Phosphate-dependent aggregation of [KL]$_{n}$ peptides affects their membranolytic activity

In this study, we investigate how the length of amphiphilic β-sheet forming peptides affects their interaction with membranes. Four polycationic model peptides with lengths from 6 to 18 amino acids were constructed from simple Lys-Leu repeats, giving [KL]$_{n=3,5,7,9}$. We found that (1) they exhibit a pronounced antimicrobial activity with an intriguing length dependent maximum for [KL]$_{5}$ with 10 amino acids; (2) their hemolytic effect, on the other hand, increases steadily with peptide length. CD analysis (3) and TEM (4) show that all peptides-except for the short [KL]$_{3}$-aggregate into amyloid-like fibrils in the presence of phosphate ions, which in turn has a critical effect on the results in (1) and (2). In fact, (5) vesicle leakage reveals an intrinsic membrane-perturbing activity (at constant peptide mass) of [KL]$_{5}$ > [KL]$_{9}$ > [KL]7 in phosphate buffer, which changes to [KL]$_{5}$ ≈ [KL]$_{7}$ ≈ [KL]$_{9}$ in PIPES. A specific interaction with phosphate ions thus explains the subtle balance between two counteracting effects: phosphate-induced unproductive pre-aggregation in solution versus monomeric membrane binding and vigorous lipid perturbation due to self-assembly of the bound peptides within the bilayer. ... mehrThis knowledge can now be used to control and optimize the peptides in further applications.