Abstract:
Material properties of lipid bilayers were studied on the mesoscopic scale using deuterium nuclear magnetic resonance spectroscopy. The fluid phase of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) was compared with DMPC containing a nonionic detergent as an additive. Order parameter profiles were obtained from the deuterium NMR spectra of DMPC having perdeuterated acyl chains (DMPC-J54). A reduction of the order parameters of DMPC-d54 in the presence of the detergent octaethyleneglycol-mono-/i-dodecyl ether (C12E8) was observed, consistent with an increased configurational freedom of the phospholipid acyl chains. Relaxation rates R1z(i) and R1Q(i) were measured and spectral densities Jm(mω0) where in = 1,2 were calculated from the combined relaxation results. Profiles of the observables, i.e., order parameters, relaxation rates, and spectral densities were interpreted within the framework of a new composite membrane deformation model, which describes characteristic properties of the membrane in terms of a continuum picture. According to this model, the influence of the nonionic detergent (C12E8) on the electroneutral DMPC membrane is to increase the membrane flexibility as manifested by the functional dependence of the R1z(i) and R1Q(i) rates, i.e., the dependence of the spectral densities on the corresponding profiles of the orientational order parameters |SCD(i)|. Within the theoretical framework the increased flexibility of the detergent-containing membranes corresponds to a decrease of the elastic constants for continuum (elastic) deformations of the membrane bilayer. In the case of splay fluctuations the elastic constant and the bilayer thickness are related to the macroscopic bending rigidity, which qualitatively yields a correspondence to studies of macroscopic bending fluctuations thus yielding support for the model. In general, these findings indicate a softening of the membrane bilayer by the presence of a nonionic detergent, which corresponds to a decrease of the elastic constants for continuum deformations of the membrane. © 2000 American Chemical Society.
PMID: 20923645;PMCID: PMC3042581;Abstract:
Alpha-synuclein (aS) is a 140-amino-acid protein that is involved in a number of neurodegenerative diseases. In Parkinson's disease, the protein is typically encountered in intracellular, high-molecular-weight aggregates. Although αSis abundant in the presynaptic terminals of the central nervous system, its physiological function is still unknown. There is strong evidence for the membrane affinity of the protein. One hypothesis is that lipid-induced binding and helix folding may modulate the fusion of synaptic vesicles with the presynaptic membrane and the ensuing transmitter release. Here we show that membrane recognition of the N-terminus is essential for the cooperative formation of helical domains in the protein. We used circular dichroism spectroscopy and isothermal titration calorimetry to investigate synthetic peptide fragments from different domains of the full-length aS protein. Site-specific truncation and partial cleavage of the full-length protein were employed to further characterize the structural motifs responsible for helix formation and lipid-protein interaction. Unilamellar vesicles of varying net charge and lipid compositions undergoing lateral phase separation or chain melting phase transitions in the vicinity of physiological temperatures served as model membranes. The results suggest that the membrane-induced helical folding of the first 25 residues may be driven simultaneously by electrostatic attraction and by a change in lipid ordering. Our findings highlight the significance of the aS N-terminus for folding nucleation, and provide a framework for elucidating the role of lipid-induced conformational transitions of the protein within its intracellular milieu. © 2010 by the Biophysical Society.
PMID: 15803674;Abstract:
The influences of lanosterol on lipid bilayers have been compared to those of cholesterol by combining deuterium ( 2H) NMR spin relaxation studies with segmental order parameter measurements. For bilayers of 1,2-diperdeuteriomyristoyl-sn-glycero-3-phosphocholine (DMPC-d 54), the results are consistent with a square-law dependence of the 2H Zeeman relaxation rates (R 1Z) on the corresponding order parameters (S CD). This behavior is indicative of relatively slow order fluctuations, for example, due to quasi-elastic bilayer disturbances. Significant differences are found in the influences of lanosterol versus cholesterol on the microscopic NMR observables; although lanosterol produces smaller order parameters than cholesterol, it leads to larger relaxation rates. By correlating the NMR relaxation behavior with the order parameters, the results are explained by a progressive reduction of the bilayer elasticity, which parallels the biosynthetic pathway from lanosterol to cholesterol.
PMID: 2025300;Abstract:
Bovine rhodopsin was recombined with various phospholipids in which the lipid acyl chain composition was held constant at that of egg phosphatidylcholine (PC), while the identity of the headgroups was varied. The ratio of MII / MI produced in the recombinant membrane vesicles by an actinic flash was studied as a function of pH, and compared to the photochemical activity observed for rhodopsin in native ROS membranes. MI and MII were found to coexist in a pH-dependent, acid-base equilibrium on the millisecond timescale. Recombinants made with phospholipids containing unsaturated acyl chains were capable of full native-like MII production, but demonstrated titration curves with different pK values. The presence of phosphoethanolamine or phosphoserine headgroups increased the amount of MII produced. In the case of phosphatidylserine this may result from alteration of the membrane surface potential, leading to an increase in the local H+ activity. The results indicate that the Gibbs free energies of the MI and MII conformational states are influenced by the membrane bilayer environment, suggesting a possible role of lipids in visual excitation. © 1991 Academic Press, Inc.
