but a larger value for the ratio Ri/Ra. Electron micrographs of freeze-etched enzyme crystals showed approximately spherical molecules, which were arranged in hexagonal layers. The lattice constants found from the micrographs are in good agreement with the values derived from X-ray diffraction data. Rotation function calculations in Patterson space showed a set of peaks for 2-fold, 3-fold and 5-fold local rotation axes, accurately consistent with icosahedral symmetry and with the particle orientation A shown in the Appendix. The crystal packing can be described as follows: enzyme particles with icosahedral symmetry (point group 532) are located at points 32 of the hexagonal cell, corresponding to positions (0, 0, 0) and (0, 0, 1/2) on the 6-fold screw axes. From the data reported, it may be concluded that the enzyme structure can be described as an icosahedral capsid of 60 beta-subunits with the triangulation number T = 1. The alpha-subunits are located in the central core space of the capsid, but their spatial orientation is incompletely understood.
authors.forename: R
authors.initials: R
authors.lastname: Ladenstein
authors.forename: B
authors.initials: B
authors.lastname: Meyer
authors.forename: R
authors.initials: R
authors.lastname: Huber
authors.forename: H
authors.initials: H
authors.lastname: Labischinski
authors.forename: K
authors.initials: K
authors.lastname: Bartels
authors.forename: H D
authors.initials: HD
authors.lastname: Bartunik
authors.forename: L
authors.initials: L
authors.lastname: Bachmann
authors.forename: H C
authors.initials: HC
authors.lastname: Ludwig
authors.forename: A
authors.initials: A
authors.lastname: Bacher
mesh_terms: D001412:Bacillus subtilis / Q000201:enzymology* / Q000648:ultrastructure; D055598:Chemical Phenomena; D002621:Chemistry; D003461:Crystallography; D005613:Freeze Etching; D046911:Macromolecular Substances; D008433:Mathematics; D008854:Microscopy, Electron; D012258:Riboflavin Synthase / Q000302:isolation & purification / Q000378:metabolism*; D012542:Scattering, Radiation; D014166:Transferases / Q000378:metabolism*; D014961:X-Ray Diffraction
chemical_list: D046911:Macromolecular Substances; D014166:Transferases; D012258:Riboflavin Synthase