Fluorescence quenching of spectrin and other red cell membrane cytoskeletal proteins. Relation to hydrophobic binding sites

Structural Flexibility of Proteins Dramatically Alters Membrane Stability─A Novel Aspect of Lipid-Protein Interaction

Protein isoforms are structural variants with changes in the overall flexibility predominantly at the tertiary level. For membrane associated proteins, such structural flexibility or rigidity affects membrane stability by playing modulatory roles in lipid-protein interaction. Herein, we investigate the protein chain flexibility mediated changes in the mechanistic behavior of phospholipid model membranes in the presence of two well-known isoforms, erythroid (ER) and nonerythroid (NER) spectrin. We show dramatic alterations of membrane elasticity and stability induced by spectrin in the Langmuir monolayers of phosphatidylocholine (PC) and phosphatidylethanolamine (PE) by a combination of isobaric relaxation, surface pressure-area isotherm, X-ray scattering, and microscopy measurements. The NER spectrin drives all monolayers to possess an approximately equal stability, and that required 25-fold increase and 5-fold decrease of stability in PC and PE monolayers, respectively. The untilting transition of the PC membrane in the presence of NER spectrin observed in X-ray measurements can explain better membrane packing and stability.

Effect of pH on stability, conformation, and chaperone activity of erythroid & non-erythroid spectrin

Spectrin, a major component of the eukaryotic membrane skeleton, has been shown to have chaperone like activity. Here we investigate the pH induced changes in the structure and stability of erythroid and brain spectrin by spectroscopic methods. We also correlate these changes with modulations of chaperone potential at different pH. We have followed the pH induced structural changes by circular dichroism spectroscopy and intrinsic tryptophan fluorescence. It is seen that lowering the pH from 9 has little effect on structure of the proteins till about pH6. At pH4, there is significant change of the secondary structure of the proteins, along with a 5nm hypsochromic shift of the emission maxima. Below pH4 the proteins undergo acid denaturation. Probing exposed hydrophobic patches on the proteins using protein-bound 8-anilinonaphthalene-1-sulfonate fluorescence demonstrates that there is higher solvent accessibility of hydrophobic surfaces in both forms of spectrin at around pH4. Dynamic light scattering and 90° light scattering studies show that the both forms of spectrin forms oligomers at pH~4. Chemical unfolding data shows that these oligomers are less stable than the tetrameric form. Aggregation studies with BSA show that at pH4, both spectrins exhibit better chaperone activity. This enhancement of chaperone like activity appears to result from an increase in regions of solvent-exposed hydrophobicity and oligomeric state of the spectrins which in turn are induced by moderately acid pH. This may have in-vivo implications in cells facing stress conditions where cytoplasmic pH is lowered.

Spectrin and phospholipids - the current picture of their fascinating interplay

The spectrin-based membrane skeleton is crucial for the mechanical stability and resilience of erythrocytes. It mainly contributes to membrane integrity, protein organization and trafficking. Two transmembrane protein macro-complexes that are linked together by spectrin tetramers play a crucial role in attaching the membrane skeleton to the cell membrane, but they are not exclusive. Considerable experimental data have shown that direct interactions between spectrin and membrane lipids are important for cell membrane cohesion. Spectrin is a multidomain, multifunctional protein with several distinctive structural regions, including lipid-binding sites within CH tandem domains, a PH domain, and triple helical segments, which are excellent examples of ligand specificity hidden in a regular repetitive structure, as recently shown for the ankyrin-sensitive lipid-binding domain of beta spectrin. In this review, we summarize the state of knowledge about interactions between spectrin and membrane lipids.

Binding of polarity-sensitive hydrophobic ligands to erythroid and nonerythroid spectrin: fluorescence and molecular modeling studies

