Interpretation of 100- and 360-MHz proton magnetic resonance spectra of retinal rod outer segment disk membranes

The Myth of The Annular Lipids

In the early 1970s, the existence of a "lipid annulus" stably surrounding the individual intrinsic protein molecules was proposed by several authors. They referred to a number of lipid molecules in slow exchange with the bulk lipid in the bilayer, i.e., more or less protein-bound, and more ordered than the bulk lipid. The annular lipids would control enzyme activity. This idea was uncritically accepted by most scientists working with intrinsic membrane proteins at the time, so that the idea operated like a myth in the field. However, in the following decade, hard spectroscopic and biochemical evidence showed that the proposed annular lipids were not immobilized for a sufficiently long time to influence enzyme or transporter activity, nor were they ordered by the protein. Surprisingly, forty years later, the myth survives, and the term 'annular lipid' is still in use, in a different, but even more illogical sense.

An NMR database for simulations of membrane dynamics

Computational methods are powerful in capturing the results of experimental studies in terms of force fields that both explain and predict biological structures. Validation of molecular simulations requires comparison with experimental data to test and confirm computational predictions. Here we report a comprehensive database of NMR results for membrane phospholipids with interpretations intended to be accessible by non-NMR specialists. Experimental ¹³C-¹H and ²H NMR segmental order parameters (S(CH) or S(CD)) and spin-lattice (Zeeman) relaxation times (T(1Z)) are summarized in convenient tabular form for various saturated, unsaturated, and biological membrane phospholipids. Segmental order parameters give direct information about bilayer structural properties, including the area per lipid and volumetric hydrocarbon thickness. In addition, relaxation rates provide complementary information about molecular dynamics. Particular attention is paid to the magnetic field dependence (frequency dispersion) of the NMR relaxation rates in terms of various simplified power laws. Model-free reduction of the T(1Z) studies in terms of a power-law formalism shows that the relaxation rates for saturated phosphatidylcholines follow a single frequency-dispersive trend within the MHz regime. We show how analytical models can guide the continued development of atomistic and coarse-grained force fields. Our interpretation suggests that lipid diffusion and collective order fluctuations are implicitly governed by the viscoelastic nature of the liquid-crystalline ensemble. Collective bilayer excitations are emergent over mesoscopic length scales that fall between the molecular and bilayer dimensions, and are important for lipid organization and lipid-protein interactions. Future conceptual advances and theoretical reductions will foster understanding of biomembrane structural dynamics through a synergy of NMR measurements and molecular simulations.

Evidence for specificity in lipid-rhodopsin interactions

The interaction of bovine rhodopsin with poly- and monounsaturated lipids was studied by (1)H MAS NMR with magnetization transfer from rhodopsin to lipid. Experiments were conducted on bovine rod outer segment (ROS) disks and on recombinant membranes containing lipids with polyunsaturated, docosahexaenoyl (DHA) chains. Poly- and monounsaturated lipids interact specifically with different sites on the rhodopsin surface. Rates of magnetization transfer from protein to DHA are lipid headgroup-dependent and increased in the sequence PC < PS < PE. Boundary lipids are in fast exchange with the lipid matrix on a time scale of milliseconds or shorter. All rhodopsin photointermediates transferred magnetization preferentially to DHA-containing lipids, but highest rates were observed for Meta-III rhodopsin. The experiments show clearly that the surface of rhodopsin has sites for specific interaction with lipids. Current theories of lipid-protein interaction do not account for such surface heterogeneity.

Modulation of rhodopsin function by properties of the membrane bilayer

A prevalent model for the function of rhodopsin centers on the metarhodopsin I (MI) to metarhodopsin II (MII) conformational transition as the triggering event for the visual process. Flash photolysis techniques enable one to determine the [MII]/[MI] ratio for rhodopsin in various recombinant membranes, and thus investigate the roles of the phospholipid head groups and the lipid acyl chains systematically. The results obtained to date clearly show that the pK for the acid-base MI-MII equilibrium of rhodopsin is modulated by the lipid environment. In bilayers of phosphatidylcholines the MI-MII equilibrium is shifted to the left; whereas in the native rod outer segment membranes it is shifted to the right, i.e., at neutral pH near physiological temperature. The lipid mixtures sufficient to yield full photochemical function of rhodopsin include a native-like head group composition, viz, comprising phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylserine (PS), in combination with polyunsaturated docosahexaenoic acid (DHA; 22:6 omega 3) chains. Yet such a native-like lipid mixture is not necessary for the MI-MII conformational transition of rhodopsin; one can substitute other lipid compositions having similar properties. The MI-MII transition is favored by relatively small head groups which produce a condensed bilayer surface, viz, a comparatively small interfacial area as in the case of PE, together with bulky acyl chains such as DHA which prefer a relatively large cross sectional area. The resulting force imbalance across the layer gives rise to a curvature elastic stress of the lipid/water interface, such that the lipid mixtures yielding native-like behavior form reverse hexagonal (HII) phases at slightly higher temperatures. A relatively unstable membrane is needed: lipids tending to form the lamellar phase do not support full native-like photochemical function of rhodopsin. Thus chemically specific properties of the various lipids are not required, but rather average or material properties of the entire assembly, which may involve the curvature free energy of the membrane-lipid water interface. These findings reveal that the membrane lipid bilayer has a direct influence on the energetics of the conformational states of rhodopsin in visual excitation.

