Peptides that mimic the pseudosubstrate region of protein kinase C bind to acidic lipids in membranes

The electrostatics of lipid surfaces

Charged lipids constitute a substantial fraction of all membrane lipids. Their charges vary in quantity and distribution within their headgroup regions. In long range interactions, their charges' value and electrostatic potential in the vicinity of the membrane surface can be approximated by the Guy-Chapman theory. This theory treats the interface as a charged structureless plain surrounded by uniform environments. However, if one considers intermolecular interactions, such assumptions need to be revised. The interface is in reality a thick region containing the residual charges of lipid headgroups. Their arrangement depends on the type of lipid present in the membrane. The variety of lipids and their biological functions suggests that charge distribution determines the extent and type of interaction with surface associated molecules. Numerous examples show that protein behavior at the lipid bilayer surface is determined by the type of lipid present, indicating protein specificity towards certain surface locations and local properties (determined by lipid composition) of a particular type. Such specificity is achieved by a combination of electrostatic, hydrophobic and enthropic effects. Comparing lipid biological activity, it can be stated that residual charge distribution is one of the factors of intermolecular recognition leading to the specific interaction of lipid molecules and selected proteins in various processes, particularly those involved with signal transduction pathways. Such specificity enables a variety of processes occurring simultaneously on the same membrane surface to function without cross-reaction interference.

Amphitropic proteins: regulation by reversible membrane interactions (review)

What do Src kinase, Ras-guanine nucleotide exchange factor, cytidylyltransferase, protein kinase C, phospholipase C, vinculin, and DnaA protein have in common? These proteins are amphitropic, that is, they bind weakly (reversibly) to membrane lipids, and this process regulates their function. Proteins functioning in transduction of signals generated in cell membranes are commonly regulated by amphitropism. In this review, the strategies utilized by amphitropic proteins to bind to membranes and to regulate their membrane affinity are described. The recently solved structures of binding pockets for specific lipids are described, as well as the amphipathic alpha-helix motif. Regulatory switches that control membrane affinity include modulation of the membrane lipid composition, and modification of the protein itself by ligand binding, phosphorylation, or acylation. How does membrane binding modulate the protein's function? Two mechanisms are discussed: (1) localization with the substrate, activator, or downstream target, and (2) activation of the protein by a conformational switch. This paper also addresses the issue of specificity in the cell membrane targetted for binding.

Mechanism of the apparent cooperativity in the interaction of protein kinase C with phosphatidylserine

Protein kinase C displays high apparent cooperativity in its activation by phosphatidylserine. This contribution uses a novel approach to address the physical basis for this apparent cooperativity. We examine the binding of protein kinase C betaII to large unilamellar vesicles as a function of increasing mole fraction phosphatidylserine and as a function of increasing total lipid concentrations. Binding data are subjected to an analysis, described in the Appendix, that allows calculation of the fractional saturation of phosphatidylserine binding sites with this ligand. This analysis reveals that (1) protein kinase C betaII binds approximately eight phosphatidylserine molecules and (2) the binding of each lipid is not cooperative. Rather, the apparent cooperativity observed in protein kinase C's interaction with multiple phosphatidylserine molecules arises from effects specific to the interaction of a multivalent macromolecule with multiple membrane-associated ligands. Nor does diacylglycerol, which has been previously shown to dramatically increase protein kinase C's affinity for phosphatidylserine-containing membranes, induce cooperativity. Thus, protein kinase C binds multiple phosphatidylserine molecules in the absence of interaction between potential binding sites. The method presented for determining the stoichiometry and cooperativity in the interaction of protein kinase C with phosphatidylserine is applicable to any multivalent molecule binding to monovalent ligands incorporated into lipid membranes.

Protein kinase C as a molecular machine for decoding calcium and diacylglycerol signals

The specificity of many signal transduction pathways relies on the temporal coordination of different second messenger signals. Here we found a molecular mechanism which guarantees that conventional protein kinase C (PKC) isoforms are sequentially activated by calcium and diacylglycerol signals. Receptor stimuli that triggered repetitive calcium spikes induced a parallel repetitive translocation of GFP-tagged PKCgamma to the plasma membrane. While calcium acted rapidly, diacylglycerol binding to PKCgamma was initially prevented by a pseudosubstrate clamp, which kept the diacylglycerol-binding site inaccessible and delayed calcium- and diacylglycerol-mediated kinase activation. After termination of calcium signals, bound diacylglycerol prolonged kinase activity. The properties of this molecular decoding machine make PKCgamma responsive to persistent diacylglycerol increases combined with high- but not low-frequency calcium spikes.

alpha-Adrenergic effects on Na+-K+ pump current in guinea-pig ventricular myocytes

1. The whole-cell patch clamp was employed to study Na+-K+ pump current (Ip) in acutely isolated myocytes. alpha-Adrenergic receptors were activated with noradrenaline (NA) after blocking beta-adrenergic receptors with propranolol. Ip was measured as the current blocked by strophanthidin (Str). 2. Activation of alpha-receptors by NA increased Ip in a concentration-dependent manner. The K0.5 depended on intracellular calcium ([Ca2+]i), however maximal stimulation did not. At 15 nM [Ca2+]i the K0.5 was 219 nM NA whereas at 1.4 microM [Ca2+]i it was 3 nM. 3. The voltage dependence of Ip was not shifted by NA at either high or low [Ca2+]i. At each voltage, maximal stimulation of Ip was 14-15 %. 4. Staurosporine (St), an inhibitor of protein kinase C (PKC), eliminated the alpha-receptor-mediated stimulation of Ip at either high or low[Ca2+]i. 5. The stimulation of Ip was independent of changes in intracellular sodium or external potassium concentrations, and did not reflect a change in affinity for Str. 6. Phenylephrine, methoxamine and metaraminol, three selective alpha1-adrenergic agonists, stimulate Ip in a similar manner to NA. Stimulation of Ip by NA was eliminated by prazosin, an alpha1-antagonist, but was unaffected by yohimbine, an alpha2-antagonist. 7. We conclude noradrenaline activates ventricular alpha1-receptors, which are specifically coupled via PKC to increase Na+-K+ pump current. The sensitivity of the coupling is [Ca2+]i dependent, however the maximal increase in pump current is [Ca2+]i and voltage independent.

Synergistic effects of diacylglycerols and fatty acids on membrane structure and protein kinase C activity

The synergistic effects of diacylglycerol (DAG) and fatty acid (FA) in activating protein kinase C have been investigated by correlating their individual and combined effects on enzymatic activity and on membrane structure in phosphatidylcholine/phosphatidylserine (4:1) lipid mixtures using a combination of specific enzymatic assays and 31P and 2H NMR. Addition of DAGs and unsaturated FAs to the bilayers synergistically increased the tendency of the lipids to form nonbilayer phases with a concomitant increase in PKC activity until a maximum was achieved. Further increases in the DAG/FA concentration led to the formation of the nonbilayer lipid phases under the conditions of the PKC activity assays and correlated with decreased activity. The nonbilayer lipid phases still supported PKC activity, although with less than 50% efficiency as compared with the bilayer lipids. Long-chain saturated FA increased DAG-induced PKC activity by causing a lateral phase separation of gel (Lbeta) and liquid-crystalline (Lalpha) domains. Due to the preferential partitioning of DAGs into liquid-crystalline domains, the local DAG concentration increased in these domains, leading to an increase in PKC activity. Because a wide range of lipophilic compounds is capable of altering curvature stress, and therefore the tendency for nonbilayer phase formation in cellular membranes, these compounds would be expected to modulate PKC activity and the activities of a number of other membrane-associated enzymes that are sensitive to biophysical properties of lipid membranes.