We have used three polarity-sensitive fluorescence probes, 6-propionyl 2-(N,N-dimethyl-amino) naphthalene (Prodan), pyrene and 8-anilino 1-naphthalene sulphonic acid, to study their binding with erythroid and nonerythroid spectrin, using fluorescence spectroscopy. We have found that both bind to prodan and pyrene with high affinities with apparent dissociation constants (Kd) of .50 and .17 μM, for prodan, and .04 and .02 μM, for pyrene, respectively. The most striking aspect of these bindings have been that the binding stoichiometry have been equal to 1 in erythroid spectrin, both in dimeric and tetrameric form, and in tetrameric nonerythroid spectrin. From an estimate of apparent dielectric constants, the polarity of the binding site in both erythroid and nonerythroid forms have been found to be extremely hydrophobic. Thermodynamic parameters associated with such binding revealed that the binding is favored by positive change in entropy. Molecular docking studies alone indicate that both prodan and pyrene bind to the four major structural domains, following the order in the strength of binding to the Ankyrin binding domain > SH3 domain > Self-association domain > N-terminal domain of α-spectrin of both forms of spectrin. The binding experiments, particularly with the tetrameric nonerythroid spectrin, however, indicate more toward the self association domain in offering the unique binding site, since the binding stoichiometry have been 1 in all forms of dimeric and tetrameric spectrin, so far studied by us. Further studies are needed to characterize the hydrophobic binding sites in both forms of spectrin.

Spectrin organization and dynamics: new insights

Spectrin is the major constituent protein of the erythrocyte cytoskeleton which forms a filamentous network on the cytoplasmic face of the membrane by providing a scaffold for a variety of proteins. In this review, several aspects of spectrin organization are highlighted, particularly with respect to its ability to bind hydrophobic ligands and its interaction with membrane surfaces. The characteristic binding of the fluorescent hydrophobic probes Prodan and pyrene to spectrin, which allows an estimation of the polarity of the hydrophobic probe binding site, is illustrated. In addition, the contribution of uniquely localized and conserved tryptophan residues in the 'spectrin repeats' in these processes is discussed. A functional implication of the presence of hydrophobic binding sites in spectrin is its recently discovered chaperone-like activity. Interestingly, spectrin exhibits residual structural integrity even after denaturation which could be considered as a hallmark of cytoskeletal proteins. Future research could provide useful information about the possible role played by spectrin in cellular physiology in healthy and diseased states.

Specificity of Prodan for the Self-associating Domain of Spectrin: A Molecular Docking Study

The hydrophobic fluorescent probe Prodan binds to the self-associating domain of spectrin with 1:1 stoichiometry. A model of the self-associating domain was generated based on its homology with other domains of spectrin. Prodan was then docked onto the model, and several sites with low interaction energy were identified. To verify whether the binding of Prodan is specific towards the self-associating domain of spectrin, it was docked on to several other domains of spectrin, having a known three-dimensional structure. Analysis of the docking results suggests that the binding of Prodan to the self-associating domain of spectrin will involve hydrophobic and hydrophilic groups of Prodan. The results clearly indicate the preference of Prodan for a particular binding site of the self-associating domain.

Spectrin-phospholipid interactions. Existence of multiple kinds of binding sites?

The object of this paper is to review briefly the studies on the interactions of erythroid and non-erythroid spectrins with lipids in model and natural membranes. An important progress on the identification of lipid-binding sites has recently been made although many questions remain still unanswered. In particular, our understanding of the physiological role of such interactions is still limited. Another important issue is the occurrence of spectrins in membrane rafts, how they are attached to the raft and what is their function in rafts.

Conformational study of spectrin in presence of submolar concentrations of denaturants

The presence of very low concentrations of the commonly used chemical denaturants, guanidinium chloride (GdmCl) and urea brought about conformational changes in the erythrocyte membrane skeletal protein, spectrin. Evidences in support of changes in the quaternary structure of spectrin have been put forward from quenching study of tryptophan fluorescence, by both steady state and time-resolved measurements, using acrylamide as the quencher. It revealed significant differences between the Stern-Volmer quenching constants (K(SV)) and the fraction of accessible tryptophans (f(e)) observed in absence and presence of GdmCl and urea concentrations below 1 M at which the association of the two subunits remains intact. The steady state anisotropy of both the spectrin tryptophans and the spectrin-bound fluorescence probe, Prodan also indicate changes in the overall flexibility of the spectrin dimer, originating from changes in the quaternary structure of spectrin. Studies on the binding of Prodan, further indicate that conformational changes also occur in spectrin near the Prodan-binding site at the terminal domain of the protein which is reflected in 3-4 fold decrease in the affinity of binding of Prodan to spectrin in the presence of GdmCl and urea compared to that observed in the absence of the denaturants. The dissociation constant (K(d)) of Prodan to spectrin is 0.43 microM at 25 degrees C.

