Both the phosphatidylinositol-hydrolysing and the phosphatidylcholine-hydrolysing phospholipases C have been implicated in the generation of second messengers in mammalian cells. The phosphatidylcholine-hydrolysing phospholipase C (PLC) from Bacillus cereus, a monomeric protein containing 245 amino-acid residues, is similar to some of the corresponding mammalian proteins. This, together with the fact that the bacterial enzyme can mimic the action of mammalian PLC in causing, for example, enhanced prostaglandin biosynthesis, suggests that B. cereus PLC can be used as a model for the hitherto poorly characterized mammalian PLCs. We report here the three-dimensional structure of B. cereus PLC at 1.5 A resolution. The enzyme is an all-helix protein belonging to a novel structural class and contains, at least in the crystalline state, three Zn2+ in the active site. We also present preliminary results from a study at 1.9 A resolution of the complex between PLC and inorganic phosphate (Pi) which indicate that the substrate binds directly to the metal ions.

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The ?tructure of triclinic hen-egg white lysozyme is refined at 21 resolution to an Rvalue of 0.169. In addition to 1001 protein atoms, the present model consists of 239 solvent molecules ranging in occupancies from 1.0 to 0.5. Individual isotropic B-values are refined for all the atoms. B-values are found to be generally smaller than those obtained in most of the other protein structuies. Average B-values for the main-chain atoms, side-chain atoms and 2 for the entire protein are 8.2, 9.6

Introduction. Intensity data from a single crystal, 0.35 × 0.35 × 0.20 mm, were recorded at room temperature on a Nonius CAD-4 computer-controlled diffractometer with graphite-monochromatized Mo radiation and the 09--20 scan mode. Measurements of 3458 independent reflexions, including 969 with negative intensities, were made with 0 < 24 °. All reflexions were included in the structure refinement, the unobserved (mentioned above) were given I = 0 and appeared in the refinement with AF = -F c. Two standard reflexions were measured regularly throughout the data collection and the intensity of both dropped by 22%. 15% of this decay was in the first half of data collection when the surface of the crystal became opaque. Once the crystal became coated, the intensities decayed at a much slower rate. Lp corrections were applied, but an absorption correction was ignored.

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