Molecular Mechanism of Synergy between the Antimicrobial Peptides PGLa and Magainin 2 in Membranes

Abstract

The two antimicrobial peptides PGLa and magainin 2 (MAG2), both found in the skin of the African frog Xenopus laevis, are known to exhibit strong synergistic effects in bacterial killing, lipid vesicle leakage, and other membrane-related activities [1]. Here, we studied the synergy of the two peptides in an attempt to find the molecular mechanism behind their synergistic effect [2]. A large number of mutants of the two peptides were investigated, and the synergy tested was using three complementary methods: (i) a checkerboard assay against two bacteria; (ii) a vesicle leakage assay; and (iii) 15N-NMR, showing the insertion of PGLa into a transmembrane orientation (suggesting the formation of a pore) in the presence of MAG2. Several mutations were identified that reduced or inhibited the synergy of the peptides, and a single mutation on PGLa from Gly to Ala was enough to completely abolish synergy. At the same time, the designer-made and related but completely non-synergistic peptide MSI-103 could be modified by two point mutations to become strongly synergistic with MAG2. Also in this case Gly residues were involved, indicating that a GxxxG motif on PGLa is critical and necessary for synergy. Based on these results, a 3D model could be developed of the PGLa-MAG2 complex in the membrane that is synergistically active. An oligomeric pore is likely formed from antiparallel dimers of transmembrane PGLa peptides, wherein each PGLa molecule makes contact to a MAG2 peptide that is aligned on the membrane surface.