Effect of bacteriorhodopsin on the orientation of the headgroup of 1,2-dimyristoyl-sn-glycero-3-phosphocholine in bilayers: a 31P- and 2H-NMR study

Combined effect of the head groups and alkyl chains of archaea lipids when interacting with bacteriorhodopsin

Bacteriorhodopsin (bR), a transmembrane protein with seven α-helices, is highly expressed in the purple membrane (PM) of archaea such as Halobacterium salinarum. It is well known that bR forms two-dimensional crystals with acidic lipids such as phosphatidylglycerol phosphate methyl ester (PGP-Me)-a major component of PM lipids bearing unique chemical structures-methyl-branched alkyl chains, ether linkages, and divalent anionic head groups with two phosphodiester groups. Therefore, we aimed to determine which functional groups of PGP-Me are essential for the boundary lipids of bR and how these functionalities interact with bR. To this end, we compared various well-known phospholipids (PLs) that carry one of the structural features of PGP-Me, and evaluated the affinity of PLs to bR using the centerband-only analysis of rotor-unsynchronized spin echo (COARSE) method in solid-state NMR measurements and thermal shift assays. The results clearly showed that the branched methyl groups of alkyl chains and double negative charges in the head groups are important for PL interactions with bR. We then examined the effect of phospholipids on the monomer-trimer exchange of bR using circular dichroism (CD) spectra. The results indicated that the divalent negative charge in a head group stabilizes the trimer structure, while the branched methyl chains significantly enhance the PLs' affinity for bR, thus dispersing bR trimers in the PM even at high concentrations. Finally, we investigated the effects of PL on the proton-pumping activity of bR based on the decay rate constant of the M intermediate of a bR photocycle. The findings showed that bR activities decreased to 20% in 1,2-dimyristoyl-sn-glycero-3-phosphate (DMPA), and in 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC) bilayers as compared to that in PM. Meanwhile, 1,2-Diphytanoyl-sn-glycero-3-phosphate (DPhPA) bilayers bearing both negative charges and branched methyl groups preserved over 80% of the activity. These results strongly suggest that the head groups and alkyl chains of phospholipids are essential for boundary lipids and greatly influence the biological function of bR.

Development of a CP 31P NMR broadline simulation methodology for studying the interactions of antihypertensive AT1 antagonist losartan with phospholipid bilayers

A cross-polarization (CP) (31)P NMR broadline simulation methodology was developed for studying the effects of drugs in phospholipids bilayers. Based on seven-parameter fittings, this methodology provided information concerning the conformational changes and dynamics effects of losartan in the polar region of the dipalmitoylphosphatidylcholine bilayers. The test molecule for this study was losartan, an antihypertensive drug known to exert its effect on AT(1) transmembrane receptors. The results were complemented and compared with those of differential scanning calorimetry, solid-state (13)C NMR spectroscopy, Raman spectroscopy, and electron spin resonance. More specifically, these physical chemical methodologies indicated that the amphipathic losartan molecule interacts with the hydrophilic-head zone of the lipid bilayers. The CP (31)P NMR broadline simulations showed that the lipid molecules in the bilayers containing losartan displayed greater collective tilt compared to the tilt displayed by the load-free bilayers, indicating improved packing. The Raman results displayed a decrease in the trans/gauche ratio and increased intermolecular interactions of the acyl chains in the liquid crystalline phase. Additional evidence, suggesting that losartan possibly anchors in the realm of the headgroup, was derived from upfield shift of the average chemical shift sigma(iso) of the (31)P signal in the presence of losartan and from shift of the observed peak at 715 cm(-1) attributed to C-N stretching in the Raman spectra.

The infectivity of transmissible spongiform encephalopathy agent at low doses: the importance of phospholipid

The issue of whether the mechanism of infection is independent or co-operative for low doses of transmissible spongiform encephalopathy (TSE) agent is critical for risk assessment. The susceptibility (and hence ID(50)) of individuals with the same prion protein (PrP) genotype may vary considerably with a small proportion being very susceptible. Assuming independent action, the incubation period (IP) would continue to increase until the dose is below the ID(50) of the most susceptible individuals in the experiment, at which point it would become constant. This may explain the observed increase in IP with decreasing dose below the apparent ID(50) in experiments with untreated TSE agent. In contrast, IPs for autoclaved or NaOH-treated TSE agent increase greatly at doses <ID(50) suggesting strong co-operative action, or even a threshold. It is proposed here that the unit of infectivity for prion disease is a nucleation seed comprised of PrP and host phospholipid (PL). PL would play a structural role through mediating protein/lipid interactions with PrP. Heating or alkali treatment destroys the PL breaking up the nucleation seeds, which require long IPs to reform at low doses. Replenishing those inactivated PLs with host PL would explain how the phenotypic effect of long IP at low dose is lost on subpassage. It is proposed here that strain thermostability is controlled by the nature and strength of the PrP/PL interactions, which are independent of the host PrP genotype. Although repeated oral exposure to low doses of scrapie is less harmful than a single large exposure, this effect may reflect interference by competition rather than diminished risks due to a co-operative effect, and is of little importance for 'one-off' low-dose environmental exposures.

