Effect of sterol structure on acyl chain ordering in phosphatidylcholine vesicles: a deuterium nuclear magnetic resonance and electron spin resonance study

Membrane Binding of HIV-1 Matrix Protein: Dependence on Bilayer Composition and Protein Lipidation

By assembling in a protein lattice on the host's plasma membrane, the retroviral Gag polyprotein triggers formation of the viral protein/membrane shell. The MA domain of Gag employs multiple signals--electrostatic, hydrophobic, and lipid-specific-to bring the protein to the plasma membrane, thereby complementing protein-protein interactions, located in full-length Gag, in lattice formation. We report the interaction of myristoylated and unmyristoylated HIV-1 Gag MA domains with bilayers composed of purified lipid components to dissect these complex membrane signals and quantify their contributions to the overall interaction. Surface plasmon resonance on well-defined planar membrane models is used to quantify binding affinities and amounts of protein and yields free binding energy contributions, ΔG, of the various signals. Charge-charge interactions in the absence of the phosphatidylinositide PI(4,5)P2 attract the protein to acidic membrane surfaces, and myristoylation increases the affinity by a factor of 10; thus, our data do not provide evidence for a PI(4,5)P2 trigger of myristate exposure. Lipid-specific interactions with PI(4,5)P2, the major signal lipid in the inner plasma membrane, increase membrane attraction at a level similar to that of protein lipidation. While cholesterol does not directly engage in interactions, it augments protein affinity strongly by facilitating efficient myristate insertion and PI(4,5)P2 binding. We thus observe that the isolated MA protein, in the absence of protein-protein interaction conferred by the full-length Gag, binds the membrane with submicromolar affinities.

IMPORTANCE

Like other retroviral species, the Gag polyprotein of HIV-1 contains three major domains: the N-terminal, myristoylated MA domain that targets the protein to the plasma membrane of the host; a central capsid-forming domain; and the C-terminal, genome-binding nucleocapsid domain. These domains act in concert to condense Gag into a membrane-bounded protein lattice that recruits genomic RNA into the virus and forms the shell of a budding immature viral capsid. In binding studies of HIV-1 Gag MA to model membranes with well-controlled lipid composition, we dissect the multiple interactions of the MA domain with its target membrane. This results in a detailed understanding of the thermodynamic aspects that determine membrane association, preferential lipid recruitment to the viral shell, and those aspects of Gag assembly into the membrane-bound protein lattice that are determined by MA.

Conformational analysis of 9beta,19-cyclopropyl sterols: Detection of the pseudoplanar conformer by nuclear Overhauser effects and its functional implications

Nuclear Overhauser difference spectroscopy and variable temperature studies of the 9beta,19-cyclopropyl sterols 24,25-dehydropollinastanol (4,4-desmethyl-5alpha-cycloart-24-en-3beta-ol) and cyclolaudenol [(24S)-24-methyl-5alpha-cycloart-25(27)-en-3beta-ol] have shown the solution conformation of the B/C rings to be twist-chair/twist-boat rather than boat/chair as suggested in the literature. This is very similar to the known crystal structure conformation of 9beta,19-cyclopropyl sterols. The effect of these conformations on the molecular shape is highly significant: the first conformation orients into a pseudoplanar or flat shape analogous to lanosterol, whereas the latter conformation exhibits a bent shape. The results are interpreted to imply that, for conformational reasons, cyclopropyl sterols can be expected to maintain the pseudoplanar shape in membrane bilayers.

