Comparative studies on the primary structures and inhibitory properties of subtilisin-trypsin inhibitors from Streptomyces.

Three novel proteinaceous inhibitors of serine proteases which had been identified as Streptomyces subtilisin inhibitor-like (SIL) inhibitors were isolated from culture supernatant of Streptomyces; SIL2 from Streptomyces parvulus, SIL3 from Streptomyces coelicolor and SIL4 from Streptomyces lavendulae. They exhibited not only strong inhibitory activity toward subtilisin BPN' but also less strong inhibition of trypsin. Their primary sequences were determined by sequence analysis of peptides obtained by specific cleavage at the reactive site and subsequent proteolytic digestion. Each inhibitor consisted of about 110 amino acids, and was considered to form a dimer. The reactive site of the inhibitors was identified as Arg-Glu for SIL2 and SIL3, and Lys-Leu for SIL4, from sequence analysis of modified forms of the inhibitors produced from the inhibitor-subtilisin complex under acidic conditions. The presence of an arginine/lysine residue at the P1 site was in agreement with their trypsin-inhibition property. Sequence comparison with other members of the Streptomyces subtilisin inhibitor family revealed that amino acid replacements in the three isolated SIL inhibitors were frequently localized on the surface region, and many of the amino acid residues in beta-sheets and the hydrophobic core were highly conserved. Values of the inhibitor constant (Ki) toward subtilisin BPN' and trypsin were also measured, and the differences were discussed on the basis of the determined structures of the inhibitors.