Influence of lipid on the structure and phosphorylation of protein kinase C alpha substrate peptides

The structure and phosphorylation of two protein kinase C (PKC) alpha substrate peptides were investigated in varying lipid systems using enzyme activity assays and circular dichroism (CD) spectroscopy. The alpha-peptide, which exhibits the typical PKC alpha substrate motif and is based on the pseudosubstrate region of PKCalpha, was phosphorylated to a similar extent in bovine brain phosphatidylserine vesicles or diheptanoylphosphatidylcholine (PC7) micelles (both with 5 mol % 1,2-dioleoyl-sn-glycerol), whereas neuromodulin (NM)-peptide, which does not exhibit this motif by virtue of its primary structure, was phosphorylated to a much lesser extent in the PC7 micellar system. CD spectra of the peptides indicated that NM-peptide underwent a dramatic structural change in the presence of dimyristoylphosphatidylserine (DMPS) vesicles, whereas spectra acquired in PC7 micelles were similar to those acquired in buffer alone. No significant structural change was observed in the alpha-peptide in the presence of either lipid. PKC activity assays conducted with a series of NM-peptides successively substituted with nitroxide spin labels at each residue position suggested that several residues distal to the phosphorylation site are necessary for substrate recognition. The effect of these substitutions is not consistent with the binding of the NM-peptide to PKC in an extended structure, but is consistent with the binding of this peptide in a helical conformation. Furthermore, the docking of a helical NM-peptide to the substrate binding site of PKC suggests that the interaction is energetically feasible. These results suggest that PKC may recognize some non-linear substrate motifs and that lipid binding may convert a protein into a better PKC substrate.

Binding and dissociation of cytochrome c to and from membranes containing acidic phospholipids

Membrane association and detachment of cytochrome c (cyt c) in millisecond to second time domain were investigated by stopped-flow fluorescence spectroscopy monitoring the efficiency of energy transfer from a pyrene-fatty acid containing phospholipid derivative, 1-palmitoyl-2-[10-(pyren-1-yl)-decanoyl]-sn-glycero-3-phosphoglyce rol (PPDPG, mole fraction X = 0.01) to the heme of the cyt c. Large unilamellar liposomes composed of egg phosphatidylcholine (eggPC) with varying content of the acidic phospholipid phosphatidylglycerol (eggPG) were employed. Unexpectedly, the rate of binding of cyt c to membranes was attenuated upon increasing the mole fraction of the acidic phospholipid (XPG). For example, at 50 microM phospholipid and 5 microM cyt c, when XPG was increased from 0.20 to 0.40 the half-time for the single-exponential decay in fluorescence increased from 4.7 to 8.6 ms. A similar observation was made for the membrane binding of another cationic protein, histone H1. We suggest that the formation of cooperative hydrogen-bonded networks by deprotonated and protonated PG in the vesicle surface retards the binding of cyt c to the liposome surface. However, once formed, the complex of cyt c with acidic phospholipids is stabilized by increasing XPG. Accordingly, significantly prolonged half-times of dissociation of cyt c from liposomes by NaCl, ATP, and different cationic proteins are measured upon increasing XPG. Differences between the latter cationic membrane binding ligands most likely reflect the varying relative contributions of hydrophobicity and Coulombic forces to their attachment to liposomes. Our data on the release and binding of cyt c to liposomes as a function of XPG and in the presence of ATP also provide the first direct experimental evidence for multiple lipid binding sites in cyt c.

Age-related alteration of PKC, a key enzyme in memory processes: physiological and pathological examples

Brain aging is characterized by a progressive decline of the cognitive and memory functions. It is becoming increasingly clear that protein phosphorylation and, in particular, the activity of the calcium-phospholipid-dependent protein kinase C (PKC) may be one of the fundamental cellular changes associated with memory function. PKC is a multigene family of enzymes highly expressed in brain tissues. The activation of kinase C is coupled with its translocation from the cytosol to different intracellular sites and recent studies have demonstrated the key role played by several anchoring proteins in this mechanism. PKC-phosphorylating activity appears to be impaired during senescence at brain level in a strain-dependent fashion in rodents. Whereas the levels of the various isoforms do not show age-related alterations, the enzyme translocation upon phorbol-ester treatment is deficitary among all strains investigated. Anchoring proteins may contribute to this activation deficit. We discuss also modifications of the PKC system in Alzheimer's disease that may be related to pathological alterations in neurotransmission. A better insight of the different factors controlling brain-PKC activation may be important not only for elucidating the molecular basis of neuronal transmission, but also for identifying new approaches for correcting or even preventing age-dependent changes in brain function.

Effects of dipalmitoylglycerol and fatty acids on membrane structure and protein kinase C activity

The individual and combined effects of the saturated diacylglycerol (DAG) dipalmitin (DP) and saturated or polyunsaturated unesterified fatty acids (PUFAs) on both the structure of phosphatidylcholine/phosphatidylserine (PC/PS; 4:1 mol/mol) bilayers and on protein kinase C (PKC) activity were studied using 2H nuclear magnetic resonance (NMR) and enzyme activity assays. In the absence of DP, PUFAs only slightly activated PKC whereas palmitic acid had no effect. In the absence of fatty acids, DP induced lateral phase separation of the bilayer into liquid-crystalline and gel phases. Under these conditions virtually all DP was sequestered into the gel phase and no activation of PKC was observed. The addition of polyunsaturated arachidonic or docosahexaenoic acids to the DP-containing bilayers significantly increased the relative amounts of DP and other lipid components in the liquid-crystalline phase, correlating with a dramatic increase in PKC activity. Furthermore, the effect was greater with PS, resulting in an enrichment of PS in the liquid-crystalline domains. In the presence of DP, palmitic acid did not decrease the amount of gel phase lipid and had no effect on PKC activity. The results explain the observed lack of PKC-activating capacity of long-chain saturated DAGs as due to the sequestration of DAG into gel domains wherein it is complexed with phospholipids and thus not available for the required interaction with the enzyme.

Spin-label electron spin resonance studies on the interactions of lysine peptides with phospholipid membranes

The interactions of lysine oligopeptides with dimyristoyl phosphatidylglycerol (DMPG) bilayer membranes were studied using spin-labeled lipids and electron spin resonance spectroscopy. Tetralysine and pentalysine were chosen as models for the basic amino acid clusters found in a variety of cytoplasmic membrane-associating proteins, and polylysine was chosen as representative of highly basic peripherally bound proteins. A greater motional restriction of the lipid chains was found with increasing length of the peptide, while the saturation ratio of lipids per peptide was lower for the shorter peptides. In DMPG and dimyristoylphosphatidylserine host membranes, the perturbation of the lipid chain mobility by polylysine was greater for negatively charged spin-labeled lipids than for zwitterionic lipids, but for the shorter lysine peptides these differences were smaller. In mixed bilayers composed of DMPG and dimyristoylphosphatidylcholine, little difference was found in selectivity between spin-labeled phospholipid species on binding pentalysine. Surface binding of the basic lysine peptides strongly reduced the interfacial pK of spin-labeled fatty acid incorporated into the DMPG bilayers, to a greater extent for polylysine than for tetralysine or pentalysine at saturation. The results are consistent with a predominantly electrostatic interaction with the shorter lysine peptides, but with a closer surface association with the longer polylysine peptide.

