An anti-microbial peptide derivative of flesh fruit fly mimics secretory signal sequence and inhibits signal peptidase-I in the export pathway

Combinatorial search of the antimicrobial peptide R7SLCLLHCRLK from flesh fruit fly yielded a substantially more active peptide of the sequence KLKL5KLK-NH2 that had signal sequence character as revealed by Neural-network survey. Bioinformatics survey of KLKLnKLK revealed a sigmoidal relationship between SSP and the intervening Leu stretch. Synthetic enantiomeric KLKLnKLK peptides inhibited Escherichia coli signal peptidase-I, in vitro, in correlation with their SSPs; KLKL6(7)KLK exterted maximum inhibition. Both (l)-and (d)-forms were bactericidal to Gram-positive and Gram-negative bacteria. However, the protease-resistant (d)-KLKL6KLK-NH2 proved more potent than (d)-KLKL6KLK-NH2 at inhibiting the bacterial protein secretion prior to inducing bacterial lysis. Kinetic analyses of the interaction of these peptides with the signal peptidase-I revealed competitive inhibition with Ki of 10 μM and 35 μM for the (d)- and (l)-forms, respectively. The left and right-handed helicity of the respective peptides assessed by CD concurs with their probable interaction at the active site of signal peptidase-I.