Proteolytic cleavage by neutrophil elastase converts inactive storage proforms to antibacterial bactenecins

Bac5 and Bac7, antibiotics of the bactenecin (proline/arginine‐rich peptide) family, are stored as proforms in the large granules of bovine neutrophils [Zanetti, M., Litteri, L., Gennaro, R., Horstmann, H. and Romeo, D. (1990) J. Cell Biol. 111 , 1363–1371]. These proforms have been purified to homogeneity from granule extracts by immunoaffinity and reverse‐phase chromatography. While mature bactenecins efficiently kill Escherichia coli, Klebsiella pneumoniae and Salmonella typhimurium with minimal inhibitory concentrations of 6–12 μg/ml, proBac5 and proBac7 do not affect the growth of the same microorganisms, even at 500 μg/ml. Previous investigations have suggested that the conversion of probactenecins into mature antimicrobial peptides is catalyzed by a neutral serine protease stored in the azurophil granules. Purified proBac5 and proBac7 were thus treated with elastase, cathepsin G or proteinase 3, which constitute the pool of neutral serine proteases of the azurophils, and the reaction products were identified by Western blot analysis, mass spectrometry, and N‐terminal sequence analysis. Of the three proteases, only elastase is able to catalyze the stepwise cleavage of probactenecins into the corresponding mature peptides, which have the same mass, N‐terminal sequence and antibiotic activity of authentic Bac5 and Bac7. These results point to the importance of cooperation between azurophils and large granules in mounting a defense reaction.