High-resolution mono- and multidimensional magic angle spinning 1H nuclear magnetic resonance of membrane peptides in nondeuterated lipid membranes and H2O

High-speed (14 kHz) solid-state magic angle spinning (MAS) 1H NMR has been applied to several membrane peptides incorporated into nondeuterated dilauroyl or dimyristoylphosphatidylcholine membranes suspended in H2O. It is shown that solvent suppression methods derived from solution NMR, such as presaturation or jump-return, can be used to reduce water resonance, even at relatively high water content. In addition, regioselective excitation of 1H peptide resonances promotes an efficient suppression of lipid resonances, even in cases where these are initially two orders of magnitude more intense. As a consequence, 1H MAS spectra of the peptide low-field region are obtained without interference from water and lipid signals. These display resonances from amide and other exchangeable 1H as well as from aromatic nonexchangeable 1H. The spectral resolution depends on the specific types of resonance and membrane peptide. For small amphiphilic or hydrophobic oligopeptides, resolution of most individual amide resonance is achieved, whereas for the transmembrane peptide gramicidin A, an unresolved amide spectrum is obtained. Partial resolution of aromatic 1H occurs in all cases. Multidimensional 1H-MAS spectra of membrane peptides can also be obtained by using water suppression and regioselective excitation. For gramicidin A, F2-regioselective 2D nuclear Overhauser effect spectroscopy (NOESY) spectra are dominated by intermolecular through-space connectivities between peptide aromatic or formyl 1H and lipid 1H. These appear to be compatible with the known structure and topography of the gramicidin pore. On the other hand, for the amphiphilic peptide leucine-enkephalin, F2-regioselective NOESY spectra mostly display cross-peaks originating from though-space proximities of amide or aromatic 1H with themselves and with aliphatic 1H. F3-regioselective 3D NOESY-NOESY spectra can be used to obtain through-space correlations within aliphatic 1H. Such intrapeptide proximities should allow determination of the conformation of the peptide in membranes. It is suggested that high-speed MAS multidimensional 1H NMR of peptides in nondeuterated membranes and in H2O can be used for studies of both peptide structure and lipid-peptide interactions.

Lipid lateral heterogeneity in phosphatidylcholine/phosphatidylserine/diacylglycerol vesicles and its influence on protein kinase C activation

To test the hypothesis that the activation of protein kinase C (PKC) is influenced by lateral heterogeneities of the components of the lipid bilayer, the thermotropic phase behavior of dimyristoylphosphatidylcholine (DMPC)/dimyristoylphosphatidylserine (DMPS)/dioleoylglycerol (DO) vesicles was compared with the activation of PKC by this system. Differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy were used to monitor the main transition (i.e., the gel-to-fluid phase transition) as a function of mole fraction DO (chi(DO)) in DMPC/DO, DMPS/DO, and [DMPC/DMPS (1:1, mol/mol)]/DO multilamellar vesicles (MLVs). In each case, when chi(DO) < or approximately 0.3, DO significantly broadened the main transition and shifted it to lower temperatures; but when chi(DO) > approximately 0.3, the main transition became highly cooperative, i.e., narrow, again. The coexistence of overlapping narrow and broad transitions was clearly evident in DSC thermograms from chi(DO) approximately 0.1 to chi(DO) approximately 0.3, with the more cooperative transition growing at the expense of the broader one as chi(DO) increased. FTIR spectroscopy, using analogs of DMPC and DMPS with perdeuterated acyl chains, showed that the melting profiles of all three lipid components in [DMPC/DMPS (1:1, mol/mol)]/DO MLVs virtually overlay when chi(DO) = 0.33, suggesting that a new type of phase, with a phospholipid/DO mole ratio near 2:1, is formed in this system. Collectively, the results are consistent with the coexistence of DO-poor and DO-rich domains throughout the compositions chi(DO) approximately 0.1 to chi(DO) approximately 0.3, even at temperatures above the main transition. Comparison of the phase behavior of the binary mixtures with that of the ternary mixtures suggests that DMPS/DO interactions may be more favorable than DMPC/DO interactions in the ternary system, especially in the gel state. PKC activity was measured using [DMPC/DMPS (1:1, mol/mol)]/DO MLVs as the lipid activator. At 35 degrees C (a temperature above the main transition of the lipids), PKC activity increased gradually with increasing chi(DO) from chi(DO) approximately 0.1 to chi(DO) approximately 0.4, and activity remained high at higher DO contents. In contrast, at 2 degrees C (a temperature below the main transition), PKC activity exhibited a maximum between chi(DO) approximately 0.1 and chi(DO) approximately 0.3, and at higher DO contents activity was essentially constant at 20-25% of the activity at the maximum. We infer from these results that the formation of DO-rich domains is related to PKC activation, and when the lipid is in the gel state, the coexistence of DO-poor and DO-rich phases also contributes to PKC activation.

Dissociation of cytochrome c from liposomes by histone H1. Comparison with basic peptides

The basic chromosomal protein histone H1 binds avidly to liposomes containing acidic phospholipids and with characteristics somewhat resembling the lipid association of cytochrome c (cyt c) [Koiv et al. (1995) Biochemistry 34, 8018-8027]. Membrane association of histone H1 strongly attenuates the lipid lateral diffusion in large unilamellar vesicles containing phosphatidylglycerol (PG) as revealed by the decrease in the excimer to monomer ratio Ie/Im of the pyrene fatty acid-containing phospholipid derivative 1-palmitoyl-2-[10-(pyren-1-yl)decanoyl]-sn-glycero-3-phosphogly cer ol (PPDPG) fluorescence. Similarly, an increase in fluorescence anisotropy of the membrane-incorporated probe, diphenylhexatriene (DPH), due to histone H1 indicates that the membrane becomes more rigid. Increasing the mole fraction of PG (XPG) increases in a linear manner the concentration [H1]s required for the maximal decrease in Ie/Im or increase in fluorescence anisotropy, thus allowing to estimate the binding site for H1 to be constituted by approximately 20 PG molecules. Domain formation is also supported by differential scanning calorimetry measurements. Subsequently, we studied the detachment of cyt c from PG-containing liposomes by H1 by measuring its efficiency in decreasing resonance energy transfer between PPDPG and the heme of cyt c. The A-site interaction of 1 microM cyt c with 25 microM PG/PC (XPG = 0.20) liposomes is fully inhibited by low (0.1 microM) histone concentrations. Upon XPG being increased, the concentration [H1]D required for complete detachment of cyt c increases. Irrespective of the [cyt c] present (varying between 0.1 and 10 microM), the C-site-mediated binding of cyt c to neat PG liposomes (XPG = 1.0) is fully prevented at [H1] = 0.6 microM. These measurements indicate that the affinity of histone H1 to liposomes exceeds that of cyt c. The above effects of H1 were subsequently compared with those of different basic membrane-associating peptides. Notably, the effects of HI were remarkably well-reproduced by polylysine (K19). The high affinity of H1 to acidic phospholipids suggests that this feature might also contribute to its physiological function.

