No abstract

The functional significance of the lipid-protein interface in photosynthetic membranes, mainly in thylakoids, is reviewed with emphasis on membrane structure and dynamics. The lipid-protein interface is identified primarily by the restricted molecular dynamics of its lipids as compared with the dynamics in the bulk lipid phase of the membrane. In a broad sense, lipid-protein interfaces comprise solvation shell lipids that are weakly associated with the hydrophobic surface of transmembrane proteins but also include lipids that are strongly and specifically bound to membrane proteins or protein assemblies. The relation between protein-associated lipids and the overall fluidity of the thylakoid membrane is discussed. Spin label electron paramagnetic resonance spectroscopy has been identified as the technique of choice to characterize the protein solvation shell in its highly dynamic nature; biochemical and direct structural methods have revealed an increasing number of protein-bound lipids. The structural and functional roles of these protein-bound lipids are mustered, but in most cases they remain to be determined. As suggested by recent data, the interaction of the non-bilayer-forming lipid, monogalactosyldyacilglycerol (MGDG), with the main light-harvesting chlorophyll a/b-binding protein complexes of photosystem-II (LHCII), the most abundant lipid and membrane protein components on earth, play multiple structural and functional roles in developing and mature thylakoid membranes. A brief outlook to future directions concludes this review.

Gramicidin A was incorporated at a peptide/lipid ratio of 1:10 mol/mol in aligned bilayers of dimyristoyl phosphatidylcholine (DMPC), phosphatidylserine (DMPS), phosphatidylglycerol (DMPG), and phosphatidylethanolamine (DMPE), from trifluoroethanol. Orientations of the peptide and lipid chains were determined by polarized attenuated total reflection infrared spectroscopy. Lipid-peptide interactions with gramicidin A in DMPC bilayers were studied with different spin-labeled lipid species by using electron spin resonance spectroscopy. In DMPC membranes, the orientation of the lipid chains is comparable to that in the absence of peptide, in both gel and fluid phases. In gel-phase DMPC, the effective tilt of the peptide exceeds that of the lipid chains, but in the fluid phase both are similar. For gramicidin A in DMPS, DMPG, and DMPE, the degree of orientation of the peptide and lipid chains is less than in DMPC. In the fluid phase of DMPS, DMPG, and DMPE, gramicidin A is also less well oriented than are the lipid chains. In DMPE especially, gramicidin A is largely disordered. In DMPC membranes, three to four lipids per monomer experience direct motional restriction on interaction with gramicidin A. This is approximately half the number of lipids expected to contact the intramembranous perimeter of the gramicidin A monomer. A selectivity for certain negatively charged lipids is found in the interaction with gramicidin A in DMPC. These results are discussed in terms of the integration of gramicidin A channels in lipid bilayers, and of the interactions of lipids with integral membrane proteins.

The development of the thylakoid membrane was studied during illumination of dark-grown barley seedlings by using biochemical methods, and Fourier transform infrared and spin label electron paramagnetic resonance spectroscopic techniques. Correlated, gross changes in the secondary structure of membrane proteins, conformation, composition, and dynamics of lipid acyl chains, SDS͞PAGE pattern, and thermally induced structural alterations show that greening is accompanied with the reorganization of membrane protein assemblies and the protein-lipid interface. Changes in overall membrane fluidity and noncovalent proteinlipid interactions are not monotonic, despite the monotonic accumulation of chlorophyll, LHCII [light-harvesting chlorophyll a͞b-binding (polypeptides) associated with photosystem II] apoproteins, and 18:3 fatty acids that follow a similar time course with highest rates between 12-24 h of greening. The 18:3 fatty acid content increases 2.8-fold during greening. This appears to both compensate for lipid immobilization by membrane proteins and facilitate packing of larger protein assemblies. The increase in the amount of protein-solvating immobile lipids, which reaches a maximum at 12 h, is caused by 40% decrease in the membranous mean diameter of protein assemblies at constant protein͞lipid mass ratio. Alterations in the SDS͞PAGE pattern are most significant between 6 -24 h. The size of membrane protein assemblies increases Ϸ4.5-fold over the 12-48-h period, likely caused by the 2-fold gain in LHCII apoproteins. The thermal stability of thylakoid membrane proteins increases monotonically, as detected by an increasing temperature of partial protein unfolding during greening. Our data suggest that a structural coupling between major protein and lipid components develops during greening. This protein-lipid interaction is required for the development and protection of thylakoid membrane protein assemblies.barley (Hordeum vulgare) ͉ FTIR ͉ photosynthesis ͉ protein-lipid interaction ͉ spin label EPR

