PepN is the major aminopeptidase in Escherichia coli: insights on substrate specificity and role during sodium-salicylate-induced stress

Abstract

PepN and its homologues are involved in the ATP-independent steps (downstream processing) during cytosolic protein degradation. To obtain insights into the contribution of PepN to the peptidase activity inEscherichia coli, the hydrolysis of a selection of endopeptidase and exopeptidase substrates was studied in extracts of wild-type strains and twopepNmutants, 9218 and DH5αΔpepN. Hydrolysis of three of the seven endopeptidase substrates tested was reduced in bothpepNmutants. Similar studies revealed that hydrolysis of 10 of 14 exopeptidase substrates studied was greatly reduced in bothpepNmutants. This decreased ability to cleave these substrates ispepN-specific as there is no reduction in the ability to hydrolyse exopeptidase substrates inE. colimutants lacking other peptidases,pepA,pepBorpepE. PepN overexpression complemented the hydrolysis of the affected exopeptidase substrates. These results suggest that PepN is responsible for the majority of aminopeptidase activity inE. coli. Furtherin vitrostudies with purified PepN revealed a preference to cleave basic and small amino acids as aminopeptidase substrates. Kinetic characterization revealed the aminopeptidase cleavage preference ofE. coliPepN to be Arg>Ala>Lys>Gly. Finally, it was shown that PepN is a negative regulator of the sodium-salicylate-induced stress inE. coli, demonstrating a physiological role for this aminoendopeptidase under some stress conditions.