Effect of ionic strength on the organization and dynamics of tryptophan residues in erythroid spectrin: A fluorescence approach

The ionic strength of the medium plays an important role in the structure and conformation of erythroid spectrin. The spectrin dimer is a flexible rod at physiological ionic strength. However, lower ionic strength results in elongation and rigidification (stiffening) of spectrin as shown earlier by electron microscopy and hydrodynamic studies. The ionic strength induced structural transition does not involve any specific secondary structural changes. In this article, we have used a combination of fluorescence spectroscopic approaches that include red edge excitation shift (REES), fluorescence quenching, time-resolved fluorescence measurements, and chemical modification of the spectrin tryptophans to assess the environment and dynamics of tryptophan residues of spectrin under different ionic strength conditions. Our results show that while REES, fluorescence anisotropy, lifetime, and chemical modification of spectrin tryptophans remain unaltered in low and high ionic strength conditions, quenching of tryptophan fluorescence by the aqueous quencher acrylamide (but not the hydrophobic quencher trichloroethanol) and resonance energy transfer to a dansyl-labeled fatty acid show differences in tryptophan environment. These results, which report tertiary structural changes in spectrin upon change in ionic strength, are relevant in understanding the molecular details underlying the conformational flexibility of spectrin.

Chaperone activity and prodan binding at the self-associating domain of erythroid spectrin

Spectrin, the major constituent protein of the erythrocyte membrane skeleton, exhibits chaperone activity by preventing the irreversible aggregation of insulin at 25 degrees C and that of alcohol dehydrogenase at 50 degrees C. The dimeric spectrin and the two subunits, alpha-spectrin and beta-spectrin prevent such aggregation appreciably better, 70% in presence of dimeric spectrin at an insulin:spectrin ratio of 1:1, than that in presence of the tetramer of 25%. Our results also show that spectrin binds to denatured enzymes alpha-glucosidase and alkaline phosphatase during refolding and the reactivation yields are increased in the presence of the spectrin derivatives when compared with those refolded in their absence. The unique hydrophobic binding site on spectrin for the fluorescence probe, 6-propionyl-2-(dimethylamino)naphthalene (Prodan) has been established to localize at the self-associating domain with the binding stoichiometry of one Prodan/both dimeric and tetrameric spectrin. The other fluorescence probe, 1-anilinonaphthalene-8-sulfonic acid, does not show such specificity for spectrin, and the binding stoichiometry is between 3 and 5 1-anilinonaphthalene-8-sulfonic acid/dimeric and tetrameric spectrin, respectively. Regions in alpha- and beta-spectrins have been found to have sequence homology with known chaperone proteins. More than 50% similarities in alpha-spectrin near the N terminus with human Hsp90 and in beta-spectrin near the C terminus with human Hsp90 and Escherichia coli DnaJ have been found, indicating a potential chaperone-like sequence to be present near the self-associating domain that is formed by portions of alpha-spectrin near the N terminus and the beta-spectrin near the C terminus. There are other patches of sequences also in both the spectrin polypeptides, at the other termini as well as in the middle of the rod domain having significant homology with well known chaperone proteins.

Organization and dynamics of tryptophan residues in erythroid spectrin: novel structural features of denatured spectrin revealed by the wavelength-selective fluorescence approach

We have investigated the organization and dynamics of the functionally important tryptophan residues of erythroid spectrin in native and denatured conditions utilizing the wavelength-selective fluorescence approach. We observed a red edge excitation shift (REES) of 4 nm for the tryptophans in the case of spectrin in its native state. This indicates that tryptophans in spectrin are localized in a microenvironment of restricted mobility, and that the regions surrounding the spectrin tryptophans offer considerable restriction to the reorientational motion of the water dipoles around the excited state tryptophans. Interestingly, spectrin exhibits a REES of 3 nm even when denatured in 8 M urea. This represents the first report of a denatured protein displaying REES. Observation of REES in the denatured state implies that some of the structural and dynamic features of this microenvironment around the spectrin tryptophans are retained even when the protein is denatured. Fluorescence quenching data of denatured spectrin support this conclusion. In addition, we have deduced the organization and dynamics of the hydrophobic binding site of the polarity-sensitive fluorescent probe PRODAN that binds erythroid spectrin with high affinity. When bound to spectrin, PRODAN exhibits a REES of 9 nm. Because PRODAN binds to a hydrophobic site in spectrin, such a result would directly imply that this region of spectrin offers considerable restriction to the reorientational motion of the solvent dipoles around the excited state fluorophore. The results of our study could provide vital insight into the role of tryptophans in the stability and folding of spectrin.