Membrane structure of protein kinase C and calmodulin binding domain of myristoylated alanine rich C kinase substrate determined by site-directed spin labeling

Cysteine-substituted peptides based on the membrane, calmodulin, and protein kinase C binding domain of the myristoylated alanine rich C kinase substrate (MARCKS) were synthesized and derivatized with a sulfhydryl reactive proxyl nitroxide. These spin-labeled peptides were used in combination with continuous wave power saturation electron paramagnetic resonance (EPR) spectroscopy to determine the position and structure of the peptide on membranes containing phosphatidylserine. These peptides bind at the membrane interface, with nitroxide side chains in the central and C-terminal regions lying several angstroms below the level of the head group. In contrast, the N-terminus of the peptide is extended out of the membrane interface so that the two N-terminal residues are positioned on the aqueous side of the head group. When bound to the membrane, the N-terminal segment of this peptide is sensitive to the membrane surface charge density. Higher charge densities decrease the amplitude of side chain motions at the N-terminus and bring this end of the peptide closer to the membrane interface. When the location of successive residues along the bilayer normal is compared, no helical trend is seen, and no evidence for aggregation of the peptide is found. The EPR spectra of double spin-labeled peptides also show no evidence for a helical structure. Thus, these basic peptides are in an extended configuration at the membrane interface with hydrophobic side chains oriented inward toward the membrane hydrocarbon.

Characterization of electrostatic and nonelectrostatic components of protein--membrane binding interactions

A general method was developed to determine the thermodynamic parameters for the interaction of protein with membranes. Protein intrinsic tryptophan fluorescence was quenched by titration with large unilamellar vesicles containing 9,10-dibrominated distearoylphosphatidylcholine (Br4-DSPC) or a small amount of trinitrophenylphosphatidylethanolamine (TNP-PE), Binding was modeled as a bimolecular reaction of free protein with a unit of "n" lipid molecules and a dissociation constant, Kd. The contribution of residual fluorescence and light scattering could be eliminated by using the second derivative of the titration function as the basis for calculations. For the binding of C-terminal channel domain polypeptides(178-190 residues) of the colicin El ion channel, n=50-60 and Kd=2-3 nM at pH 4, ionic strength, I=0.12 M, and anionic lipid content = 40% (surface potential, psi o =-30 mV), conditions for which the protein has high activity. Values of n = 95 and 210 for the binding of a C-terminal 293-residue colicin fragment and the 522 residue intact colicin E1 molecule scale qualitatively according to the increase in molecular size. General methods are presented to distinguish the electrostatic (delta G el) and nonelectrostatic (delta G nel) components of the total delta G for binding. Using Br4DSPC as the quencher, the binding of the channel polypeptide, P178, was characterized by delta G approximately -9.8 kcal/mol, and delta G el approximately -7.0 kcal/mol, and delta G el= -2.8 kcal/mol (psi o = -30 mV). Using TNP-PE as the quencher, similar values of delta G approximately -9.3 to -9.9 kcal/mol were determined, a somewhat smaller value for delta G nel approximately -5.0 kcal/mol, and a correspondingly larger value for deltaGnel approximately -4.9 kcal/mol. The existence of a delta G nel component of this magnitude may distinguish proteins that have the potential to insert into the membrane.

Reorientational properties of fluorescent analogues of the protein kinase C cofactors diacylglycerol and phorbol ester

The reorientational properties of the fluorescently labelled protein kinase C (PKC) cofactors diacylglycerol (DG) and phorbol ester (PMA) in vesicles and mixed micelles have been investigated using time-resolved polarised fluorescence. The sn-2 acyl chain of DG was replaced by diphenylhexatriene- (DPH) propionic acid, while a dansyl labelled analogue of phorbol ester was used. The extent of ordering of DPH-DG in vesicles turned out to be slightly different from that of the control choline lipid DPH-PC. Addition of PKC to vesicles containing 30 mole% brain PS considerably slowed down the DPH-DG anisotropy decay. This was not observed when DPH-DG was replaced by DPH-PC. Analysis of the fluorescence anisotropy decays of these DPH-lipids in micelles polyoxyethylene-9-laurylether mixed with 10 mole% of the essential phosphatidylserine allowed estimation of their lateral diffusion, orientation distribution and reorientational dynamics within the micelles. Addition of PKC resulted in a significantly slower decay of the fluorescence anisotropy of both DPH-DG and DPH-PC even in the absence of calcium, indicating a calcium independent complexation of PKC with the PS containing micelles. Addition of calcium resulted in a further reduction of the decay of anisotropy of DPH-DG but not of DPH-PC indicating that the Ca2+ dependent immobilisation is cofactor-specific. Similar specific interactions with PKC resulted in a slower decay of dansylated PMA when calcium and PS were present.

Adsorption of ruthenium red to phospholipid membranes

We have measured the distribution of the hexavalent ruthenium red cation (RuR) between water and phospholipid membranes, have shown the critical importance of membrane negative surface charge for RuR binding, and determined the association constant of RuR for different phospholipid bilayers. The studies were performed with liposomes made of mixtures of zwitterionic L-alpha-phosphatidylcholine (PC), and one of the negatively charged phospholipids: L-alpha-phosphatidylserine (PS), L-alpha-phosphatidylinositol (PI), or L-alpha-phosphatidylglycerol (PG). Lipid composition of PC:PX membranes was 1:0, 19:1, 9:1, and 4:1. Liposomes were processed using freeze-and-thaw treatment, and their size distribution was characterized by light scattering and electron microscopy. Experimental distribution isotherms of RuR obtained by ultracentrifugation and spectrophotometry can be reproduced with the Langmuir-Stern-Grahame model, assuming that RuR behaves in the diffuse double layer as an ion with effective valency < 6. In terms of this model, PC-PS, PC-PI, and PC-PG membranes were found to be electrostatically equivalent and the intrinsic association constants of RuR were obtained. RuR has highest affinity to PS-containing membranes; its association constant for PC-PI and PC-PG membranes is about 5 times smaller than that for PC-PS membranes. From the comparison of RuR binding to mixed negatively charged phospholipid membranes and RuR binding to sarcoplasmic reticulum (SR), we conclude that the low-affinity RuR binding sites may indeed be associated with the lipid bilayer of SR.

C2 region-derived peptides inhibit translocation and function of beta protein kinase C in vivo

RACK1 is a protein kinase C (PKC)-binding protein that fulfills the criteria previously established for a receptor for activated C-kinase (RACK). If binding of PKC to RACK anchors the activated enzyme near its protein substrates, then inhibition of this binding should inhibit translocation and function of the enzyme in vivo. Here, we have identified such inhibitors that mimic the RACK1-binding site on beta PKC. We first found that a C2-containing fragment, but not a C1-containing fragment of beta PKC, bound to RACK1 and inhibited subsequent beta PKC binding. The RACK1-binding site was further mapped; peptides beta C2-1 (beta PKC(209-216), beta C2-2 (beta PKC(186-198)), and beta C2-4 (beta PKC(218-226), but not a number of control peptides, bound to RACK1 and inhibited the C2 fragment binding to RACK1. Peptides beta C2-1, beta C2-2, and beta C2-4 specifically inhibited phorbol ester-induced translocation of the C2-containing isozymes in cardiac myocytes and insulin-induced beta PKC translocation and function in Xenopus oocytes. Therefore, peptides corresponding to amino acids 186-198, and 209-226 within the C2 region of the beta PKC are specific inhibitors for functions mediated by beta PKC.

