The amino acid sequence-specific protease (termed GPR) in the bacterium Bacillus megaterium initiates the rapid degradation of small, acid-soluble spore proteins during the germination of spores of this organism. GPR is synthesized during spore formation as an inactive zymogen termed P-46, which later autoprocesses to a smaller active form termed P-41, which acts during spore germination. However, GPR exhibits no obvious mechanistic or amino acid sequence similarity to any of the known classes of proteases. To initiate the determination of the mechanisms of P-46 to P-41 conversion, P-46 inactivity, and P-41 catalysis, B. megaterium GPR has been overexpressed in Escherichia coli and purified to homogeneity by anion-exchange and size exclusion chromatography, and crystals of both P-46 and P-41 have been obtained by the vapor diffusion method. P-46 crystals diffracted x rays to 3.5 Angstrom but the crystals of P-41 diffracted x rays to only 6.5 Angstrom. A native x-ray diffraction data set of P-46 has been collected; the unit cell parameters are a = b = 76.8, c = 313.1 Angstrom, alpha = beta = gamma = 90 degrees; the space group is tetragonal P4(1)2(1)2 or P4(3)2(1)2. The asymmetric unit contains two monomeric molecules with a crystal volume per unit protein mass of 2.85 Angstrom(3)/Da and a solvent content of about 57%. An isomorphous heavy atom derivative data set has also been obtained for P46 crystals with potassium dicyanoaurate (I).