Divalent cations modulate membrane binding and pore formation of a potent antibiotic peptide analog of alamethicin

The Ca2+ modulation of pore formation (and disaggregation) kinetics of a synthetic analog of alamethicin F50/5 ([l-Glu(OMe)7,18,19]), a potent antibiotic peptide, was investigated in situ and in vitro. The in situ experiments consisted in whole-cell recording from isolated retinal rod outer segments (OS), because once blocking the only OS endogenous conductance with saturating light, the current flows entirely through the (exogenous) channels formed by the peptide. The kinetics of current change induced by peptide application and removal (in ∼50 ms) on the OS extracellular side was measured in the presence of divalent cations at different concentrations. The in vitro experiments consisted on the divalent cations modulation of [l-Glu(OMe)7,18,19] binding to a mimetic OS membrane immobilized on a sensor chip surface, employing surface plasmon resonance spectroscopy (SPR). The presence of even low mM Ca2+ or Mg2+ sufficed to increase the [l-Glu(OMe)7,18,19] apparent affinity for the mimetic OS membrane up to ∼4-fold, which accelerated the activation of the peptide-induced current in OS by ∼10-fold with respect to low Ca2+. In situ and in vitro experiments indicate that high concentrations of divalent cations increased also membrane rigidity, contrasting their effect on increasing the pore formation rate.