A common landscape for membrane-active peptides

Abstract

Three families of membrane-active peptides are commonly found in nature and are classified according to their initial apparent activity. Antimicrobial peptides are ancient components of the innate immune system and typically act by disruption of microbial membranes leading to cell death. Amyloid peptides contribute to the pathology of diverse diseases from Alzheimer's to type II diabetes. Preamyloid states of these peptides can act as toxins by binding to and permeabilizing cellular membranes. Cell-penetrating peptides are natural or engineered short sequences that can spontaneously translocate across a membrane. Despite these differences in classification, many similarities in sequence, structure, and activity suggest that peptides from all three classes act through a small, common set of physical principles. Namely, these peptides alter the Brownian properties of phospholipid bilayers, enhancing the sampling of intrinsic fluctuations that include membrane defects. A complete energy landscape for such systems can be described by the innate membrane properties, differential partition, and the associated kinetics of peptides dividing between surface and defect regions of the bilayer. The goal of this review is to argue that the activities of these membrane-active families of peptides simply represent different facets of what is a shared energy landscape.

Figure 1

Helical wheel diagrams for three representative peptides discussed in this review. These are α-synuclein, which forms a PAT associated with disease progression in Parkinson's disease, LL-37, a human AMP, and Pep-1, a designed CPP. Members of these three classes of peptide generally possess an amphipathic structure, as seen here, with nonpolar and polar/charged residues segregated on opposite faces of an α-helix. This allows peptides to partition onto membranes, with the nonpolar regions residing in the acyl core and the polar regions contacting the lipid headgroups and solvent. Green circles represent nonpolar residues, yellow polar uncharged residues, red acidic residues, and blue basic residues. Arrows represent the hydrophobic moment of the shown helix. Helical wheel diagrams produced using MPEx.

Figure 2

Cross-cooperative membrane leakage induced by l- and d-enantiomers of amyloid protein IAPP. Liposomes were prepared so as to encapsulate 70 kDa fluorescently labeled dextrans. These were incubated with the indicated concentration of protein for 48 h. Membrane integrity was then interrogated through the extraluminal introduction of the fluorescence quencher DPX, which transits into the liposome and quenches the entrapped fluors. Doubling the concentration of IAPP or d-IAPP individually (blue bold, purple bold) leads to an ∼100-fold increase in leakage rate constant over the initial concentrations (blue thin, purple thin). Instead, mixing equal activities of the two peptides brings about the same highly cooperative increase in leakage rate (orange). This observed leakage is more than an order of magnitude faster than the expected leakage rate if the peptides acted noncooperatively (black). Figure adapted from Ref..

Figure 3

Schematic of the effect of protein binding on lipid bilayer integrity. (i) The reference state for energy change is an intact phospholipid bilayer. (ii) Spontaneous fluctuations result in the sampling of membrane defects. These are energetically unfavorable and therefore sampled infrequently. (iii) Widening of the defect to permit leakage results in a further energetic penalty. (iv) In the presence of surface-bound protein (magenta), membrane tension is induced. (v) Protein binding increases the frequency of defect formation. (vi) Surface tension is released by pore formation and stabilized by peptide binding resulting in equilibrium poration (vii). Note, many forms of defect, such as chaotic pores, can be accommodated by this model, and defect characteristics may differ between alternate peptides or the same peptide under alternate conditions.