What happens if cholesterol is made smoother: importance of methyl substituents in cholesterol ring structure on phosphatidylcholine-sterol interaction

Although sterols constitute one of the most important molecular species in cells, the reasons for their structure-function relationships in lipid membranes are not well understood. The main objective of this work is to elucidate the recently suggested possibility that the ordering and condensing effects of sterols on phospholipid membranes are related to the smoothness of a sterol. We focus on cholesterol, which has two methyl groups attached to its beta-face, and compare its properties to those of demethylated cholesterol (Dchol), from which the two methyl groups have been removed. Atomic-scale molecular dynamics simulations of lipid membranes comprised of saturated lipids and sterols, either cholesterol or Dchol, provide compelling evidence that despite its smoother structure, the ordering and condensing effects of Dchol are less effective than those of cholesterol. The ordering capability of both cholesterol and Dchol is highly asymmetric with respect to their ring structure, but whereas cholesterol favors the alpha-face, Dchol favors the beta-face. The origin and implications of this difference are analyzed in detail. The picture that emerges from this study supports a view that the two methyl groups at the steroid ring system of cholesterol play an important role in cholesterol-lipid interactions by reducing sterol tilt in the bilayer and hence allowing for an optimal orientation for cholesterol.

Comparative calorimetric and spectroscopic studies of the effects of lanosterol and cholesterol on the thermotropic phase behavior and organization of dipalmitoylphosphatidylcholine bilayer membranes

We carried out comparative DSC and Fourier transform infrared spectroscopic studies of the effects of cholesterol and lanosterol on the thermotropic phase behavior and organization of DPPC bilayers. Lanosterol is the biosynthetic precursor of cholesterol and differs in having three rather than two axial methyl groups projecting from the beta-face of the planar steroid ring system and one axial methyl group projecting from the alpha-face, whereas cholesterol has none. Our DSC studies indicate that the incorporation of lanosterol is more effective than cholesterol is in reducing the enthalpy of the pretransition. Lanosterol is also initially more effective than cholesterol in reducing the enthalpies of both the sharp and broad components of the main phase transition. However, at sterol concentrations of 50 mol %, lanosterol does not abolish the cooperative hydrocarbon chain-melting phase transition as does cholesterol. Moreover, at higher lanosterol concentrations ( approximately 30-50 mol %), both sharp and broad low-temperature endotherms appear in the DSC heating scans, suggestive of the formation of lanosterol crystallites, and of the lateral phase separation of lanosterol-enriched phospholipid domains, respectively, at low temperatures, whereas such behavior is not observed with cholesterol at comparable concentrations. Our Fourier transform infrared spectroscopic studies demonstrate that lanosterol incorporation produces a less tightly packed bilayer than does cholesterol, which is characterized by increased hydration in the glycerol backbone region of the DPPC bilayer. These and other results indicate that lanosterol is less miscible in DPPC bilayers than is cholesterol, but perturbs their organization to a greater extent, probably due primarily to the rougher faces and larger cross-sectional area of the lanosterol molecule and perhaps secondarily to its decreased ability to form hydrogen bonds with adjacent DPPC molecules. Nevertheless, lanosterol does appear to produce a lamellar liquid-ordered phase in DPPC bilayers, although this phase is not as tightly packed as comparable cholesterol/DPPC mixtures.

Non-polar interactions between cholesterol and phospholipids: a molecular dynamics simulation study

A 15-ns molecular dynamics simulation of the fully hydrated dimyristoylphosphatidylcholine-cholesterol (DMPC-Chol) bilayer containing approximately 22 mol% Chol was carried out. An 8-ns trajectory was analysed to investigate the effect of Chol on the chain packing in the bilayer core. While the packing of DMPC chains on the smooth alpha-face side of the Chol ring is similar to that in the pure DMPC bilayer, the packing on the rough beta-face side is less regular and less tight. Two methyl groups located on the Chol beta-face disturb the packing; in effect, van der Waals (vdW) interactions between Chol rings and DMPC chains are weaker than the ones between sole DMPC chains. VdW interactions between an alkyl chain of DMPC and an isooctyl tail of Chol are similarly strong as those between two DMPC chains.

