Identification of OmpT as the protease that hydrolyzes the antimicrobial peptide protamine before it enters growing cells of Escherichia coli

The influence of extracytoplasmic proteases on the resistance of Escherichia coli to the antimicrobial peptide protamine was investigated by testing strains with deletions in the protease genes degP, ptr, and ompT. Only DeltaompT strains were hypersusceptible to protamine. This effect was abolished by plasmids carrying ompT. Both at low and at high Mg2+ concentrations, ompT+ strains cleared protamine from the medium within a few minutes. By contrast, at high Mg2+ concentrations, protamine remained present for at least 1 h in the medium of an ompT strain. These data indicate that OmpT is the protease that degrades protamine and that it exerts this function at the external face of the outer membrane.