Identifying regions of membrane proteins in contact with phospholipid head groups: covalent attachment of a new class of aldehyde lipid labels to cytochrome c oxidase

Structure, dynamics and composition of the lipid-protein interface. Perspectives from spin-labelling

Implications of the data on lipid-protein interactions involving integral proteins that are obtained from EPR spectroscopy with spin-labelled lipids in membranes are reviewed. The lipid stoichiometry, selectivity and exchange dynamics at the lipid-protein interface can be determined, in addition to information on the configuration and rotational dynamics of the protein-associated lipid chains. These parameters, particularly the stoichiometry and selectivity, are directly related to the intramembranous structure and degree of oligomerisation of the integral protein, and conversely may be used to study the state of assembly of such proteins in the membrane. Insertion of proteins into membranes can be studied by analogous methods. Comparison with the results obtained from integral proteins helps to define the extent of membrane penetration and degree of transmembrane crossing that are relevant to protein translocation mechanisms.

Preparation of Schiff base adducts of phosphatidylcholine core aldehydes and aminophospholipids, amino acids, and myoglobin

We have prepared Schiff base adducts of the core aldehydes of phosphatidylcholine and aminophospholipids, free amino acids, and myoglobin. The Schiff bases of the ethanolamine and serine glycerophospholipids were obtained by reacting sn-1-palmitoyl(stearoyl)-2-[9-oxo]nonanoyl-glycerophosphocholine (PC-Ald) with a twofold excess of the aminophospholipid in chloroform/methanol 2:1 (vol/vol) for 18 h at room temperature. The Schiff bases of the amino acids and myoglobin were obtained by reacting the aldehyde with an excess of isoleucine, valine, lysine, methyl ester lysine and myoglobin in aqueous methanol for 18 h at room temperature. Prior to isolation, the Schiff bases were reduced with sodium cyanoborohydride in methanol for 30 min at 4 degrees C. The reaction products were characterized by normal-phase high-performance liquid chromatography and on-line mass spectrometry with electrospray ionization. The amino acids and aminophospholipids yielded single adducts. A double adduct was obtained for myoglobin, which theoretically could have accepted up to 23 PC-Ald groups. The yields of the products ranged from 12 to 44% for the aminophospholipids and from 15-57% for the amino acids, while the Schiff base of the myoglobin was estimated at 5% level. The new compounds are used as reference standards for the detection of high molecular weight Schiff bases in lipid extracts of natural products.

Bacterial solute transport proteins in their lipid environment

The cytoplasmic membrane of bacteria is a selective barrier that restricts entry and exit of solutes. Transport of solutes across this membrane is catalyzed by specific membrane proteins. Integral membrane proteins usually require specific lipids for optimal activity and are inhibited by other lipid species. Their activities are also sensitive to the lipid bilayer dynamics and physico-chemical state. Bacteria can adapt to changes in the environments (respective temperature, hydrostatic pressure, and pH) by altering the lipid composition of the membrane. Homeoviscous adaptation results in the maintenance of the liquid-crystalline phase through alterations in the degree of acyl chain saturation and branching, acyl chain length and the sterol content of the membrane. Homeophasic adaptation prevents the formation of non-bilayer phases, which would disrupt membrane organization and increase permeability. A balance is maintained between the lamellar phase, preferring lipids, and those that adopt a non-bilayer organization. As a result, the membrane proteins are optimally active under physiological conditions. The molecular basis of lipid-protein interactions is still obscure. Annular lipids stabilize integral membrane proteins. Stabilization occurs through electrostatic and possibly other interactions between the lipid headgroups and the charged amino acid residues close to the phospholipid-water interface, and hydrophobic interactions between the fatty acyl chains and the membrane-spanning segments. Reconstitution techniques allow manipulation of the lipid composition of the membrane in a way that is difficult to achieve in vivo. The physical characteristics of membrane lipids that affect protein-mediated transport functions have been studied in liposomal systems that separate an inner and outer compartment. The activity of most transport proteins is modulated by the bulk physical characteristics of the lipid bilayer, while specific lipid requirements appear rare.

Lipid-protein interactions in membranes

The interactions of lipids with integral and peripheral proteins can be studied in reconstituted and natural membranes using spin label electron spin resonance (ESR) spectroscopy. The ESR spectra reveal a reduction in mobility of the spin-labelled lipid species, and in certain cases evidence is obtained for a partial penetration of the peripheral proteins into the membrane. The latter may be relevant to the import mechanism of apocytochrome c into mitochondria. Integral proteins induce a more direct motional restriction of the spin-labelled lipid chains, allowing the stoichiometry and specificity of the interaction, and the lipid exchange rate at the protein interface to be determined from the ESR spectra. In this way, a population of very slowly exchanging cardiolipin associated with the mitochondrial ADP-ATP carrier has been identified. The residues involved in the specificity for charged lipids of the myelin proteolipid protein have been localized to the deletion in the DM-20 mutant, and the difference in lipid-protein interactions with the beta-sheet and alpha-helical conformations of the M-13 coat protein, has been characterized.