Lipid headgroup and acyl chain composition modulate the MI-MII equilibrium of rhodopsin in recombinant membranes

A current paradigm for visual function centers on the metarhodopsin I (MI) to metarhodopsin II (MII) conformational transition as the trigger for an intracellular enzyme cascade leading to excitation of the retinal rod. We investigated the influences of the membrane lipid composition on this key triggering event in visual signal transduction using flash photolysis techniques. Bovine rhodopsin was combined with various phospholipids to form membrane recombinants in which the lipid acyl chain composition was held constant at that of egg phosphatidylcholine (PC), while the identity of the lipid headgroups was varied. The ratio of MII/MI produced in these recombinants by an actinic flash at 28 degrees C was studied as a function of pH. The results were compared to the photochemical function observed for rhodopsin in native retinal rod outer segment (ROS) membranes, in total native ROS lipid recombinants, and in dimyristoylphosphatidylcholine (DMPC) recombinants. In membrane recombinants incorporating lipids derived from egg PC, as well as in the total ROS lipids control and the native ROS disk membranes, MI and MII were found to coexist in a pH-dependent, acid-base equilibrium on the millisecond time scale. The recombinants of rhodopsin with egg PC, either alone or in combination with egg PC-derived phosphatidylethanolamine (PE) or phosphatidylserine (PS), exhibited substantially reduced photochemical activity at pH 7.0. However, all recombinants comprising phospholipids with unsaturated acyl chains were capable of full native-like MII production at pH 5.0, confirming previous results [Gibson, N.J.. & Brown, M.F. (1990) Biochem. Biophys. Res. Commun. 169, 1028-1034]. It follows that energetic constraints on the MI and MII states imposed by egg PC-derived acyl chains can be offset by increased activity of H+ ions. The data reveal that the major effect of the membrane lipid composition is to alter the apparent pK for the MI-MII conformational equilibrium of rhodopsin [Gibson, N.J., & Brown, M.F. (1991) Biochem. Biophys. Res. Commun. 176, 915-921]. Recombinants containing only phosphocholine headgroups exhibited the lowest apparent pK values, whereas the presence of either 50 mol % PE or 15 mol % PS increased the apparent pK. The inability to obtain full native-like function in recombinants having egg PC-derived chains and a native-like headgroup composition indicates a significant role of the polyunsaturated docosahexaenoic acid (DHA) chains (22:6 omega 3) of the native retinal rod membrane lipids. Temperature studies of the MI-MII transition enabled an investigation of lipid influences on the thermodynamic parameters of a membrane protein conformational change linked directly to function.(ABSTRACT TRUNCATED AT 400 WORDS)

Analysis of membrane lipids by 500 MHz 1H NMR

A nondestructive method has been developed for rapid analysis of lipid content of membrane extracts based on high field proton NMR spectroscopy. Lipid extraction is done by stepwise sonication of purified membranes into chloroform:methanol:water mixtures, and 1H spectra are recorded at 35 degrees C on final preparations consisting of at least 1 mg dried lipid solubilized in 2:1 CD3OD:CDCl3. Spectral peaks of lipid mixtures are assigned to lipid classes using a data base of standard lipid characteristic resonances derived from purified single membrane lipids and known mixtures of them. Peak intensities of characteristic peaks yield ratios of various lipids such as cholesterol:phospholipid and phosphatidylcholine:phosphatidylethanolamine, degree of unsaturation, average acyl chain length, total glycerol lipid content, and presence or absence of particular lipids, such as glycolipids or lysolipids. This procedure of membrane lipid analysis has been verified using known mixtures of purified standard lipids.

