The Three-dimensional Structure of Plasminostreptin, a Bacterial Protein Protease Inhibitor, at 2.8Å Resolution

Abstract

Abstract The three-dimensional structure of plasminostreptin, a bacterial protein protease inhibitor from Streptomyces antifibrinolyticus, was determined at 2.8Å resolution by the multiple isomorphous replacement technique. A dimer with the molecular weight of 11,402×2 was included in an asymmetric unit of the crystal. The monomer, composed of 109 amino acid residues, consisted of an antiparallel twisted β-sheet with five strands, two short helices, and irregular polypeptide-chain segments. The reactive site of plasminostreptin belonged to one of these irregular polypeptide-chain segments. The two monomers in the dimer were related by a non-crystallographic pseudo-two-fold axis, and the two β-sheets were arranged face-to-face with each other. The averaged angle between the strands of the two β-sheets was approximately 30° clockwise. In spite of 37 amino acid substitutions including the reactive site residues, the tertiary and quaternary structures of plasminostreptin were similar to those of Streptomyces subtilisin inhibitor (SSI). The main-chain conformations of the reactive site of plasminostreptin were similar to those of SSI and other protease inhibitors analysed previously. Moreover, the conformations of the side chain of the evolutionary conservative asparagine in the so-called “secondary contact region” resembled those found in the inhibitors of the pancreatic secretory trypsin inhibitor family.