Effect of chemical modification of lysine amino groups on redox and protonmotive activity of bovine heart cytochrome c oxidase reconstituted in phospholipid membranes

A study is presented of the effect of chemical modification of lysine amino groups on the redox and protonmotive activity of bovine heart cytochrome c oxidase. Treatment of soluble oxidase with succinic acid anhydride resulted in succinylation of lysines in all the subunits of the enzyme. The consequent change of surface charges from positive to negative resulted in inversion of the orientation of the reconstituted enzyme from right-side-out to inside-out. Reconstitution of the oxidase in phospholipid vesicles prevented succinylation of subunits III and Vb and depressed that of other subunits with the exception of subunits II and IV which were predominantly labeled in a concentration-dependent manner by succinic acid anhydride. This modification of lysines produced a decoupling effect on redox-linked proton ejection, which was associated with a decrease of the respiratory control exerted by the delta pH component of PMF. The decoupling effect was directly shown to be exerted at the level of the pH-dependent rate-limiting step in intramolecular electron flow located on the oxygen side of heme a.

Influence of polar residue deletions on lipid-protein interactions with the myelin proteolipid protein. Spin-label ESR studies with DM-20/lipid recombinants

The lipid specificities of two related integral membrane proteins of central nervous system myelin, the proteolipid (PLP) and DM-20 proteins, which differ only by the deletion of a polar stretch of 35 contiguous amino acid residues, were studied with spin-labeled lipids after reconstitution into dimyristoyl-phosphatidylcholine. The selectivity in populating lipid association sites at the protein interface and in modulating the lipid exchange between protein and bulk lipid sites was quantitated by the relative association constants and the off-rate constants for exchange, respectively, for both proteins. The sequence deleted in DM-20 (residues 116-150 of PLP) is found to play a major role in determining the lipid selectivity for the parent PLP protein.

Use of electron spin resonance spectroscopy of spin labels for studying drug-induced membrane perturbation

The use of electron spin resonance spectroscopy of spin labels is reviewed in the context of drug-induced membrane perturbation. The correlation between membrane perturbation and biological effects is also considered.

Models of protein lateral arrangements in lipid bilayer membranes. Application to electron spin resonance studies of cytochrome c oxidase

We consider the situation of integral membrane proteins in a lipid bilayer matrix where the size of the polar group of the protein is important in determining the lateral packing of the proteins. We represent the cross-section of the protein hydrophobic core as a hexagon moving on a lattice, and represent the projection of the polar group onto the plane of the bilayer as a shape, parts of which overlap the hexagon. Lattice sites represent lipid molecules. We calculate the fraction of lipid molecules which are adjacent to the hydrophobic core of at least one protein. We use this data to consider the "motion restricted" spectrum observed in electron spin resonance (ESR) probe studies, and compute the dependence of the "motion restricted" fraction upon protein concentration. The resulting curves can be used to analyse ESR data in order to deduce the size and shape of the proteins' polar segment. We have used the range of models examined to study the dependence upon protein concentration of the particular case of the "motion restricted" spectrum of a spin-labelled lipid freely diffusing or, alternatively, covalently bound to cytochrome c oxidase. We find that our calculations are in accord with a model where approximately 60 lipid molecules can fit around an isolated such protein in both halves of the bilayer, and where the polar segment is substantially anisotropic and extends laterally beyond the limits of the hydrophobic core. The latter is in accord with what is known about the structure of cytochrome c oxidase. We indicate further measurements that should be performed in order to establish more definitively the dependence of the "motion restricted" component upon protein concentration, giving the lipid protein ratios at which they should be performed, and we make predictions concerning the results. Finally we argue for a particular unified way of plotting experimental data.

Spin-label studies on the specificity of interaction of cardiolipin with beef heart cytochrome oxidase

The selectivity of interaction of various cardiolipin analogues with beef heart cytochrome oxidase in reconstituted complexes with dimyristoylphosphatidylcholine has been studied by electron spin resonance spectroscopy, using lipids spin-labeled in the acyl chains. No difference in selectivity is observed between cardiolipin and its monolyso derivative, and similarly no selectivity is observed between phosphatidylcholine and lysophosphatidylcholine. Removal of the cardiolipin charge by methylation of the phosphate groups reduces but does not eliminate selectivity relative to phosphatidylcholine. The dependence of the lipid selectivity on head group and chain composition is in the order cardiolipin approximately equal to monolysocardiolipin greater than acylcardiolipin greater than dimethylcardiolipin greater than phosphatidylcholine approximately equal to lysophosphatidylcholine, where acylcardiolipin has the spin-label chain attached at the center -OH of the head group. The degree of association of the negatively charged cardiolipin derivatives with cytochrome oxidase decreases with increasing salt concentration, to a level comparable to that for dimethylcardiolipin. At high ionic strength there is still a marked selectivity relative to phosphatidylcholine. Li+ ions are more effective in screening the interaction than are Na+ ions, and divalent ions are more effective than monovalent ions. The selectivity for cardiolipin is only slightly reduced on titrating the protein to high pH. Alkylation of the protein with N-ethylmaleimide has little effect on the titration behavior. Covalent modification of the protein by reaction with citraconic anhydride decreases the selectivity of interaction with cardiolipin. It is concluded that cardiolipin possesses an additional specificity of interaction with cytochrome oxidase other than that of purely electrostatic origin.