Functionally Relevant Coupled Dynamic Profile of Bacteriorhodopsin and Lipids in Purple Membranes

The dynamics of bacteriorhodopsin (bR) and the lipid headgroups in oriented purple membranes (PMs) was determined at various temperatures and relative humidity (rh) using solid-state NMR spectroscopy. The 31P NMR spectra of the alpha- and gamma-phosphate groups in methyl phosphatidylglycerophosphate (PGP-Me), which is the major phospholipid in the PM, changed sensitively with hydration levels. Between 253 and 233 K, the signals from a fully hydrated sample became broadened similarly to those of a dry sample at 293 K. The 15N cross polarization (CP) NMR spectral intensities from [15N]Gly bR incorporated into fully hydrated PMs were suppressed in 15N CP NMR spectra at 293 K compared with those of dry membranes but gradually recovered at low temperatures or at lower hydration (75%) levels. The suppression of the NMR signals, which is due to interference with proton decoupling frequency (approximately 45 kHz), coupled with short spin-spin relaxation times (T2) indicates that the loops of bR, in particular, have motional components around this frequency. The motion of the transmembrane alpha-helices in bR was largely affected by the freezing of excess water at low temperatures. While between 253 and 233 K, where a dynamic phase transition-like change was observed in the 31P NMR spectra for the phosphate lipid headgroups, the molecular motion of the loops and the C- and N-termini slowed, suggesting lipid-loop interactions, although protein-protein interactions between stacks cannot be excluded. The results of T2 measurements of dry samples, which do not have proton pumping activity, were similar to those for fully hydrated samples below 213 K where the M-intermediates can be trapped. These results suggest that motions in the 10s micros correlation regime may be functionally important for the photocycle of bR, and protein-lipid interactions are motionally coupled in this dynamic regime.

Differential stiffness and lipid mobility in the leaflets of purple membranes

Purple membranes (PM) are two-dimensional crystals formed by bacteriorhodopsin and a variety of lipids. The lipid composition and density in the cytoplasmic (CP) leaflet differ from those of the extracellular (EC) leaflet. A new way of differentiating the two sides of such asymmetric membranes using the phase signal in alternate contact atomic force microscopy is presented. This method does not require molecular resolution and is applied to study the stiffness and intertrimer lipid mobility in both leaflets of the PM independently over a broad range of pH and salt concentrations. PM stiffens with increasing salt concentration according to two different regimes. At low salt concentration, the membrane Young's normal modulus grows quickly but differentially for the EC and CP leaflets. At higher salt concentration, both leaflets behave similarly and their stiffness converges toward the native environment value. Changes in pH do not affect PM stiffness; however, the crystal assembly is less pronounced at pH > or = 10. Lipid mobility is high in the CP leaflet, especially at low salt concentration, but negligible in the EC leaflet regardless of pH or salt concentration. An independent lipid mobility study by solid-state NMR confirms and quantifies the atomic force microscopy qualitative observations.

Determination of the orientation and dynamics of ergosterol in model membranes using uniform 13C labeling and dynamically averaged 13C chemical shift anisotropies as experimental restraints

A new strategy was established to determine the average orientation and dynamics of ergosterol in dimyristoylphosphatidylcholine model membranes. It is based on the analysis of chemical shift anisotropies (CSAs) averaged by the molecular dynamics. Static (13)C CSA tensors were computed by quantum chemistry, using the gauge-including atomic-orbital approach within Hartree-Fock theory. Uniformly (13)C-labeled ergosterol was purified from Pichia pastoris cells grown on labeled methanol. After reconstitution into dimyristoylphosphatidylcholine lipids, the complete (1)H and (13)C assignment of ergosterol's resonances was performed using a combination of magic-angle spinning two-dimensional experiments. Dynamically averaged CSAs were determined by standard side-band intensity analysis for isolated (13)C resonances (C(3) and ethylenic carbons) and by off-magic-angle spinning experiments for other carbons. A set of 18 constraints was thus obtained, from which the sterol's molecular order parameter and average orientation could be precisely defined. The validity of using computed CSAs in this strategy was verified on cholesterol model systems. This new method allowed us to quantify ergosterol's dynamics at three molar ratios: 16 mol % (Ld phase), 30 mol % (Lo phase), and 23 mol % (mixed phases). Contrary to cholesterol, ergosterol's molecular diffusion axis makes an important angle (14 degrees) with the inertial axis of the rigid four-ring system.

Lipid-protein interactions in the purple membrane: structural specificity within the hydrophobic domain

In the absence of native-like interactions between bacteriorhodopsin and its neighbouring lipids, the pigment chromophore is reversibly titrated from its purple 570 nm form to a blue-shifted 480 nm form in the moderately alkaline pH range. Quantitation of this acid-base chromophore equilibrium in vesicles prepared from modified lipid mixtures shows that it is absent under conditions where bacteriorhodopsin is allowed to interact with methyl-substituted alkyl chains. The peculiar homogeneous structure of purple membrane alkyl chain lipids is thus likely to be an essential requirement for maintenance of the native bacteriorhodopsin structure over a wide pH range.