The role of phosphatidylglycerol (PG) in protein-lipid interactions and membrane dynamics has been studied in the thylakoids of wild type and manipulated tobacco plants transformed with complementary DNAs for glycerol-3-phosphate acyltransferases (GPATs) from squash and Arabidopsis. The expression of the foreign enzymes resulted in the level of saturation of the PG molecules being higher in the squash and lower in the Arabidopsis transformants, as compared with the level in wild-type tobacco. For the analysis of fatty acyl chain dynamics in the thylakoid membranes, the nu(sym)CH(2) vibration bands of the infrared specta were decomposed into two components, corresponding to ordered and disordered fatty acyl chain segments. With this approach, it was shown that in squash GPAT-transformed tobacco thylakoids a rigid lipid domain exists below 25 degrees C. Above 25 degrees C, the dynamics of all thylakoid membranes were very similar, regardless of the manipulations. PG seems to tune the dynamics at the protein-lipid interface rather than to affect the structure of the proteins directly. Above 50 degrees C, the frequencies of the disordered nu(sym)CH(2) component bands were decreased. This lipid-related phenomenon correlated with protein denaturing. It is demonstrated that the protein aggregation appearing upon heat denaturing changes the conformational distribution of the disordered lipid population. The data also reveal that the protein stability does not depend on the fatty acid composition of the PG molecules; other lipids should provide the environment governing the protein stability in the thylakoid membrane. This is the first such detailed analysis of the infrared spectra of biological membranes that permits a differentiation between structurally different lipid populations within a membrane.

In this article, the assignment of the ν(CH) stretching region of lipid molecules is revisited. This region is extensively used to follow lipid phase transitions, and especially the frequency shifts and bandwidth alterations in the νsymCH2 band have been utilized in this respect. Here, we propose and prove that behind these phenomena there are pairs of component bands in the cases of both the νsymCH2 and the νasCH2 bands. The lower‐frequency components of the pairs are assigned to the vibrations of CH2 groups on trans segments of the fatty acyl chains, while the higher‐frequency components of the pairs are assigned to CH2 groups on gauche segments. To prove these assignments, we have shown that the νCH2 frequencies are characteristic of the conformation of the lipid fatty acyl chain itself, and not the state of the whole lipid matrix. Curve fitting in fact revealed the conformer‐specific components. With the use of singular value decomposition analysis we have demonstrated that the relative intensity changes in the components, and not the shifts in the whole bands, cause the observed shifts in the νCH2 bands upon lipid phase transition. The results of this approach are presented for deuterium‐saturated dioleoyl–phosphatidylcholine mixtures, for the gel → liquid‐crystalline phase transition of dipalmitoyl–phosphatidylcholine multilayers, and for a biological membrane, barley thylakoid. This refined assignment offers physically plausible reasoning for the observed phenomena and is able to explain frequency shifts and bandwidth changes observed previously upon lipid phase transitions, including their nonconcerted temperature dependences. In biological membranes, this interpretation allows the separation of protein‐ and membrane‐dynamics‐induced lipid conformational changes. © 1999 John Wiley & Sons, Inc. Biospectroscopy 5: 169–178, 1999

Dipalmitoylphosphatidylcholine (DPPC) bilayer was created on the surface of an exponentially growing poly(glutamic acid)/poly(lysine) (PGA/PLL) layer-by-layer polyelectrolyte film. The lipid bilayer decreased the surface roughness of the polyelectrolyte film. The layer-by-layer construction of the polyelectrolyte film could be continued on the top of the DPPC layer. The lipid bilayer, however, formed a barrier in the interior of the polyelectrolyte film, which blocked the diffusion (a prerequisite for exponential growth) of the polyelectrolytes. Thus, a new growth regime started in the upper part of the polyelectrolyte film, which was added to embed the DPPC bilayer. The structure and the dynamics of the DPPC bilayer on the polyelectrolyte film surface remained similar to that of its hydrated multibilayers, except that the phase transition became wider. In the case of embedded DPPC bilayers, in addition, the phase-transition temperature also decreased. This is the result of interactions with the nonconcerted movements of the barrier-separated lower and higher parts of the polyelectrolyte film. Gramicidin A (GRA) as a model of lipid-soluble peptides and proteins was successfully incorporated into such DPPC films. The DPPC films, either with or without GRA, were remarkably stable; as many heating-cooling cycles to measure phase transition could be carried out without visible alterations as wanted.

Rotary enzymes are complex, highly challenging biomolecular machines whose biochemical working mechanism involves intersubunit rotation. The true intrinsic rate of rotation of any rotary enzyme is not known in a native, unmodified state. Here we use the effect of an oscillating electric (AC) field on the biochemical activity of a rotary enzyme, the vacuolar proton-ATPase (V-ATPase), to directly measure its mean rate of rotation in its native membrane environment, without any genetic, chemical or mechanical modification of the enzyme, for the first time. The results suggest that a transmembrane AC field is able to synchronise the steps of ion-pumping in individual enzymes via a hold-and-release mechanism, which opens up the possibility of biotechnological exploitation. Our approach is likely to work for other transmembrane ion-transporting assemblies, not only rotary enzymes, to determine intrinsic in situ rates of ion pumping.