Protein kinase C epsilon subcellular localization domains and proteolytic degradation sites. A model for protein kinase C conformational changes

Protein kinase C (PCK) epsilon has been found to have unique properties among the PCK isozymes in terms of its membrane association, oncogenic potential, and substrate specificity. Recently we have demonstrated that PKC epsilon localizes to the Golgi network via its zinc finger domain and that both the holoenzyme and its zinc finger region modulate Golgi function. To further characterize the relationship between the domain organization and the subcellular localization of PKC epsilon, a series of NIH 3T3 cell lines were created, each overexpressing a different truncated version of PKC epsilon. The overexpressed proteins each were designed to contain an epsilon-epitope tag peptide at the COOH terminus to allow ready detection with an antibody specific for the tag. The subcellular localization of the recombinant proteins was analyzed by in vivo phorbol ester binding, immunocytochemistry, and cell fractionation followed by immunoblotting. Results revealed several regions of PKC epsilon that contain putative subcellular localization signals. The presence either of the hinge region or of a 33-amino-acid region including the pseudosubstrate sequence in the recombinant proteins resulted in association with the plasma membrane and cytoskeletal components. The catalytic domain was found predominantly in the cytosolic fraction. The accessibility and thus the dominance of these localization signals is likely to be affected by the overall conformation of the recombinant proteins. Regions with putative proteolytic degradation sites also were identified. The susceptibility of the overexpressed proteins to proteolytic degradation was dependent on the protein conformation. Based on these observations, a model depicting the interaction and hierarchy of the suspected localization signals and proteolytic degradation sites is presented.

Adsorption properties of polar/apolar inducers at a charged interface and their relevance to leukemia cell differentiation

The interfacial adsorption properties of polar/apolar inducers of cell differentiation (PAIs) were studied on a mercury electrode. This study, on a clean and reproducible charged surface, unraveled the purely physical interactions among these compounds and the surface, apart from the complexity of the biological membrane. The interfacial behavior of two classical inducers, hexamethylenebisacetamide (HMBA) and dimethylsulfoxide, was compared with that of a typical apolar aliphatic compound, 1-octanol, that has a similar hydrophobic moiety as HMBA but a much smaller dipolar moment. Both HMBA and Octanol adsorb flat in contact with the surface because of hydrophobic forces, with a very similar free energy of adsorption. However, the ratio of polar to apolar moieties in PAIs turned out to be crucial to drive the adsorption maximum toward physiological values of surface charge density, where octanol is desorbed. The electrostatic effects in the interfacial region reflected the adsorption properties: the changes in the potential drop across the interfacial region as a function of the surface charge density, in the physiological range, were opposite in PAIs as compared with apolar aliphatic compounds, as exemplified by octanol. This peculiar electrostatic effect of PAIs has far-reaching relevance for the design of inducers with an adequate therapeutic index to be used in clinical trials.

Calculations of the electrostatic potential adjacent to model phospholipid bilayers

We used the nonlinear Poisson-Boltzmann equation to calculate electrostatic potentials in the aqueous phase adjacent to model phospholipid bilayers containing mixtures of zwitterionic lipids (phosphatidylcholine) and acidic lipids (phosphatidylserine or phosphatidylglycerol). The aqueous phase (relative permittivity, epsilon r = 80) contains 0.1 M monovalent salt. When the bilayers contain < 11% acidic lipid, the -25 mV equipotential surfaces are discrete domes centered over the negatively charged lipids and are approximately twice the value calculated using Debye-Hückel theory. When the bilayers contain > 25% acidic lipid, the -25 mV equipotential profiles are essentially flat and agree well with the values calculated using Gouy-Chapman theory. When the bilayers contain 100% acidic lipid, all of the equipotential surfaces are flat and agree with Gouy-Chapman predictions (including the -100 mV surface, which is located only 1 A from the outermost atoms). Even our model bilayers are not simple systems: the charge on each lipid is distributed over several atoms, these partial charges are non-coplanar, there is a 2 A ion-exclusion region (epsilon r = 80) adjacent to the polar headgroups, and the molecular surface is rough. We investigated the effect of these four factors using smooth (or bumpy) epsilon r = 2 slabs with embedded point charges: these factors had only minor effects on the potential in the aqueous phase.

The interaction between protein kinase C and lipid cofactors studied by simultaneous observation of lipid and protein fluorescence

The interaction of protein kinase C (PKC) with lipids was probed by a dual approach. Pyrene-labeled lipid analogues of diacylglycerol, phosphatidylserine (PS), phosphatidylinositol (PI), phosphatidylinositol 4-phosphate (PIP), and phosphatidylcholine (PC) were used both as acceptors of tryptophan excitation energy of PKC and as membrane probes for intra- and intermolecular lipid chain collisions by measuring the ratio of excimer-to-monomer fluorescence intensity (EM). Both in micelles of polyoxyethylene 9-lauryl ether and in dioleoyl-PC vesicles, interaction of PKC with monopyrenyl PS (pyr-PS) in the absence of calcium resulted in a relatively slow decrease of the EM value. This effect on the lipid dynamics was accompanied by quenching of the tryptophan fluorescence of PKC. Addition of calcium resulted in a rapid further decrease of the EM ratio of pyr-PS and in additional quenching of the tryptophan fluorescence. When 4 mol % of pyr-PS was replaced by 0.5 mol % of dipyrenyl-labeled diacylglycerol a decrease of the intramolecular excimer formation rate and tryptophan fluorescence could only be detected in the presence of calcium and PS. Strong binding was also observed with dipyrenyl-labeled PIP (dipyr-PIP), but not with the other dipyrenyl-labeled lipids: PI, PS, or PC. In addition, the EM ratios of dipyr-PIP were not affected by phorbol 12-myristate 13-acetate, indicating that phorbol 12-myristate 13-acetate and dipyr-PIP can bind simultaneously to PKC.

Does the binding of clusters of basic residues to acidic lipids induce domain formation in membranes?

Several proteins that are important components of the calcium/phospholipid second messenger system (e.g. phospholipase C, protein kinase C, myristoylated alanine-rich C kinase substrate (MARCKS) and pp60src) contain clusters of basic residues that can interact with acidic lipids on the cytoplasmic surface of plasma membranes. We have studied the membrane binding of MARCKS and pp60src, peptides that mimic the basic regions of these proteins, and simple model peptides. Specifically, we determined how the binding of these model peptides depends on (1) the number of basic residues in the peptide (2) the fraction of acidic lipids in the membrane (3) the ionic strength of the solution (4) the chemical nature of the basic residues (Arg versus Lys) and the acidic phospholipids [phosphatidylglycerol (PG) versus phosphatidylserine (PS)] (5) the pressure and (6) the temperature. The results are consistent with a simple theoretical model: each basic residue in a peptide binds independently to an acidic lipid with an intrinsic microscopic association constant of 1-10 M-1 (binding energy congruent to 1 kcal/mol). The binding is described with a mass action formalism and the non-specific electrostatic accumulation of the peptides in the aqueous diffuse double layer is described with the Gouy-Chapman theory. This Gouy-Chapman/mass action model accounts surprisingly well for the sigmoidal dependence of binding on the percentage of acidic lipids in the membrane (apparent co-operativity or Hill coefficient > 1); the model assumes that the multivalent basic peptides bind > 1 acidic lipids and thus induce or stabilize domain formation.

