An 84-residue bactericidal peptide, PSI(, was purified from a Chrysomya megacephala fly larvae preparation. Its amino acid sequence is similar to that of a previously reported larval peptide of the Drosophila genus (SK84) noticed for its anticancer and antimicrobial properties. The PSK sequence is also homologous to mitochondrial ATPase inhibitors from insects to humans (35-65% sequence identity), indicating an intracellular protein target and possible mechanism for PSK. It contains a cluster of six glycine residues, and has several two- and three-residue repeats. It is active against both Gram-positive and Gram-negative bacteria via a mechanism apparently involving cell membrane disintegration and inhibition of ATP hydrolysis. In addition, PSI( induces an inward cationic current in pancreatic 13 cells. Together, the findings identify a bioactive peptide of the ATPase inhibitor family with specific effects on both prokaryotic and mammalian cells. (C) 2014 Elsevier Inc. All rights reserved.