High-resolution 1H NMR spectral signature from human atheroma

Coronary artery disease due to atherosclerosis takes the lives of approximately 550,000 Americans each year--an enormous toll. Put in economic terms, the cost to the United States alone has been estimated to exceed 60 billion dollars annually. We have found that well-resolved proton (1H) NMR spectra can be obtained from human atheroma (fatty plaque), despite its macroscopic solid appearance. The fraction of the total spectral intensity corresponding to the sharp 1H NMR signals is temperature dependent and approaches unity at body temperature (37 degrees C). Studies of the total lipids extracted from atheroma and cholesteryl esters were conducted to identify the chemical and physical origin of the spectral signature. The samples were characterized through assignment of their chemical shifts and by measurement of their T1 and T2 relaxation times as a function of magnetic field strength. The results suggest that the relatively sharp 1H NMR signals from human atheroma (excluding water) are due to a mixture of cholesteryl esters, whose liquid-crystalline to isotropic fluid phase transition is near body temperature. Preliminary applications to NMR imaging of human atheroma are reported, which demonstrate early fatty plaque formation within the wall of the aorta. These findings offer a basis for noninvasive imaging by NMR to monitor early and potentially reversible stages of human atherogenesis.

Thermotropic behavior of retinal rod membranes and dispersions of extracted phospholipids

High sensitivity, differential scanning calorimetry studies of bovine retinal rod outer segment (ROS) disk membranes and aqueous dispersions of the extracted ROS phospholipids have been performed. ROS disk membranes were found to exhibit a broad peak of excess heat capacity with a maximum at less than about 3 degrees C, ascribable to a gel-to-liquid crystalline phase transition of a fraction of the phospholipids. A similar thermotropic transition was observed for aqueous dispersions of the total extracted and purified ROS phospholipids. Comparison of the results obtained for the dispersion of total ROS phospholipids to those of the purified head group fractions suggests that the thermotropic behavior reflects a gel-to-liquid crystalline transition, leading to lateral phase separation, involving those phosphatidylcholine (PC) molecules containing saturated fatty acyl chains, possibly together with the highest melting ROS phosphatidylethanolamine (PE) and phosphatidylserine (PS) components. The interpretation of the thermal behavior of the ROS disk membranes depends on whether the transition is assumed to derive from the ROS PC and/or PE/PS fractions, and whether the transbilayer arrangement of the ROS phospholipids is assumed to be symmetric or asymmetric. The calorimetric data can be simply explained in terms of an asymmetric distribution of the major ROS disk membrane phospholipids (G.P. Miljanich et al., J. Membrane Biol. 60:249-255, 1981). In this case, the transition would arise from the PE/PS fractions in the outer ROS disk membrane monolayer, and the anticipated transition from the PC in the inner monolayer would be broadened due to interaction with cholesterol. For the ROS membranes at higher temperatures, two additional, irreversible transitions are observed at 57 and 72 degrees C, corresponding to the thermal denaturation of opsin and rhodopsin, respectively.

Cell-cycle-specific biosynthesis of the photosynthetic membrane of Rhodopseudomonas sphaeroides. Structural implications

Structural changes association with the intracytoplasmic membrane during the cell cycle of the photosynthetic bacterium Rhodopseudomonas sphaeroides have been studied by freeze-fracture electron microscopy. The isolated intracytoplasmic membrane vesicles, chromatophores, were fused in order to obtain large fracture faces, allowing more precise measurements and statistical analysis of both intramembrane particle density and size determinations. The intramembrane particle density of the protoplasmic face (PF) of the intracytoplasmic membrane, (from 4970 to 8290/micrometers 2), was shown to be a linear function of the protein/phospholipid ratio (from 2.5 to 5.1, w/w) of the intracytoplasmic membrane. Under constant light intensity, both the average particle size and particle size distribution remained unchanged during the cell cycle. These results provide the structural basis for the earlier reported cell-cycle-specific variations in both protein/phospholipid ratio and alternation in phospholipid structure of the intracytoplasmic membrane of R. sphaeroides during photosynthetic growth. The average particle diameter in the PF face of the intracytoplasmic membrane was 8.25, 9.08 and 9.75 nm at incident light intensities of 4000, 500 and 30 ft X cd, respectively. When chromatophores were fused with small, unilamellar liposomes, the intramembrane particle density decreased as input liposome phospholipid increased, whereas the particle size remained constant and particle distribution became random.

Lipid bilayer dynamics and rhodopsin-lipid interactions: new approach using high-resolution solid-state 13C NMR

High-resolution, solid-state 13C NMR spectra have been obtained for unsonicated multilamellar dispersions of 1,2-dilauryl-sn-glycero-3-phosphocholine (DLPC), recombinant membranes containing DLPC and rhodopsin, and native retinal rod disk membranes. The roles of 1H dipolar decoupling, 1H-13C cross-polarization, and magic-angle sample spinning have been investigated. Rotating-frame 13C relaxation times have been measured and are discussed in terms of lipid bilayer dynamics and rhodopsin-lipid interactions.

