Membrane Thinning and Thickening Induced by Membrane-Active Amphipathic Peptides

"Force-from-lipids" gating of mechanosensitive channels modulated by PUFAs

The level of fatty acid saturation in phospholipids is a crucial determinant of the biophysical properties of the lipid bilayer. Integral membrane proteins are sensitive to changes of their bilayer environment such that their activities and localization can be profoundly affected. When incorporated into phospholipids of mammalian cells, poly-unsaturated fatty acids (PUFAs) determine the mechanical properties of the bilayer thereby affecting several membrane-associated functions such as endo- and exo-cytosis and ion channel/membrane receptor signalling cascades. In order to understand how membrane tension is propagated through poly-unsaturated bilayers, we characterized the effect of lipid saturation on liposome reconstituted MscS and MscL, the two bacterial mechanosensitive ion channels that have for many years served as models of ion- channel-mediated mechanotransduction. The combination of NMR and patch clamp experiments in this study demonstrate that bilayer thinning is the main responsible factor for the modulation of the MscL threshold of activation while a change in transbilayer pressure profile is indicated as the main factor behind the observed modulation of the MscS kinetics. Together, our data offer a novel insight into how the structural shape differences between the two types of mechanosensitive channels determine their differential modulation by poly-unsaturated phospholipids and thus lay the foundation for future functional studies of eukaryotic ion channels involved in the physiology of mechanosensory transduction processes in mammalian cells.

SUMMARY

Mechanosensitive channels MscL and MscS are differentially modulated by poly-unsaturated fatty acids in lipid bilayers. MscL becomes sensitized because of increased hydrophobic mismatch while MscS open state is stabilized due to changes in the bilayer lateral pressure profile determined by NMR.

Antimicrobial peptide cWFW kills by combining lipid phase separation with autolysis

The synthetic cyclic hexapeptide cWFW (cyclo(RRRWFW)) has a rapid bactericidal activity against both Gram-positive and Gram-negative bacteria. Its detailed mode of action has, however, remained elusive. In contrast to most antimicrobial peptides, cWFW neither permeabilizes the membrane nor translocates to the cytoplasm. Using a combination of proteome analysis, fluorescence microscopy, and membrane analysis we show that cWFW instead triggers a rapid reduction of membrane fluidity both in live Bacillus subtilis cells and in model membranes. This immediate activity is accompanied by formation of distinct membrane domains which differ in local membrane fluidity, and which severely disrupts membrane protein organisation by segregating peripheral and integral proteins into domains of different rigidity. These major membrane disturbances cause specific inhibition of cell wall synthesis, and trigger autolysis. This novel antibacterial mode of action holds a low risk to induce bacterial resistance, and provides valuable information for the design of new synthetic antimicrobial peptides.

Orientation and Location of the Cyclotide Kalata B1 in Lipid Bilayers Revealed by Solid-State NMR

Cyclotides are ultra-stable cyclic disulfide-rich peptides from plants. Their biophysical effects and medically interesting activities are related to their membrane-binding properties, with particularly high affinity for phosphatidylethanolamine lipids. In this study we were interested in understanding the molecular details of cyclotide-membrane interactions, specifically with regard to the spatial orientation of the cyclotide kalata B1 from Oldenlandia affinis when embedded in a lipid bilayer. Our experimental approach was based on the use of solid-state 19F-NMR of oriented bilayers in conjunction with the conformationally restricted amino acid L-3-(trifluoromethyl)bicyclopent-[1.1.1]-1-ylglycine as an orientation-sensitive 19F-NMR probe. Its rigid connection to the kalata B1 backbone scaffold, together with the well-defined structure of the cyclotide, allowed us to calculate the protein alignment in the membrane directly from the orientation-sensitive 19F-NMR signal. The hydrophobic and polar residues on the surface of kalata B1 form well-separated patches, endowing this cyclotide with a pronounced amphipathicity. The peptide orientation, as determined by NMR, showed that this amphipathic structure matches the polar/apolar interface of the lipid bilayer very well. A location in the amphiphilic headgroup region of the bilayer was supported by 15N-NMR of uniformly labeled protein, and confirmed using solid-state 31P- and 2H-NMR. 31P-NMR relaxation data indicated a change in lipid headgroup dynamics induced by kalata B1. Changes in the 2H-NMR order parameter profile of the acyl chains suggest membrane thinning, as typically observed for amphiphilic peptides embedded near the polar/apolar bilayer interface. Furthermore, from the 19F-NMR analysis two important charged residues, E7 and R28, were found to be positioned equatorially. The observed location thus would be favorable for the postulated binding of E7 to phosphatidylethanolamine lipid headgroups. Furthermore, it may be speculated that this pair of side chains could promote oligomerization of kalata B1 through electrostatic intermolecular contacts via their complementary charges.