Effect of 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7) on HSP70 and HSP28 gene expression and thermotolerance development in human colon carcinoma cells

The effect of 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), a potent protein kinase C (PKC) inhibitor, on the development of thermotolerance and expression of heat shock genes (HSP70 and HSP28) was investigated in human colon carcinoma HT-29 cells. After acute heating at 45 degrees for 15 min, cells became resistant to a challenge heat shock. The development of thermotolerance was suppressed by adding H-7 after heat shock. Northern blots show that the levels of HSP70 and HSP28 mRNA increased rapidly and reached maximal values within 6 hr. H-7 suppressed the accumulation of HSP70 and HSP28 mRNA as well as their protein synthesis, and the level of suppression was concentration dependent. However, little effect was observed if the drug was added 1 hr before and during heat shock. These results suggest that PKC is involved in the regulation of heat shock gene expression after acute heat shock.

Characterization of apocytochrome C binding to human erythrocytes

The binding of 125I-labeled apocytochrome c to human erythrocytes was determined for free apocytochrome c concentrations at 10(-10)-10(-6) M. At about 2 x 10(-9) M, maximum cell association of apocytochrome c occurs at 50 mM NaCl and at 22 degrees C. Intact erythrocytes at 22 degrees C have three classes of apocytochrome c binding sites: one high-affinity noncooperative site (n1 = 728 per cell, Kd1 = 1.5 x 10(-9) M) and two positively cooperative sites (n2 = 3.7 x 10(4) per cell, Kd2 = 1.2 x 10(-7) M, alpha 2 = 2.0, and n3 = 2.5 x 10(5) per cell, Kd3 = 7.1 x 10(-7) M, alpha 3 = 12). Erythrocytes at 37 degrees C, and erythrocyte ghosts at 22 degrees C, also have three classes of apocytochrome c binding sites, and most sites are positively cooperative.

Lipid dynamics and peripheral interactions of proteins with membrane surfaces

A large body of evidence strongly indicates biomembranes to be organized into compositionally and functionally specialized domains, supramolecular assemblies, existing on different time and length scales. For these domains and intimate coupling between their chemical composition, physical state, organization, and functions has been postulated. One important constituent of biomembranes are peripheral proteins whose activity can be controlled by non-covalent binding to lipids. Importantly, the physical chemistry of the lipid interface allows for a rapid and reversible control of peripheral interactions. In this review examples are provided on how membrane lipid (i) composition (i.e., specific lipid structures), (ii) organization, and (iii) physical state can each regulate peripheral binding of proteins to the lipid surface. In addition, a novel and efficient mechanism for the control of the lipid surface association of peripheral proteins by [Ca2+], lipid composition, and phase state is proposed. The phase state is, in turn, also dependent on factors such as temperature, lateral packing, presence of ions, metabolites and drugs. Confining reactions to interfaces allows for facile and cooperative large scale integration and control of metabolic pathways due to mechanisms which are not possible in bulk systems.

Membrane interaction of small N-myristoylated peptides: implications for membrane anchoring and protein-protein association

The effect of the covalent attachment of a myristolyl moiety to the N-terminal glycine residue in proteins, N-myristoylation, on lipid-protein interactions was investigated in a model system using magnetic resonance spectroscopic methods. Two peptides with sequences conserved among known N-myristoylated proteins were chosen for this study. Using two-dimensional nuclear magnetic resonance techniques, it was shown that N-myristolylation results in an aggregation of both peptides in solution, although they lack well defined folded conformations in solution either when chemically N-myristolyated or when nonacylated. The interaction of the acylated peptides with lipid bilayers was investigated using spin label electron spin resonance and 2H NMR techniques. The results show that when bound to membranes, the covalently linked myristoyl chain of one of the peptides is directly inserted into or anchored to the lipid bilayer. The binding of the other peptide with membranes is effected by interactions between amino acid residues and the phospholipid headgroups. In this case, the covalently linked myristoyl moiety is most likely not in direct contact with the acyl chains of the host lipid bilayer. Rather, the N-myristoyl chains stabilize the peptide aggregate by forming a hydrophobic core. Measurements of peptide binding to membranes showed that N-myristoylation affects both the lipid:peptide stoichiometry at saturation and the equilibrium binding constant, in a manner that is consistent with the structural information obtained by magnetic resonance methods.

Phosphorylation reverses the membrane association of peptides that correspond to the basic domains of MARCKS and neuromodulin

Several groups have observed that phosphorylation causes the MARCKS (Myristoylated Alanine-Rich C Kinase Substrate) protein to move off cell membranes and phospholipid vesicles. Our working hypothesis is that significant membrane binding of MARCKS requires both hydrophobic insertion of the N-terminal myristate into the bilayer and electrostatic association of the single cluster of basic residues in the protein with acidic lipids and that phosphorylation reverses this electrostatic association. Membrane binding measurements with myristoylated peptides and phospholipid vesicles show this hydrophobic moiety could, at best, barely attach proteins to plasma membranes. We report here membrane binding measurements with basic peptides that correspond to the phosphorylation domains of MARCKS and neuromodulin. Binding of these peptides increases sigmoidally with the percent acidic lipid in the phospholipid vesicle and can be described by a Gouy-Chapman/mass action theory that explains how electrostatics and reduction of dimensionality produce apparent cooperativity. The electrostatic affinity of the MARCKS peptide for membranes containing 10% acidic phospholipids (10(4) M-1 = chi/[P], where chi is the mole ratio of peptide bound to the outer monolayer of the vesicles and [P] is the concentration of peptide in the aqueous phase) is the same as the hydrophobic affinity of the myristate moiety for bilayer membranes. Phosphorylation decreases the affinity of the MARCKS peptide for membranes containing 15% acidic lipid about 1000-fold and produces a rapid (t1/2 < 30 s) dissociation of the peptide from phospholipid vesicles.

The exocytotic fusion pore and neurotransmitter release

Membrane fusion is ubiquitous in biological systems, occurring in the simplest of unicellular eukaryotes as well as higher eukaryotes. As soon as the first primitive eukaryotic cell utilized a lipid bilayer as an outer membrane, membrane fusion (and fission) became necessary for the traffic of material from the outside to the inside, the inside to the outside, and between different intracellular membrane-bounded compartments. The earliest cells would have made use of the intrinsic ability of lipid bilayers to fuse under certain conditions. Although this fusogenic property of bilayers has been known for some time, it is has become clear only relatively recently that two phospholipid bilayers will fuse spontaneously, owing to a hydrophobic force, when the bilayers are brought close together under conditions of membrane tension or high curvature (Helm and Israelachvili, 1993). The primeval cell would have used proteins to develop the appropriate architecture in which such fusion would occur in a regulated manner. During the course of evolution, ever more sophisticated ways of regulating this basic process would evolve, but the underlying fusion mechanism would remain unchanged. We have proposed that a macromolecular scaffold of proteins is responsible for bringing the plasma membrane close to the secretory granule membranes and creating the architecture that enables the hydrophobic force to cause fusion (Figure 1; Nanavati et al., 1992; Monck and Fernandez, 1992; Oberhauser and Fernandez, 1993). Evidence is now accumulating that there are several highly conserved families of proteins associated with vesicle fusion events, from yeast to mammalian cells, and with intracellular traffic, as well as with regulated exocytosis and synaptic transmission (Bennett and Scheller, 1993; Sollner et al., 1993; Südhof et al., 1993). The molecular structures (or scaffolds) that regulate membrane fusion are likely to contain related proteins and share certain fundamental properties.

