A novel proteinaceous Kex 2 proteinase inhibitor, kexstatin, from Streptomyces platensis Q268

We found a novel proteinaceous Kex 2 proteinase inhibitor, named kexstatin, in the culture supernatant of Streptomyces platensis Q268. The purified kexstatin was homogeneous by SDS-PAGE and the molecular weight was estimated to be 13,000. The N-terminal amino acid sequence of kexstatin has high similarity to Streptomyces subtilisin inhibitor (SSI), suggesting that kexstatin belongs to the SSI family. Kexstatin was a strong inhibitor of Kex 2 proteinase and subtilisin but not thermolysin, trypsin, or chymotrypsin. The IC50 value of kexstatin against 1 mu g of Kex 2 proteinase was 1.4 mu g.