Crystal structure of an intracellular protease from Pyrococcus horikoshii at 2-angstrom resolution

The intracellular protease from Pyrococcus horikoshii (PH1704) and PfpI fro m Pyrococcus furiosus are members of a class of intracellular proteases tha t have no sequence homology to any other known protease family. We report t he crystal structure of PH1704 at 2.0-Angstrom resolution. The protease is tentatively identified as a cysteine protease based on the presence of cyst eine (residue 100) in a nucleophile elbow motif. In the crystal, PH1704 for ms a hexameric: ring structure, and the active sites are formed at the inte rfaces between three pairs of monomers.