Identification of a membrane-binding domain within the amino-terminal region of human immunodeficiency virus type 1 Gag protein which interacts with acidic phospholipids

Retroviral Gag proteins are targeted to the plasma membrane, where they play the central role in virion formation. Several studies have suggested that the membrane-binding signal is contained within the amino-terminal matrix sequence; however, the precise location has never been determined for the Gag protein of any retrovirus. In this report, we show that the first 31 residues of human immunodeficiency virus type 1 Gag protein can function independently as a membrane-targeting domain when fused to heterologous proteins. A bipartite membrane-targeting motif was identified, consisting of the myristylated N-terminal 14 amino acids and a highly basic region that binds acidic phospholipids. Replacement of the N-terminal membrane-targeting domain of pp60v-src with that of human immunodeficiency virus type 1 Gag elicits efficient membrane binding and a transforming phenotype. Removal of myristate or the basic region results in decreased membrane binding of Gag-Src chimeras in vitro and impaired virion formation by Pr55gag in vivo. We propose that the N-terminal Gag sequence functions as a targeting signal to direct interaction with acidic phospholipids on the cytoplasmic leaflet of the plasma membrane.

The role of electrostatic charge in the membrane insertion of colicin A. Calculation and mutation

The bacterial toxin colicin A binds spontaneously to the surfaces of negatively charged membranes. The surface-bound toxin must subsequently, however, become an acidic 'molten globule' before it can fully insert into the lipid bilayer. Clearly, electrostatic interactions must play a significant role in both events. The electrostatic field around the toxin in solution was calculated using the finite-difference Poisson-Boltzmann method of the Delphi programme and the known X-ray structure. A large positively charged surface was identified which could be involved in the binding of colicin to negatively charged membranes. The applicability of the result was tested by also calculating the fields around modelled structures of the closely related colicins B and N. Surprisingly, colicin N showed a similar charge distribution in spite of its isoelectric point of pI 10.20 (colicin A has pI 5.44). One reason for this is the strong conservation of certain negative charges in all colicins. There is a single highly conserved aspartate residue (Asp78) on the positively charged face which provides a small but discrete region of negative charge. This residue, Asp78, was replaced by asparagine in the mutant D78N. D78N binds faster to negatively charged vesicles but inserts only half as fast as the wild-type protein into the membrane core. This indicates that, first, the initial membrane binding has a significant electrostatic component and, second, that the isolated charge on Asp78 plays a role in the formation of the insertion intermediate.

Protein kinase C as mediator of arachidonic acid-induced decrease of neuronal M current

The M current, IM, of NG108-15 neuroblastoma x glioma hybrid cells, a non-inactivating K+ current, is decreased by arachidonic acid (5-25 microM), often after an initial transitory increase. To test the possibility that the decrease is caused by activation of protein kinase C (PKC) we used the PKC 19-31 peptide, which is an effective inhibitor of PKC. With 1 microM peptide in the pipette solution the normally observed strong reduction of IM by 1 microM phorbol 12,13-dibutyrate (PDB) was almost totally prevented, indicating that PKC is completely inhibited; also the voltage dependence of the M conductance, gM(V), was shifted to more negative membrane potentials. In the presence of 1 microM peptide the effect of 25 microM arachidonic acid on IM was significantly reduced, suggesting that the effect, or at least a large part of it, is mediated by PKC.

Protein kinase C and annexins: unusual calcium response elements in the cell

Protein kinase C and the annexins appear to share some unusual and potentially important membrane- and calcium-binding properties. While these proteins are calcium response elements, they are not calcium-binding proteins in the formal sense; at intracellular calcium concentrations, they only bind significant amounts of calcium when membranes or other suitable surfaces are present. The number of calcium ions bound per protein is large (> 8) and this stoichiometry, at the protein-membrane interface, may provide the large number of contact points needed for the very high-affinity interaction that is observed. The further ability of annexins and PKC to form structures with properties of integral membrane proteins may be important to provide a type of long-term cell signalling that produces a constitutively active kinase or ion channel activity. Selectivity for phospholipids in bilayer form is modest with respect to the acidic phospholipids but there is a surprising preference for phosphatidylethanolamine as the neutral phospholipid matrix. Along with other unusual properties, these proteins offer the potential for unique types of cell regulation events.

Internal electrostatic potentials in bilayers: measuring and controlling dipole potentials in lipid vesicles

The binding and translocation rates of hydrophobic cation and anion spin labels were measured in unilamellar vesicle systems formed from phosphatidylcholine. As a result of the membrane dipole potential, the binding and translocation rates for oppositely charged hydrophobic ions are dramatically different. These differences were analyzed using a simple electrostatic model and are consistent with the presence of a dipole potential of approximately 280 mV in phosphatidylcholine. Phloretin, a molecule that reduces the magnitude of the dipole potential, increases the translocation rate of hydrophobic cations, while decreasing the rate for anions. In addition, phloretin decreases the free energy of binding of the cation, while increasing the free energy of binding for the anion. The incorporation of 6-ketocholestanol also produces differential changes in the binding and translocation rates of hydrophobic ions, but in an opposite direction to those produced by phloretin. This is consistent with the view that 6-ketocholestanol increases the magnitude of the membrane dipole potential. A quantitative analysis of the binding and translocation rate changes produced by ketocholestanol and phloretin is well accounted for by a point dipole model that includes a dipole layer due to phloretin or 6-ketocholestanol in the membrane-solution interface. This approach allows dipole potentials to be estimated in membrane vesicle systems and permits predictable, quantitative changes in the magnitude of the internal electrostatic field in membranes. Using phloretin and 6-ketocholestanol, the dipole potential can be altered by over 200 mV in phosphatidylcholine vesicles.

Polar/apolar compounds induce leukemia cell differentiation by modulating cell-surface potential

The mechanism of action of polar/apolar inducers of cell differentiation, such as dimethyl sulfoxide and hexamethylene-bisacetamide, is still obscure. In this paper evidence is provided that their effects on murine erythroleukemia cells are modulated by various extracellular cations as a precise function of the cation effects on membrane surface potential. The interfacial effects of the inducers were directly measured on the charged electrode, showing that both dimethyl sulfoxide and hexamethylene-bisacetamide, at the effective concentrations for cell differentiation and within the physiological range of charge density, adsorb at the charged surface and produce a potential shift. A linear correlation was found between this shift and the inducer effects on cell differentiation. Besides offering a different interpretation of the mechanism of action of the inducers, these findings indicate that surface potential has a signaling function. They may also be relevant to cancer treatments based on tumor-cell commitment to terminal differentiation.

