A subtilisin inhibitor produced by Streptomyces bikiniensis possesses a glutamine residue at reactive site P1

We determined the complete amino acid sequence of a novel subtilisin inhibitor, SIL15, which had been isolated from the culture supernatant of Streptomyces bikiniensis and shown to be a member of the Streptomyces subtilisin inhibitor (SSI)-like (SIL) protein family, and then identified its reactive site. SIL15 is composed of 113 amino acids and exists as a dimer. Compared with other SSI-family inhibitors, SIL15 was found to be unique in that it possesses a Gin residue at the PI site of the reactive site and has two-residue insertions in two regions, one in the a \-helix and the other in the flexible loop region near the reactive site. Inhibition of subtilisin BPN ' by SIL15 (inhibitor constant, 2.7 X10" " M) was due to the presence of a Gin residue at the PI site, which was well consistent with the results obtained for Pl-site mutants of SSI and turkey ovomucoid domain 3. Key words: amino acid sequence, PI site, protease inhibitor, Streptomyces subtilisin inhibitor-like protein. Recently, we isolated many proteinaceous protease in-hibitors, which show strong inhibition of subtilisin BPN', from the culture supernatants of various Streptomyce