Interaction of dystrophin rod domain with membrane phospholipids. Evidence of a close proximity between tryptophan residues and lipids

Dystrophin is assumed to act via the central rod domain as a flexible linker between the amino-terminal actin binding domain and carboxyl-terminal proteins associated with the membrane. The rod domain is made up of 24 spectrin-like repeats and has been shown to modify the physical properties of lipid membranes. The nature of this association still remains unclear. Tryptophan residues tend to cluster at or near to the water-lipid interface of the membrane. To assess dystrophin rod domain-membrane interactions, tryptophan residues properties of two recombinant proteins of the rod domain were examined by (1)H NMR and fluorescence techniques in the presence of membrane lipids. F114 (residues 439-553) is a partly folded protein as inferred from (1)H NMR, tryptophan fluorescence emission intensity, and the excited state lifetime. By contrast, F125 (residues 439-564) is a folded compact protein. Tryptophan fluorescence quenching shows that both proteins are characterized by structural fluctuations with their tryptophan residues only slightly buried from the surface. In the presence of negatively charged small vesicles, the fluorescence characteristics of F125 change dramatically, indicating that tryptophan residues are in a more hydrophobic environment. Interestingly, these modifications are not observed with F114. Fluorescence quenching experiments confirm that tryptophan residues are shielded from the solvent in the complex F125 lipids by a close contact with lipids. The use of membrane-bound quenchers allowed us to conclude that dystrophin rod domain lies along the membrane surface and may be involved in a structural array comprising membrane and cytoskeletal proteins as well as membrane lipids.

Erythroid spectrin in miceller detergents

We have studied the interaction of spectrin, the major protein of the erythrocyte cytoskeleton, with four commonly used detergents at concentrations above their critical miceller concentrations (cmc). Fluorescence spectroscopic studies on the emission intensity, steady state polarization, quenching with acrylamide, and time-resolved fluorescence measurements were done with spectrin in anionic detergents, e.g., SDS, deoxycholate, and nonionic detergents, e.g., Triton-X-100 and octylglucoside at concentrations double their respective cmc's. The spectrin-detergent complexes in all four systems have been characterized by far-UV CD and measurements on tryptophan fluorescence in combination with fluorescence of the extrinsic probe, pyrene. Tryptophan fluorescence studies revealed quaternary structural changes due to unzipping of the spectrin subunits in Triton-X-100 without complete dissociation. Both Triton-X-100 and SDS were found to partially denature spectrin indicated by the far-UV CD. Octylglucoside and deoxycholate are shown to have the least structural perturbations on the cytoskeletal protein, rationalizing the use of octylglucoside, in particular and also deoxycholate to be the most effective in preparing cytoskeletal fractions from erythrocytes rather than the Triton-X-100 that has long been used for preparing the Triton shells.

The tertiary amine local anesthetic dibucaine binds to the membrane skeletal protein spectrin

The quinoline-based tertiary amine dibucaine has been shown to bind the membrane skeletal protein spectrin with a dissociation constant of 3.5x10(-5) M at 25 degrees C. Such binding is detected by monitoring the quenching of the tryptophan fluorescence intensity with increasing concentrations of dibucaine only and not with the benzene-based local anesthetics procaine, tetracaine and lidocaine. Binding of dibucaine also indicated changes in the tertiary structure of spectrin indicated by a circular dichroism spectrum in the near-UV region due to absorption of the aromatic side chains. The thermodynamic parameters associated with the binding indicated the interaction of dibucaine and spectrin to be enthalpy-driven and insensitive to an increase in the ionic strength of the buffer.

Phospholipid-cytochrome c interaction: evidence for the extended lipid anchorage

Binding of cytochrome c (cyt c) to fatty acids and acidic phospholipid membranes produces pronounced and essentially identical changes in the spectral properties of cyt c, revealing conformational changes in the protein. The exact mechanism of the interaction of fatty acids and acidic phospholipids with cyt c is unknown. Binding of cyt c to liposomes with high contents (mole fraction X > 0.7) of acidic phospholipids caused spectral changes identical to those due to binding of oleic acid. Fluorescence spectroscopy of a cyt c analog containing a Zn(2+) substituted heme moiety and brominated lipid derivatives (9,10)-dibromostearate and 1-palmitoyl-2-(9,10)-dibromo-sn-glycero-3-phospho-rac-glycerol demonstrated a direct contact between the fluorescent [Zn(2+)-heme] group and the brominated acyl chain. These data constitute direct evidence for interaction between an acyl chain of a membrane phospholipid and the inside of the protein containing the heme moiety and provide direct evidence for the so-called extended-lipid anchorage of cyt c to phospholipid membranes. In this mechanism, one of the phospholipid acyl chains protrudes out of the membrane and intercalates into a hydrophobic channel in cyt c while the other chain remains in the bilayer.

