Mastoparan peptide causes mitochondrial permeability transition not by interacting with specific membrane proteins but by interacting with the phospholipid phase

The mastoparan peptide is known as an inducer of the mitochondrial permeability transition. Although mastoparan was suggested to interact with a proteinaceous target in mitochondria to induce this transition, the action sites of mastoparan have not yet been investigated. To clarify whether specific interactions of mastoparan with receptors or enzymes are associated with the induction of this permeability transition, we examined the effects of d-isomeric peptides, which were synthesized using d-amino acids assembled in endogenous (inverso mastoparan) and reverse (retro-inverso mastoparan) orientations. When we added inverso mastoparan to isolated mitochondria, the peptide caused the permeability transition in a partially cyclosporin A-sensitive manner at lower doses and in a cyclosporin A-insensitive manner at higher ones. The manners of action and the potencies of inverso mastoparan were close to those of parent mastoparan, indicating that the targets of mastoparan for induction of the permeability transition were neither receptors, nor enzymes in the mitochondria. Retro-inverso mastoparan also had the same effect on the mitochondria as mastoparan, although the potencies of the effect were weaker. Not only on mitochondria, but also on phospholipid vesicles, mastoparan and inverso mastoparan showed massive permeabilization effects at the same potencies, although retro-inverso mastoparan showed weaker ones. These results indicate that mastoparan interacted with the phospholipid phase of the mitochondrial membrane (and not with specific proteins) to induce the permeabilization in cyclosporin A-sensitive and -insensitive manners.