Cholesterol effects on the phosphatidylcholine bilayer nonpolar region: a molecular simulation study

A 15-ns molecular dynamics (MD) simulation of the fully hydrated dimyristoylphosphatidylcholine-cholesterol (DMPC-Chol) bilayer in the liquid-crystalline state was carried out to investigate the effect of Chol on the hydrocarbon chain region of the bilayer. The last 8-ns fragment of the generated trajectory was used for analyses. As a reference system, a pure DMPC bilayer (M. Pasenkiewicz-Gierula, Y. Takaoka, H. Miyagawa, K. Kitamura, and A. Kusumi, 1999, Biophys. J. 76:1228-1240) simulated for 14 ns was used. The study shows that a Chol-induced increase of the bulk molecular order parameter along both beta- and gamma-chain is mainly caused by a decrease of the average tilt of the chains, because the bulk average number of gauche rotamers/myristoyl chain is not significantly changed by Chol. Nevertheless, for DMPCs located near Chol molecules both the number of gauche rotamers/chain and the chain tilt are decreased. The magnitude of the Chol effect on the PC alkyl chains depends, in addition to the PC-Chol distance, on the side of the Chol molecule (alpha- or beta-face) that the chains are in contact with. This study provides some new insight into the properties of the coexistence region of the partial phase diagram for DMPC-Chol bilayers.

Cholesterol and cataracts

Inherited defects in enzymes of cholesterol metabolism and use of drugs which inhibit lens cholesterol biosynthesis can be associated with cataracts in animals and man. The basis of this relationship apparently lies in the need of the lens to satisfy its sustained requirement for cholesterol by on-site synthesis, and impairing this synthesis can lead to alteration of lens membrane structure. Lens membrane contains the highest cholesterol content of any known membrane. The Smith-Lemli-Opitz syndrome, mevalonic aciduria, and cerebrotendinous xanthomatosis all involve mutations in enzymes of cholesterol metabolism, and affected patients can develop cataracts. Two established models of rodent cataracts are based on treatment with inhibitors of cholesterol biosynthesis. The long-term ocular safety of the very widely used vastatin class of hypocholesterolemic drugs is controversial. Some vastatins are potent inhibitors of cholesterol biosynthesis by animal lenses, can block cholesterol accumulation by these lenses and can produce cataracts in dogs. Whether these drugs inhibit cholesterol biosynthesis in human lenses at therapeutic doses is unknown. Results of clinical trials of 1-5 years duration in older patient populations indicate high ocular safety. However, considering the slow life-long growth of the lens and its continuing need for cholesterol, longterm safety of the vastatins should perhaps be viewed in units of 10 or 20 years, particularly with younger patients.

Molecular order and dynamics of phosphatidylcholine bilayer membranes in the presence of cholesterol, ergosterol and lanosterol: a comparative study using 2H-, 13C- and 31P-NMR spectroscopy

