Zinc metalloproteases hydrolyzing the peptide bonds have been widely studied. We determined the crystal structure of a zinc metalloprotease from Streptomyces caespitosus (ScNP) at 1.6 9 resolution previously by the multiple isomorphous replacement method. ScNP was a small zinc protease composed of 132 amino acid residues with one disulfide bond. Its molecular mass is about 14,500. Structure of ScNP reveals that it is a structurally novel metalloprotease with an aspartate as a zinc ligand.Crystals of ScNP were prepared using acetone as a precipitant at 4°C in the refrigerator. Since the acetone is volatile, the mother liquor for the crystals was gradually replaced with a 50% MPD solution containing 5mM calcium acetate before X-ray experiments. Crystals in this mother liquor belong to the orthorhombic space group, P2 1 2 1 2 1 (a=55.21, b=55.27 and c=37.60 9) and diffract up to 1.5 9 resolution at room temperature. However, when crystals in original mother liquor were carefully mounted in a X-ray capillary and attached to the imaging-plate detector (R-AXIS CS) equipped on the MoKα rotating anode X-ray generator, we found that they diffracted to atomic resolution.In order to determine the more precise structure of ScNP, especially the structure around catalytic zinc and the positions of hydrogen atoms participating in the enzymatic reaction, we began the structural analysis of ScNP at atomic resolution. Whole diffraction data could be collected up to 1.0 9 resolution at 4 °C using one crystal without radiation damage. The image data was processed using the PROCESS package (Rigaku). A summary of data collection and processing is given in TABLE 1. Atomic positions and isotropic temperature factors are being refined against 1.0 9 data by restrained least-squares minimization using the PROLS Q. Currently, crystallographic and free R-factors are 0.20 and 0.22, respectively. Electron density map calculated at this point is very clear, the C β carbons is recognized as the sphere and the nitrogen and oxygen is easily distinguished. In order to collect higher resolution data, we collected the diffraction data up to 0.8 9 at KEK-PF (BL-18B). Now more accurate refinement is in progress to assign the positions of the hydrogen atoms.