Crystallization and molecular symmetry of ornithine decarboxylase from Lactobacillus 30a

Ornithine decarboxylase from Lactobacillus 30a is representative of the large subunit (80 kDa), oligomeric, pyridoxal phosphate-dependent amino-acid decarboxylases. Yellow crystals of ornithine decarboxylase are obtained from polyethylene glycol solutions and belong to space group P6 with unit cell constants a = b = 194.9 and c = 97.44 A, alpha = beta = 90 degrees and gamma = 120 degrees, V = 3.21 x 10(6) A3. Still photographs show reflections at better than 2.4-A resolution. Electron micrographs reported by Guirard and Snell (Guirard, B.M., and Snell, E.E. (1980) J. Biol. Chem. 255, 5960-5964) reveal that the ornithine decarboxylase dodecamer is a hexagonally shaped particle with a point-to-point distance of approximately 210 A and a thickness of approximately 70 A. The crystallographic unit cell can thus accommodate one 10(6)-Da dodecamer (Vm = 3.2 A3/Da), implying that a dimer occupies an asymmetric unit. Tanaka rotation function analysis, using native data (5-7 A) collected from three crystals, reveals that the particle has the expected 622 molecular symmetry with molecular 2-fold axes lying at 20 degrees and 50 degrees from a in the a-b plane. A search for suitable heavy atom derivatives is underway.