Dynamical and temperature-dependent effects of lipid-protein interactions. Application of deuterium nuclear magnetic resonance and electron paramagnetic resonance spectroscopy to the same reconstitutions of cytochrome c oxidase

2H NMR and EPR spectra have been obtained as a function of temperature and protein concentration from the same samples of beef heart cytochrome c oxidase reconstituted into 1-(16,16,16-trideuteriopalmitoyl)-2-palmitoleoyl-sn-glycero-3-phosphocholine. At all temperatures, the EPR spectra show the characteristic "bound" and "free" components, while the 2H NMR spectra show only a narrow distribution of orientational order parameters. At temperatures near the phase transition of the pure lipid, the dependence of the 2H NMR average orientational order on protein concentration fits a two-stage model in which the phospholipid molecular exchange rapidly between two states tentatively identified as sites either on or off the protein surface. From this model, the 2H NMR spectra yield a value of 0.18 mg of phospholipid per mg of protein as necessary to cover the surface of cytochrome c oxidase, which is the same value as derived from the EPR spectra at -20 degrees C. Both the 2H NMR and EPR spectra vary markedly with temperature. At temperatures well above the phase transition of the pure lipid, the average orientational parameters derived from the 2H NMR spectra are independent of protein concentration and are the same as for the lipid alone. Qualitatively, the EPR spectra show large apparent decreases in the average orientational order with increasing temperature. Analysis of 2H NMR relaxation rates indicates an additional motion in the presence of protein with a correlation time of 10(-6)-10(-7) s. If this new motion is associated with exchange between the two states, a minimum value of 10(6)-10(7) s-1 for the exchange rate is obtained, assuming that the lipids on the protein surface are much more motionally restricted than the rest of the lipid. Such an exchange rate is compatible with the observed differences in 2H NMR and EPR spectra. These results are consistent with short-lived, energetically weak interactions between cytochrome c oxidase and the phospholipids used in this study.

A possible role of rhodopsin in maintaining bilayer structure in the photoreceptor membrane

31P-NMR measurements demonstrate that at 37 degrees C, independent of the photolytic state of the photopigment rhodopsin, the lipids in the photo-receptormembrane are almost exclusively organised in a bilayer. In strong contrast, the 31P-NMR spectra of the extracted lipids are characteristic for the hexagonal HII phase and an isotropic phase. The isotropic phase is characterised by freeze-fracture electron microscopy as particles and pits on smooth surfaces, possibly indicating inverted micelles. These results suggest a structural role for rhodopsin in maintaining the photoreceptor membrane lipids in a bilayer configuration.

Pressure effects on lipid-protein interactions in (NA+ + K+)-ATPase

The (Na+ + K+)-stimulated ATPase activity decreases with increasing pressure and a plot of the logarithm of the activity versus pressure shows a change in slope at a defined breakpoint pressure (Pb). The value of Pb increases linearly with increasing temperature. A dT/dP value of 27.7 +- 0.4 (S.D.) K/1000 atm is obtained. This is in very good agreement with the pressure shift for the melting transitions in phospholipids and aliphatic chains. This strongly indicates that an aliphatic chain melting process is involved in the breakpoint in the Arrhenius plot and pressure dependence of (Na+ + K+)-ATPase. The p-nitrophenyl phosphatase activity of this enzyme also decreases with pressure. In this case the plot of the logarithm of the activity versus pressure is linear without a break-point. The temperature dependence for (Na+ + K+)-ATPase was also studied in the presence of fluidizing drugs: desipramine and benzylalcohol. The presence of these drugs had no effect on the inflection point in the Arrhenius plot.

The use of the fluorescent probe alpha-parinaric acid to determine the physical state of the intracytoplasmic membranes of the photosynthetic bacterium, Rhodopseudomonas sphaeroides

alpha-Parinaric acid has been used to determine the degree of ordering of the hydrocarbon region of purified intracytoplasmic membranes of Rhodopseudomonas sphaeroides. The usefulness of alpha-parinaric acid as a probe of membrane fluidity was established by comparison of its fluorescent properties in phosphatidylcholine vesicles with those of the more commonly used fluorescent probe, 1,6-diphenyl-1,3,5-hexatriene. Both fluorescent probes were shown to monitor similar environments in the phosphatidylcholine vesicles when the phospholipids were maintained at temperatures above their phase transition temperature. The rotational mobility of alpha-parinaric acid in the intracytoplasmic membranes was determined from 0 to 50 degrees C, a region where no phase transitions were detectable. The rotational mobility of alpha-parinaric acid dissolved in vesicles formed from total extracted intracytoplasmic membrane phospholipids, was 2--3-fold greater than that measured in the intact intracytoplasmic membranes; demonstrating that the presence of protein greatly reduces the mobility of the phospholipid acyl chains of the intracytoplasmic membranes. Due to the high protein content of these membranes, the perturbing effect of protein on acyl chain mobility may extend to virtually all the intracytoplasmic membrane phospholipid.