Binding of a denatured heme protein and ATP to erythroid spectrin

Spectrin is a large, worm-like cytoskeletal protein that is abundant in all cell types. The denatured heme enzyme, horseradish peroxidase showed significant decrease in the reactivation yield, after 30 min of refolding, in presence of increasing concentrations of spectrin from that in the absence. This indicated that spectrin could bind denatured HRP and inhibit their refolding. In presence of 1 mM ATP and 10 mM MgCl(2) the spectrin binding of denatured HRP is abolished. This activity of decreasing the reactivation yield was found to be ATP-dependent and the denatured enzyme after 30 min refolding in the presence of spectrin, pretreated with Mg/ATP, showed about 40% increase in the reactivation yield compared to the same in absence of spectrin. Fluorescence spectroscopic studies indicated binding of ATP to native spectrin showing concentration-dependent quenching of tryptophan fluorescence by ATP. The apparent dissociation constant of binding of ATP to spectrin was estimated to be 1.1 mM. A high affinity binding of spectrin with denatured HRP has been characterized (K(d) = 16 nM). Since these properties are similar to those of established molecular chaperone proteins, these data indicate that spectrin might have a chaperone-like function in erythrocytes.

Rapid diffusion of spectrin bound to a lipid surface

Human erythrocyte spectrin was labelled with the probe 5, 5'-disulfato-1-(6-hexanoic acid N-hydroxysuccinimide ester)-1'-ethyl-3,3,3',3'-tetramethylindocarbocyanine (Cy3). Cy3-spectrin was bound to the outer surface of dimyristoylphosphatidylcholine (DMPC) multilamellar vesicles and its diffusion measured by fluorescence recovery after photobleaching (FRAP). It was found that at 30 degrees C, above the lipid gel to liquid-crystalline phase transition of the lipids, Cy3-spectrin had an unexpectedly high diffusion coefficient D=(2.1+/-0.6)x10(-7)) cm2/s. At the phase transition, diffusion of Cy3-spectrin was only slightly lower; D=(1.3+/-0.3)x10(-7) cm2/s, whereas at 14 degrees C, well below the lipid phase transition, diffusion was found to be much slower with D=(3.1+/-0.12)x10(-9) cm2/s. The fast diffusion of Cy3-spectrin on the lipid surface implies that the individual bonds which bind spectrin to the lipid surface must rapidly be made and broken. In the light of these results, spectrin-lipid interactions alone appear unlikely to have any significant role in supporting the cell membrane. Probably, the interactions serve only to localise the spectrin at the inner lipid surface in order to facilitate formation of the cytoskeleton.

Brain spectrin (fodrin) interacts with phospholipids as revealed by intrinsic fluorescence quenching and monolayer experiments

We demonstrate that phospholipid vesicles affect the intrinsic fluorescence of isolated brain spectrin. In the present studies we tested the effects of vesicles prepared from phosphatidylcholine (PtdCho) alone, in addition to vesicles containing PtdCho mixed with other phospholipids [phosphatidylethanolamine (PtdEtn) and phosphatidylserine] as well as from total lipid mixture extracted from brain membrane. The largest effect was observed with PtdEtn/PtdCho (3:2 molar ratio) vesicles; the effect was markedly smaller when vesicles were prepared from egg yolk PtdCho alone. Brain spectrin injected into a subphase induced a substantial increase in the surface pressure of monolayers prepared from phospholipids. Results obtained with this technique indicated that the largest effect is again observed with monolayers prepared from a PtdEtn/PtdCho mixture. The greatest effect was observed when the monolayer contained 50-60% PtdEtn in a PtdEtn/PtdCho mixture. This interaction occurred at salt and pH optima close to physiological conditions (0.15 M NaCl, pH7.5). Experiments with isolated spectrin subunits indicated that the effect of the beta subunit on the monolayer surface pressure resembled that measured with the whole molecule. Similarly to erythrocyte spectrin-membrane interactions, brain spectrin interactions with PtdEtn/PtdCho monolayer were competitively inhibited by isolated erythrocyte ankyrin. This also suggests that the major phospholipid-binding site is located in the beta subunit and indicates the possible physiological significance of this interaction.

