The thin line between cell-penetrating and antimicrobial peptides: the case of Pep-1 and Pep-1-K

Cell-penetrating peptides (CPPs) are cationic oligopeptides able to translocate across biological membranes without perturbing them, while antimicrobial peptides (AMPs) kill bacteria mainly by disrupting their membranes. The two peptide classes share several characteristics (charge, amphipathicity, helicity, and length), and therefore the molecular properties discriminating between the two different bioactivities are not clear. Pep-1-K (KKTWWKTWWTKWSQPKKKRKV) is a new AMP derived from the widely studied CPP Pep-1 (KETWWETWWTEWSQPKKKRKV), or 'Chariot', known for its ability to carry large cargoes across biological membranes. Pep-1-K was obtained from Pep-1 by substituting the three Glu residues with Lys, to increase its cationic character. Previous studies showed that these modifications endow Pep-1-K with a potent antimicrobial activity, with MICs in the low micromolar range. Here, we characterized the interaction of Pep-1 and Pep-1-K with model membranes to understand the reason for the antimicrobial activity of Pep-1-K. The data show that this peptide causes vesicle aggregation, perturbs membrane order, and induces the leakage of ions, but not of larger solutes, while these effects were not observed for Pep-1. These differences are likely due, at least in part, to the higher affinity of Pep-1-K toward anionic bilayers, which mimick the composition of bacterial membranes.