The ADP/ATP carrier is the 32-kilodalton receptor for an NH2-terminally myristylated src peptide but not for pp60src polypeptide

Membrane binding of pp60src is initiated via its myristylated NH2 terminus. To identify a candidate pp60src docking protein or receptor in the membrane, a radiolabelled peptide corresponding to the pp60src NH2-terminal membrane binding domain was cross-linked to fibroblast membranes and found to specifically label a 32-kDa protein. This protein was purified by appending an affinity tag to the peptide probe so that the cross-linked complex could be isolated via affinity chromatography. Microsequencing indicated that the 32-kDa protein was the mitochondrial ADP/ATP carrier (AAC). This result was further confirmed by the ability of an antibody to the AAC to immunoprecipitate the cross-linked complex, by the ability of certain inhibitors of the AAC to block cross-linking, and by membrane fractionation to show that complex formation occurred essentially exclusively in the mitochondrial fraction. While the AAC bound the myristyl-src peptide in a specific manner both in vitro and in vivo, its localization to the inner membrane of the mitochondrion precludes its being a pp60src binding protein. An analysis of pp60v-src binding in vitro was consistent with this expectation. Thus, use of a myristyl-src peptide revealed an unexpected and previously unidentified binding capacity of the AAC, most likely related to the ability of long-chain fatty acyl coenzyme As to serve as AAC inhibitors. The amphipathic nature of the pp60src NH2 terminus suggests alternative strategies for uncovering pp60src membrane binding species.

The ADP/ATP carrier is the 32-kilodalton receptor for an NH2-terminally myristylated src peptide but not for pp60src polypeptide

Membrane binding of pp60src is initiated via its myristylated NH2 terminus. To identify a candidate pp60src docking protein or receptor in the membrane, a radiolabelled peptide corresponding to the pp60src NH2-terminal membrane binding domain was cross-linked to fibroblast membranes and found to specifically label a 32-kDa protein. This protein was purified by appending an affinity tag to the peptide probe so that the cross-linked complex could be isolated via affinity chromatography. Microsequencing indicated that the 32-kDa protein was the mitochondrial ADP/ATP carrier (AAC). This result was further confirmed by the ability of an antibody to the AAC to immunoprecipitate the cross-linked complex, by the ability of certain inhibitors of the AAC to block cross-linking, and by membrane fractionation to show that complex formation occurred essentially exclusively in the mitochondrial fraction. While the AAC bound the myristyl-src peptide in a specific manner both in vitro and in vivo, its localization to the inner membrane of the mitochondrion precludes its being a pp60src binding protein. An analysis of pp60v-src binding in vitro was consistent with this expectation. Thus, use of a myristyl-src peptide revealed an unexpected and previously unidentified binding capacity of the AAC, most likely related to the ability of long-chain fatty acyl coenzyme As to serve as AAC inhibitors. The amphipathic nature of the pp60src NH2 terminus suggests alternative strategies for uncovering pp60src membrane binding species.

Protein kinase C activity modulates myelin gene expression in enriched oligodendrocytes

Protein kinase C (PKC) and its potential role in myelin gene expression were investigated in primary cultured rat oligodendrocytes. The major myelin genes were expressed in a developmentally coordinated manner in cultured oligodendrocytes. PKC activity in these cells was similarly regulated with differential expression of PKC isozyme mRNAs. PKC-gamma mRNA was expressed transiently and was most abundant in 9-day cells in vitro. PKC-alpha and PKC-beta mRNAs were present at low levels throughout development in these cells, and their expression increased in 18-25 day cells. Immunocytochemical colocalization of PKC with oligodendrocyte-specific markers--O4, galactosyl cerebroside, MBP, and PLP--in enriched oligodendrocyte cultures suggested that the PKC enzyme activities assayed in these cultures were predominantly contributed by oligodendrocytes. PKC inhibition resulting from long-term exposure to 4 beta-phorbol-12,13-dibutyrate (4 beta-PDB) reduced steady-state levels of MBP, PLP, MAG, CNP, and PKC-alpha mRNAs, as detected by slot blots or in situ hybridization, and downregulated the oligodendrocyte-specific markers O4, galactosyl cerebroside, and the major constituent proteins MBP and PLP, as detected by immunocytochemistry. PKC-mediated downmodulation of myelin gene expression was most profound in normally differentiating oligodendrocytes at or before the onset of myelin protein synthesis. Six-day oligodendrocytes were most susceptible to such modulation. To elucidate the mechanism of reduction in various myelin gene messages upon modulation of PKC, we analyzed mRNA levels in oligodendrocytes, which were pretreated with either the transcriptional inhibitor actinomycin D or the protein synthesis blocker cycloheximide before exposure to 4 beta-PDB. Our results demonstrate that the PKC inhibition-mediated loss in myelin mRNA levels did not require the transcription of any genes, but appeared to be at least partially dependent on continuous protein synthesis.

Interaction of peptide fragment 828-848 of the envelope glycoprotein of human immunodeficiency virus type I with lipid bilayers

The interaction of the peptide fragment 828-848, called P828, from the carboxy-terminal region of the envelope glycoprotein gp41 of HIV-I with model membranes composed of phosphatidylcholine (PC) and phosphatidylglycerol (PG) was investigated using microelectrophoretic mobility of liposomes, fluorescence polarization of labeled lipids, NMR, and differential scanning calorimetry. The peptide binds to negatively charged lipid surfaces. No interaction between P828 and neutral PC surfaces is observed. The interaction between the peptide and the lipid is exclusively electrostatic with the six positively charged arginines of P828 acting as binding sites for PG. Circular dichroism measurements of P828 indicate that the peptide undergoes a transition from a random coil to an ordered conformation upon binding to negatively charged PG bilayers or SDS micelles, but not in the presence of neutral PC bilayers. The ordered structure has an apparent helical content of 60%. IN DOPG/DOPC mixtures containing 20 mol % DOPG, the peptide causes the formation of lipid domains enriched in DOPG, as assessed by measurement of fluorescence energy transfer between labeled PG and PC. The formation of these domains requires energy and therefore reduces the strength of peptide binding to the lipid matrix. Our data support and quantitate the results from antibody binding studies [Haffar, O.K., Dowbenko, D. J., & Berman, P. W. (1988) J. Cell Biol. 107, 1677-1687] that the carboxy-terminal segment of the envelope glycoprotein gp41 interacts with microsomal membranes.

Thermodynamic characterization of the association of small basic peptides with membranes containing acidic lipids

We measured the binding of the peptide acetyl-Trp-Lys7-amide to membranes formed from mixtures of the zwitterionic lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (PC) and the acidic lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (PG). Direct filtration and equilibrium dialysis measurements demonstrate that binding increases sigmoidally with the mole fraction of PG as predicted from a simple Gouy-Chapman/mass action theoretical model. We used these measurements to calibrate two binding assays, one based on the increase in Trp fluorescence that occurs when the peptide binds to the membrane, the other on the quenching of Trp fluorescence that occurs when the peptide binds to membranes containing fluorescent lipids. Both fluorescence assays demonstrate that binding does not depend strongly on temperature, which suggests the enthalpy change, delta H, is small. Calorimetric measurements demonstrate this directly for the analogous basic peptide Lys5: delta H congruent to +1 kcal/mol for the binding of Lys5 to sonicated phospholipid vesicles and delta H congruent to 0 kcal/mol for its binding to large unilamellar vesicles. Thus, the decrease in the free energy that occurs when these peptides bind to the membrane is due to a positive change in the entropy of the system. Fluorescence measurements demonstrate the binding of the Trp-containing peptide to 4:1 PC/PG membranes is independent of pressure up to 2 kbar, which suggests that binding occurs without a significant change in volume.

Interaction of phospholipids with proteins and peptides. New advances III

1. The review deals with the recent achievements in the study of the various interactions of phospholipids with proteins and peptides. 2. The interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. 3. The effect of the interaction on the structure and biological activity of the interacting molecules is also discussed.