Spectroscopic analysis of halothane binding to the plasma membrane Ca2+-ATPase

The intrinsic tryptophan (Trp) fluorescence of the plasma membrane Ca2+-ATPase (PMCA) is significantly quenched by halothane, a volatile anesthetic common in clinical practice. It has been proposed that halothane inhibition of the Ca2+-ATPase activity results from conformational changes following anesthetic binding in the enzyme. We have investigated whether the observed quenching reflects halothane binding to PMCA. We have shown that the quenching is dose dependent and saturable and can be fitted to a binding curve with an equilibrium constant K(Hal) = 2.1 mM, a concentration at which the anesthetic approximately half-maximally inhibits the Ca2+-ATPase activity. The relatively low sensitivity of halothane quenching of Trp fluorescence to the concentration of phosphatidylcholine and detergent in the PMCA preparation concurs with the quenching resulting from anesthetic binding in the PMCA molecule. Analysis of the Trp fluorescence quenching by acrylamide indicates that the Trp residues are not considerably exposed to the solvent (Stern-Volmer quenching constant of 2.9 M(-1)) and do not differ significantly in their accessibility to halothane. Other volatile anesthetics, diethyl ether and diisopropyl ether, reduce the quenching caused by halothane in a dose-dependent manner, suggesting halothane displacement from its binding site(s). These observations indicate that halothane quenching of intrinsic Trp fluorescence of PMCA results from anesthetic binding to the protein. The analysis, used as a complementary approach, provides new information to the still rudimentary understanding of the process of anesthetic interaction with membrane proteins.

Evidence of molten globule like state(s) of interferon gamma in acidic and sodium perchlorate solutions

Recombinant porcine interferon gamma (IFN gamma) at neutral pH is characterized by a tryptophan (Trp) fluorescence maximum around 343 nm and a rigid conformation, evidenced from tryptophan polarization values. Guanidine HCl shifts the protein emission spectra further to the red and decreases the fluorescence polarization values, indicating denatured IFN gamma in these solutions. In acidic solutions (3 < pH < 4), the emission spectra show a blue shift and lower tryptophan polarization. The midpoint of transition of these fluorescence properties occurs around pH 3.5-3.6. The protein in NaClO4 solution at neutral pH is similarly characterized by a blue shift in the tryptophan fluorescence maxima and low polarization values. The extent of quenching of tryptophan fluorescence by acrylamide is less in acid and in NaClO4 solutions of IFN gamma compared to its native form. This indicates a lower accessibility of the tryptophan in the altered conformation of the protein. The emission spectra of IFN gamma in NaClO4 solution shows a decrease in the tryptophan fluorescence intensity with simultaneous shift of the emission spectra over time. The presence of two conformational forms of IFN gamma in perchlorate solution is evidenced from an isofluorescent point at 315 nm. The change in the conformational state in perchlorate solution is characterized by first order kinetics. The dye anilinonaphthalene sulfonic acid does not bind either to the native IFN gamma or to its denatured form. However, the dye binds to the acid form of IFN gamma, as well as when the protein is present in NaClO4 solution at neutral pH. These observations, together with the results from literature that IFN gamma retains its secondary structure in acid solution to a considerable degree, would suggest that the protein exists as a molten globule-like state in acidic solution. Similarities of the protein fluorescence and 1-anilino-8-naphthalene-sulfonic-acid (ANS) binding properties of the protein in NaClO4 and acid solutions indicate that IFN gamma also exists in a molten globule-like state in perchlorate solution at neutral pH.