We report the results of a comparative study of the molecular order and dynamics of phosphatidylcholine (PC) bilayer membranes in the absence and presence of cholesterol, ergosterol and lanosterol, using deuterium (2H) nuclear magnetic resonance (NMR) of deuterated phospholipid molecules, in addition to solid state 13C and 31P-NMR. Using dimyristoylphosphatidylcholines (DMPCs) specifically labeled at positions 2', 3', 4', 6', 8', 10' and 12' of the sn-2 chain together with the perdeuterated 2-[2H27]DMPC derivative, the order profile for 9 of the 13 methylene groups of the sn-2 chain was established at 25 degrees C for DMPC, DMPC/cholesterol, DMPC/ergosterol and DMPC/lanosterol membranes, at a fixed sterol/phospholipid mol ratio of 30%, and in the presence of excess water. The overall ordering effects were found to be ergosterol > cholesterol >> lanosterol. Transverse relaxation (T2e) studies of these systems indicated that while for DMPC, DMPC/cholesterol and DMPC/ergosterol the relative relaxation rates were in qualitative agreement with models which assume cooperative motions of the bilayer molecules as the main relaxation mechanism, those in DMPC/lanosterol were anomalously high, suggesting alterations of lipid packing. Using dipalmitoylphosphatidylcholine (DPPC) deuterated at the trimethylammonium group of the choline moiety, we found that the differential ordering and motional effects induced by the sterols in the acyl chains were also reflected in the headgroup, both in the gel (L beta) and liquid-crystalline phases. 13C and 1H spin dynamics studies of these systems, including cross-polarization, rotating frame longitudinal relaxation and dipolar echo relaxation rates showed that the mobility of the different regions of the phospholipid molecules in the binary lipid systems were inversely correlated with the ordering effects induced by the sterols. A novel combination of C-D bond order parameters (obtained by 2H-NMR) and 13C-1H cross polarization rates confirmed these results. The effects of the same sterols at the same molar proportion on the unsaturated lipid 1-[2H31]palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (2H31-POPC) at 25 and 35 degrees C were different from those observed on DMPC and showed ordering effects which are largest for cholesterol, while ergosterol and lanosterol produced significantly smaller effects. Transverse relaxation studies indicate that while cholesterol does not perturb cooperative motions in POPC, both ergosterol and lanosterol do. Again, high-resolution solid state 13C-NMR studies support the conclusions of the 2H-NMR experiments.(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of stigmastanol and stigmastanyl-phosphorylcholine, two plasma cholesterol lowering substances, on synthetic phospholipid membranes. A 2H- and 31P-NMR study

Cholesterol, stigmastanol, and stigmastanyl-phosphorylcholine (ST-PC) were incorporated into model membranes composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) or 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). POPC and ST-PC were deuterated at the lipid headgroup, DOPC at the cis-double bonds. The influence of the three sterols on the motion and conformation of the lipid headgroups and the hydrocarbon chains was monitored with 2H- and 31P-NMR. All three sterols were freely miscible with the lipid matrix in concentrations of up to 50 mol% without inducing phase separations or nonbilayer structures. However, the molecules exert quite different effects on the phospholipid bilayer. Cholesterol and stigmastanol are largely buried in the hydrocarbon part of the membrane, distinctly restricting the flexing motions of the fatty acyl chains whereas the conformation of the phospholipid headgroups is little affected. In contrast, ST-PC is anchored with its headgroup in the layer of phospholipid dipoles, preventing an extensive penetration of the sterol ring into the hydrocarbon layer. Hence ST-PC has almost no effect on the hydrocarbon chains but induces a characteristic conformational change of the phospholipid headgroups. The 2H- and 31P-NMR spectra of mixed phospholipid/ST-PC membranes further demonstrate that the PC headgroup of ST-PC has a similar orientation as the surrounding phosphatidylcholine headgroups. For both types of molecules the -P-N+ dipole is essentially parallel to the membrane surface. Addition of ST-PC induces a small rotation of the POPC headgroup towards the water phase.

Soybean phosphatidylcholine vesicles containing plant sterols: a fluorescence anisotropy study

The typical plant sterols (sitosterol, stigmasterol and campesterol) were compared with respect to their ability to regulate membrane fluidity of soybean phosphatidylcholine (PC) vesicles. Fluidity changes were monitored by the steady-state fluorescence anisotropy with 1,6-diphenyl-1,3,5-hexatriene as a probe and assigned to a measure of the acyl chain orientational order. Sitosterol and campesterol appear to be the most suitable sterols in ordering the acyl chains of soybean lecithin bilayers, even more efficient than cholesterol, the standard of reference for sterol effects on membranes, suggesting that they play a significant role in the regulation of plant membrane properties. Stigmasterol is shown to be much less active. Cycloartenol, a biosynthetic precursor of plant sterols, increases the acyl chain order with the same efficiency as cholesterol. We also investigated the effects of two unusual sterols, 24-methylpollinastanol and 14 alpha,24-dimethylcholest-8-en-3 beta-ol, which were shown to accumulate in plants treated with fungicides belonging to two important classes, N-substituted morpholines and triazoles, respectively. These two sterols exhibit a behavior very similar to that of stigmasterol. The results are discussed in terms of sterol effects on the molecular packing of soybean PC bilayers.

