Role of Helix Nucleation in the Kinetics of Binding of Mastoparan X to Phospholipid Bilayers^†

Many antimicrobial peptides undergo a coil-to-helix transition upon binding to membranes. While this conformational transition is critical for function, little is known about the underlying mechanistic details. Here, we explore the membrane-mediated folding mechanism of an antimicrobial peptide, mastoparan X. Using stopped-flow fluorescence techniques in conjunction with a fluorescence resonance energy transfer (FRET) pair, p-cyanophenylalanine (donor) and tryptophan (acceptor), we were able to probe, albeit in an indirect manner, the membrane-mediated folding kinetics of this peptide. Our results show that the association of mastoparan X with model lipid vesicles proceeds with biphasic kinetics. The first step shows a large change in the FRET signal, indicating that the helix forms early in the time course of the interaction, while the second step where a further increase in tryptophan fluorescence is observed presumably reflects deeper insertion of the peptide into the bilayer. Additional kinetic studies on a double mutant of mastoparan X, designed to form a nucleation site for α-helix formation through coordination with a metal ion (e.g., Zn²⁺ or Ni²⁺), indicate that while the coil-to-helix transition occurs in the first step, it follows the rate-determining docking of the peptide onto the membrane surface. Taken together, these results indicate that the initial association of the peptide with the membrane occurs in a nonhelical conformation, which rapidly converts to a helical state within the anisotropic environment of the bilayer surface.