Reversibility of the binding of cytochrome c to liposomes. Implications for lipid-protein interactions

Studies on the membrane binding of cytochrome c revealed liposome-associated and soluble cytochrome c not to be in rapid equilibrium. In brief, cytochrome c attached to pyrene phospholipid-labeled, fluorescent liposomes containing either 17.6 mol % cardiolipin (CL) or 30 mol % egg phosphatidylglycerol (PG) is practically not at all or very slowly, respectively, detached by a subsequently added excess (up to 20-fold) of nonlabeled liposomes containing these acidic lipids. Cytochrome c was fully dissociated from PG-containing liposomes by increasing the ionic strength by NaCl, whereas dissociation from CL-containing membranes was less complete, presumably because of the scavenging of the protein within inverted intramembrane micelles. Importantly, the apparent irreversibility of the binding of cytochrome c to liposomes is strongly dependent on the structure of the acidic phospholipid. Cytochrome c bound to lyso-PG/PC liposomes could be dissociated with an excess of nonlabeled PG-containing liposomes. Cytochrome c was also efficiently bound to membranes containing the negatively charged dicetylphosphate yet could be readily dissociated by nonlabeled PG-containing liposomes. We conclude both proper geometry of the phosphate group and the presence of two acyl chains to be required for the tight binding of cytochrome c to acidic phospholipids. These data provide evidence for the membrane association of cytochrome c by an acidic phospholipid in the extended conformation (Kinnunen, P. K. J., Köiv, A., Lehtonen, J. Y. A., Rytömaa, M., and Mustonen, P. (1994) Chem. Phys. Lipids 73, 181-207) in which one of the acyl chains of the lipid becomes accommodated within a hydrophobic cavity of the protein. Based on the crystal structure of cytochrome c we putatively assign the invariant Asn-52 (horse heart cytochrome c) as the site liganding the protonated phosphate of the lipid, whereas Lys-72 and -73 should bind the deprotonated form.

Back and forth: the regulation and function of transbilayer phospholipid movement in eukaryotic cells

That some membranes restrict certain lipid species to one side of the bilayer and others to the opposite side has been known for two decades. However, how this asymmetric transbilayer distribution is generated and controlled, how many and what type of membranes are so structured, and even the reason for its existence is just now beginning to be understood. It has been a decade since the discovery of an activity which transports in an ATP-dependent manner only the aminophospholipids from the outer to the inner leaflet of the plasma membrane. This aminophospholipid translocase has yet to be isolated, reconstituted, and identified molecularly. Elevating intracellular Ca2+ allows all the major classes of phospholipids to move freely across the bilayer, scrambling lipids and dissipating asymmetry. The nature of this pathway and its mode of activation by Ca2+ remain to be determined. Though loss of transbilayer asymmetry by blood cells clearly produces a procoagulant surface and increases interactions with the reticuloendothelial system, it remains to be elucidated whether maintenance of blood homeostasis is just one expression of a more general raison d'être for lipid asymmetry. It is these persisting uncertainties and gaps in our knowledge which make the field such an interesting and exciting challenge at the present time.

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.

Effects of dehydroabietic acid on the physical state of cytoskeletal proteins and the lipid bilayer of erythrocyte membranes

Dehydroabietic acid (DHAA) is a major aquatic toxic resin acid usually found in unbleached pulp mill effluents. This compound has been reported to accumulate in the red cells of rainbow trout and to cause hemolysis. To elucidate further understanding to the mechanism of action of this resin, the interaction of DHAA with human erythrocyte membranes has been monitored by electron paramagnetic resonance techniques of spin labeling. Results presented in this paper indicate that DHAA, in a concentration-dependent manner, significantly altered both the motion and order of the lipid bilayer and the physical state of cytoskeletal proteins, while DHAA had no effect on isolated lipids. It is proposed that the increase in the 'fluidity' of the lipid bilayer induced by DHAA is a secondary effect of primary changes in the physical state of the cytoskeletal proteins of the membrane, and that the latter effect is critically associated with the toxicity of DHAA.

Ankyrin inhibits binding of erythrocyte spectrin to phospholipid vesicles

The studies on binding of erythrocyte spectrin to frozen and thawed phospholipid liposomes and its inhibition by ankyrin were performed. It was found that ankyrin inhibited up to 60% binding of spectrin by phosphatidylethanolamine/phosphatidylcholine vesicles. It was able to dissociate up to 40% of spectrin from this complex. Ankyrin inhibition of binding of phosphatidylserine/phosphatidylcholine vesicles by spectrin, although much lower, was also observed.