Effects of steroid molecules on the dynamical structure of dioleoylphosphatidylcholine and digalactosyldiacylglycerol bilayers

The ESR spectra of cholestane spin labels (CSL) in dioleoylphosphatidylcholine (DOPC) bilayers containing 20 wt% of cholesterol, 7-dehydrocholesterol, beta-sitosterol, stigmasterol and lanosterol exhibit a marked similarity, thus indicating that these steroids induced the same effects on the lipid bilayer over the temperature range 21-55 degrees C. The incorporation of these steroids into the DOPC bilayers enhances the orientational order of the CSL molecules at every temperature studied, but only induces a pronounced slow-down in their rotational motions at temperatures above 35 degrees C. Similar results were obtained in DOPC/ergosterol multilamellar liposomes, but the changes are now less pronounced than in the other five DOPC/steroid systems. In contrast, the addition of stigmasterol to digalactosyldiacylglycerol (DGDG) bilayers appears to increase the order parameter mean value of P2, without affecting the diffusion coefficients. Furthermore, the incorporation of 7-dehydrocholesterol to DGDG bilayers causes a large enhancement in the orientational order, but has only a small effect on D perpendicular of the CSL molecules. Importantly, this latter effect appears to be independent of temperature. The marked changes in the rates of the rotational motion brought about by the addition of steroids, contrasts with the lack of a significant effect of unsaturation on the bilayer dynamics reported by us previously (Korstanje et al. (1989), Biochim. Biophys. Acta 980, 225-233, and 982, 196-204).

Alteration of lipid order profile and permeability of plasma membranes from Trypanosoma cruzi epimastigotes grown in the presence of ketoconazole

Highly purified preparations of plasma membranes from control and ketoconazole-treated (1 microM, 120 h) epimastigotes of Trypanosoma cruzi have been obtained by cell disruption using abrasion with glass beads, differential centrifugation and isopycnic centrifugation in continuous, self-generating Percoll gradients. The purity of the preparation was ascertained by the specific activity 125I bound to the membranes obtained from enzymatically radiolabeled epimastigotes and by the alpha-methyl-mannoside sensitive binding of 125I-concanavalin A. The membranes form closed vesicles of 0.2-0.4 micron in diameter which display Mg2+ ATPase and acid phosphatase activities, but are devoid of 5'-nucleotidase and succinate-cytochrome c oxidoreductase; these vesicles can be strongly agglutinated by concanavalin A. The lipid order profiles of membranes from control and treated cells were compared with that present in egg phosphatidylcholine/ergosterol liposomes (84:16, mol/mol) by electron spin resonance spectroscopy of doxylstearic acid probes with the nitroxide group bound to carbon 5, 10, 12 and 16 of the stearic acid chain. Membranes from treated epimastigotes have a lipid order profile which resembles that of control plasma membranes near the polar surface (positions 5 and 10) but there is an abrupt decrease of order at position 12 and from there to the center of bilayer is highly disordered, even more than in pure lipid membranes. Consistent with these results, the leakage of L-[14C]glucose from membrane vesicles of ketoconazole-treated cells is much faster than that observed in vesicles obtained from control cells. These results indicate a strong alteration of the plasma membrane physical and biological properties due to the incubation of the parasite with the drug; this alteration is consistent with the accumulation of methylated precursors of ergosterol, which affects both lipid-lipid and lipid-protein interactions in the membrane.

