Abstract
PGLa and magainin 2 (MAG2) are amphiphilic α-helical membranolytic peptides from frog skin with known synergistic antimicrobial activity. By systematically mutating residues in the two peptides it was possible to identify the ones crucial for the synergy, as monitored by biological assays, fluorescence vesicle leakage, and solid-state 15N-NMR. Electrostatic interactions between anionic groups in MAG2 and cationic residues in PGLa enhance synergy but are not necessary for the synergistic effect. Instead, two Gly residues (7 and 11) in a so-called GxxxG motif in PGLa are necessary for synergy. Replacing either of them with Ala or another hydrophobic residue completely abolishes synergy according to all three methods used. The designer-made peptide MSI-103, which has a similar sequence as PGLa, shows no synergy with MAG2, but by introducing two Gly mutations it was possible to make it synergistic. A molecular model is proposed for the functionally active PGLa-MAG2 complex, consisting of a membrane-spanning antiparallel PGLa dimer that is stabilized by intimate Gly-Gly contacts, and where each PGLa monomer is in contact with one MAG2 molecule at its C-terminus.
Highlights
All organisms are exposed to a large number of microbial species
We have recently shown that the membrane-bound PGLa-magainin 2 (MAG2) complex is thermodynamically more stable than PGLa-PGLa oligomers, which are in turn more stable than MAG2-MAG2 oligomers[30]
A suitable method to study synergistic antimicrobial activity is the so-called checker-board assay, described in the Methods section. We performed such experiments on one Gram-negative bacterium, E. coli, and one Gram-positive one, S. aureus. From this method a fractional inhibitory concentration index (FICI) is determined, and when this is below 0.5, the peptides are said to have a synergistic effect against bacteria
Summary
All organisms are exposed to a large number of microbial species. Innate immune defense systems have emerged as a first line of protection against potential pathogens. Antimicrobial peptides (AMPs), known as host defense peptides, are found in almost all types of organisms as part of this innate immunity[1,2,3] These peptides are often short (10-30 amino acids), cationic, and can form amphipathic structures, and many of them are membrane-active[1,2,3,4,5,6]. The fact that synergy is observed in all these different assays indicates that the effect is based on the primary activity of the peptides, i.e. the permeabilization of membranes This mode of action has been proposed to involve the formation of water-filled pores that are lined by the membrane-spanning amphiphilic peptides[16,23,24,25,26,27]. The membrane permeabilization activity of these cross-linked parallel heterodimers was considerably higher than for each peptide alone, supporting the hypothesis that synergy is linked to heterodimer formation
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