Invariant tryptophan at a shielded site promotes folding of the conformational unit of spectrin

The tryptophan that is highly conserved among repeating structural units of spectrin is reported to promote the conformational stability of one such unit of chicken brain alpha-spectrin. Four constructs were inserted into pET vectors for overexpression in Escherichia coli of the following spectrin peptides: (i) two adjacent but separately expressed "conformationally phased" repeating units, R16 and R17, one of which (R17) contains a single tryptophan; (ii) a mutant, M17, of the single tryptophan-containing unit with alanine substituted for the tryptophan; and (iii) a conformationally unphased unit, 1617, composed of half of each of the phased units. Both the mutant unit and the unphased unit were much more readily digested by chymotrypsin and by elastase than the phased units and exhibited only 38% and 54% as much alpha-helical structure, respectively, as the phased units by their far UV CD spectra; 90 degrees light scattering measurements revealed the folded peptides to be predominantly monomeric in solution, whereas the unfolded, protease-sensitive peptides consisted of dimers and/or trimers. This trend was corroborated by their dynamic light scattering. Both the blue-shifted wavelength of maximal emission and the relative inaccessibility to acrylamide of the single tryptophan in the folded unit indicate that the invariant tryptophan occupies a site that is shielded from the aqueous phase.

The effects of PMA and TFP and alterations in intracellular pH and calcium concentration on the membrane associations of phospholipid-binding proteins fodrin, protein kinase C and annexin II in cultured MDCK cells

Annexin II, alpha-fodrin and protein kinase C (PKC) are associated with the cytoplasmic surface of the plasma membranes. When assayed with liposomes, they show affinity for acidic phospholipids and bind calcium ions. They also respond to or participate in cell signal transduction by altered membrane binding properties. In the present work we have studied the properties of these proteins in epithelial MDCK cells in response to elevated intracellular calcium ion concentration, lowered pH, treatment with tumor promoter phorbol myristoyl acetate (PMA) and calmodulin inhibitor trifluoperazine (TFP). In untreated polarized MDCK cells annexin II was seen both along the lateral walls and membranes of intracellular vesicles, fodrin was located along the lateral walls, whereas PKC was seen in the cytoplasm. There was no observable translocation of these proteins upon elevation of the intracellular calcium concentration using a calcium ionophore A23187. On the other hand, treatment with TFP led to a release of annexin II from the plasma membranes which was accompanied by a transient peak in the intracellular calcium. Treatment with PMA led to a loss of the cubic form of the cells, a slight elevation in the intracellular calcium concentration and a drop in the intracellular pH. Simultaneously fodrin was released from the lateral walls, but still remained insoluble in Triton X-100, PKC became associated with the intracellular membranes and fibers, whereas annexin II remained along the lateral walls. These changes could be prevented by clamping the intracellular pH neutral during PMA treatment. On the other hand, lowering of intracellular pH below 6.5 with the nigericin treatment led to a similar translocation of fodrin and PKC as PMA. This suggests that the protein redistribution is caused by cytoplasmic acidification and is due to an increased hydrophobicity and enhanced protonation of lipids and proteins. In contrast, no changes were seen in the annexin II distribution in response to altered pH. Hence, its release by TFP is presumably due to changes in the cationic properties of the inner phase of the plasma membrane. Thus, proteins which show similar binding properties with liposomes show different characteristics in their association with the intracellular membranes.

Fluorescence studies of spectrin and its subunits

To better understand the solution structure of spectrin, the environment of its tryptophan residues have been examined by fluorescence spectroscopy. The spectra and the extent of quenching by several quenching agents have been determined for intact spectrin and its alpha and beta subunits. The arsenal of quenchers used in the study represented both hydrophilic and hydrophobic species including anionic, cationic and neutral compounds. Effects on spectrin fluorescence of ethanol and ionic strength, which extend and/or rigidify spectrin, and of glycerol, which is commonly used in electron microscopy of the protein, have also been assessed in the presence and absence of quenchers. Most of the tryptophans of spectrin are either internally quenched or are sequestered, hindering the approach of hydrophilic quenching agents. Both the spectral shape and the extent of quenching by acrylamide indicate that some tryptophans of the beta subunit are slightly more exposed in the isolated chain than in the dimer. Similar effects on spectra and on quenching of the intact dimer and of the isolated beta chain are seen when the ionic strength is reduced. Ethanol and glycerol reduce spectrin tryptophan accessibility to 2-p-toluidinyl napthalene-6-sulfonic acid (TNS). It therefore appears that low ionic strength, alpha-beta association and neutral solute (or lowered dielectric constant) all induce a similar, but modest conformational change in the domain structure. The extent of TNS binding is not increased by lowering the ionic strength, suggesting that the expansion and/or stiffening of the molecule in low electrolyte solution does not involve exposure of significant numbers of hydrophobic sites.