The effects of cholesterol on lateral diffusion and vertical fluctuations in lipid bilayers. An electron-electron double resonance (ELDOR) study

Electron-electron double resonance (ELDOR) and saturation-recovery spectroscopy employing 14N:15N stearic acid spin-label pairs have been used to study the effects of cholesterol on lateral diffusion and vertical fluctuations in lipid bilayers. The 14N:15N continuous wave electron-electron double resonance (CW ELDOR) theory has been developed using rate equations based on the relaxation model. The collision frequency between 14N-16 doxyl stearate and 15N-16 doxyl stearate, WHex (16:16), is indicative of lateral diffusion of the spin probes, while the collision frequency between 14N-16 doxyl stearate and 15N-5 doxyl stearate, WHex (16:5), provides information on vertical fluctuations of the 14N-16 doxyl stearate spin probe toward the membrane surface. Our results show that: (a) cholesterol decreases the electron spin-lattice relaxation time Tle of 14N-16 doxyl stearate spin label in dimyristoylphosphatidylcholine (DMPC) and egg yolk phosphatidylcholine (egg PC). (b) Cholesterol increases the biomolecular collision frequency WHex (16:16) and decreases WHex (16:5), suggesting that incorporation of cholesterol significantly orders the part of the bilayer that it occupies and disorders the interior region of the bilayer. (c) Alkyl chain unsaturation of the host lipid moderates the effect of cholesterol on both vertical fluctuations and lateral diffusion of 14N-16 doxyl stearate. And (d), there are marked differences in the effects of cholesterol on lateral diffusion and vertical fluctuations between 0-30 mol% and 30-50 mol% of cholesterol that suggest an inhomogeneous distribution of cholesterol in the membrane.

Raman spectroscopy of selectively deuterated dimyristoylphosphatidylcholine: studies on dimyristoylphosphatidylcholine-cholesterol bilayers

Dimyristoylphosphatidylcholine (DMPC), selectively deuterated in the sn-2 chain at the 3, 6, and 10 positions is used to probe DMPC-cholesterol interactions in multilamellar dispersions. Using the Raman spectral linewidths of the 2100 cm-1 C2H2 stretching modes as an index of membrane disorder, we demonstrate that cholesterol tends to order, or increase the number of trans carbon-carbon bonds within the DMPC acyl chain near the headgroup region at all temperatures. At low temperatures, cholesterol disorders the acyl chains near the methyl termini by inducing gauche conformers; cholesterol orders the entire chain at higher temperatures. These determinations are qualitatively consistent with conclusions drawn from deuterium nuclear magnetic resonance studies, but specifically reflect acyl chain trans/gauche isomerization on the 10(-12)-10(-13) s vibrational time scale.

Cholesterol and the cell membrane

Recent studies concerning cholesterol, its behavior and its roles in cell growth provide important new clues to the role of this fascinating molecule in normal and pathological states.

Cardiolipin vesicles can accommodate cholesterol up to 0.80 mole fraction, i.e. one molecule per cardiolipin fatty acid chain

Cardiolipin-cholesterol interactions were studied by time-resolved fluorescence spectroscopy with 1,6-diphenyl-1,3,5-hexatriene as probe. The residual anisotropy parameter, r infinity (reflecting the fatty acid chain packing), was measured in the liquid crystalline phase as a function of cholesterol addition. Two main results are reported: (i) a slight increase of the order parameter, S, computed from the r infinity value as S = (r infinity/r0)1/2, in the physiological concentration range of cholesterol; (ii) a sharp enhancement of S from a cholesterol mole fraction (X chl) of 0.20 and up to X chl of 0.80. This is in contrast to unsaturated lecithin systems for which a continuous increase of the order parameter was monitored, culminating at X chl = 0.50, the well-known maximum level of incorporation of cholesterol into lecithin model membranes.

Lanosterol and cholesterol have different effects on phospholipid acyl chain ordering

2H nuclear magnetic resonance (2H-NMR) spectra of dioleoylphosphatidylcholine labelled at positions 9 and 10 in the acyl chains of the phospholipid were obtained in the presence of cholesterol and lanosterol. The spectra show in all cases three quadrupole splittings. One is due to the deuterium on position 10 of the sn-1 chain and another to the deuterium on position 10 of the sn-2 chain. The third deuterium quadrupole splitting arises from the deuterium at position 9 of both chains. Cholesterol, at increasing concentration, produces an increase in the quadrupole splitting from position 9, corresponding to an increase in order of that C-D bond segment arising from the inclusion of cholesterol in the membrane. Little effect is noted on the quadrupole splittings arising from position 10 of either chain. Lanosterol appears to have no effect on the quadrupole splittings from position 9. Lanosterol, likewise, has no effects on the quadrupole splittings from position 10 of both chains. These data therefore suggest little disorganization of the membrane structure due to the 14-methyl group. However, the 14-methyl group prevents lanosterol from causing the increase in motional order of the phospholipid hydrocarbon chains characteristic of cholesterol.

Effect of sterol incorporation on head group separation in liposomes

Electrophoretic mobilities of multilamellar liposomes of varying composition have been measured to determine the effect of incorporated sterols on surface charge density. Liposomes made from mixtures of zwitterionic egg phosphatidylcholine (PC) and anionic egg phosphatidylglycerol (PG) in varying proportions were shown to have electrophoretic mobilities consistent with the anticipated surface charge density. Incorporation of cholesterol up to 50 mole per cent in the bilayer produced no detectable change in surface charge density. Similar results were obtained for lanosterol and epicoprostanol. These results are interpreted to mean that incorporation of the sterols into the bilayers produced no detectable change (less than 3%) in the spacing of charged phospholipids. It is inferred that sterols are incorporated among the fatty acyl chains of these phospholipid bilayers with little or no displacement of the head groups at the surface.

Properties of the strongly immobilized signal observed in spin-labeled erythrocytes

A strongly immobilized signal from fatty acid spin labels was observed in human erythrocytes treated with oxidizing agents such as glutaraldehyde, hydrogen peroxide, phenylhydrazine and copper-ortho-phenanthroline. This signal was also observed in freshly prepared ghosts treated with potassium superoxide and in old erythrocyte ghosts. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of these samples demonstrated the diffuse, nondiscrete bands of high molecular weight due to the cross-linking of membrane proteins. The temperature and pH dependences of the outer hyperfine splitting of this signal were very similar to those of bovine serum albumin. We propose that the strongly immobilized signal reflects the interaction of the lipids with the cross-linked products of membrane proteins.

Coordinate regulation of unsaturated phospholipid, RNA, and protein synthesis in Mycoplasma capricolum by cholesterol

The effect of cholesterol, epicoprostanol, and phosphatidylcholine on phospholipid, RNA, and protein synthesis was investigated in the sterol auxotroph Mycoplasma capricolum. Cells growing poorly on lanosterol were stimulated to grow more rapidly by supplementing the medium with either 2 micrograms of cholesterol or 2.2 micrograms of egg phosphatidylcholine per ml. In such cells cholesterol caused a sequential stimulation of phospholipid, RNA, and protein synthesis. Enhanced oleate incorporation into phospholipid occurred early; the rates of RNA and protein synthesis increased later. In cells supplemented with phosphatidylcholine only RNA and protein syntheses were enhanced. The addition of 2 micrograms of epicoprostanol per ml to cells growing on lanosterol promptly inhibited the rate of unsaturated phospholipid synthesis and subsequently the rate of growth. Inhibition of both processes was relieved by supplying 2 micrograms of cholesterol or 2.2 micrograms of phosphatidylcholine per ml along with the inhibitory sterol. The results suggest that cholesterol in small amounts exerts a positive regulatory effect and epicoprostanol exerts a negative one on unsaturated phospholipid synthesis and, in turn, that RNA and protein synthesis are coordinately controlled with phospholipid synthesis. The previously reported phenomenon of sterol synergism and the postulated novel role of sterols in membranes.