Synthesis of maize chloroplast ATP-synthase beta-subunit fusion proteins in Escherichia coli and binding to the inner membrane

The maize chloroplast gene for the beta subunit (atpB) of the chloroplast CF1 component of ATPase from maize, when fused to either the lacZ or ral genes in the vectors pMC1403 or pHUB4, is expressed in Escherichia coli as a fusion protein with beta-galactosidase or with bacteriophage lambda Ral sequences. Some of the fusion proteins are converted to a membrane-bound form, as determined by differential and sucrose-gradient centrifugation. The specificity of membrane binding has been examined using E. coli unc mutants that are defective in binding of the F1 ATPase component to the F0 receptor site on the membrane, and by the use of two different length maize atpB::lacZ gene fusions. We show that the first 365 N-terminal amino acids (aa) of the maize beta subunit are involved in binding to the E. coli inner membrane, and that this binding is probably mediated by the bacterial F0 receptor.     

Calcium binding to the subunit c ofE. coli ATP-synthase and possible functional implications in energy coupling

The 8-kDa subunit c of theE. coli F₀ ATP-synthase proton channel was tested for Ca⁺⁺ binding activity using a⁴⁵Ca⁺⁺ ligand blot assay after transferring the protein from SDS-PAGE gels onto polyvinyl difluoride membranes. The purified subunit c binds⁴⁵Ca⁺⁺ strongly with Ca⁺⁺ binding properties very similar to those of the 8-kDa CF₀ subunit III of choloroplast thylakoid membranes. The N-terminal f-Met carbonyl group seems necessary for Ca⁺⁺ binding capacity, shown by loss of Ca⁺⁺ binding following removal of the formyl group by mild acid treatment. The dicyclohexylcarbodiimide-reactive Asp-61 is not involved in the Ca⁺⁺ binding, shown by Ca⁺⁺ binding being retained in twoE. coli mutants, Asp61Asn and Asp61Gly. The Ca⁺⁺ binding is pH dependent in both theE. coli and thylakoid 8-kDa proteins, being absent at pH 5.0 and rising to a maximum near pH 9.0. A treatment predicted to increase the Ca⁺⁺ binding affinity to its F₀ binding site (chlorpromazine photoaffinity attachment) caused an inhibition of ATP formation driven by a base-to-acid pH jump in whole cells. Inhibition was not observed when the Ca⁺⁺ chelator EGTA was present with the cells during the chlorpromazine photoaffinity treatment. An apparent Ca⁺⁺ binding constant on the site responsible for the UV plus chlorpromazine effect of near 80–100 nM was obtained using an EGTA-Ca⁺⁺ buffer system to control free Ca⁺⁺ concentration during the UV plus chlorpromazine treatment. The data are consistent with the notion that Ca⁺⁺ bound to the periplasimic side of theE. coli F₀ proton channel can block H⁺ entry into the channel. A similar effect occurs in thylakoid membranes, but the Ca⁺⁺ binding site is on the lumen side of the thylakoid, where Ca⁺⁺ binding can modulate acid-base jump ATP formation. The Ca⁺⁺ binding to the F₀ and CF₀ complexes is consistent with a pH-dependent gating mechanism for control of H⁺ ion flux across the opening of the H⁺ channel.This work was supported in part by grants from the Department of Energy and the U.S. Department of Agriculture.On leave from the Institute of Soil Science and Photosynthesis, Russian Academy of Science, Pushchino, Russia.     

Non-covalent binding of membrane lipids to membrane proteins

Polar lipids and membrane proteins are major components of biological membranes, both cell membranes and membranes of enveloped viruses. How these two classes of membrane components interact with each other to influence the function of biological membranes is a fundamental question that has attracted intense interest since the origins of the field of membrane studies. One of the most powerful ideas that driven the field is the likelihood that lipids bind to membrane proteins at specific sites, modulating protein structure and function. However only relatively recently has high resolution structure determination of membrane proteins progressed to the point of providing atomic level structure of lipid binding sites on membrane proteins. Analysis of X-ray diffraction, electron crystallography and NMR data over 100 specific lipid binding sites on membrane proteins. These data demonstrate tight lipid binding of both phospholipids and cholesterol to membrane proteins. Membrane lipids bind to membrane proteins by their headgroups, or by their acyl chains, or binding is mediated by the entire lipid molecule. When headgroups bind, binding is stabilized by polar interactions between lipid headgroups and the protein. When acyl chains bind, van der Waals effects dominate as the acyl chains adopt conformations that complement particular sites on the rough protein surface. No generally applicable motifs for binding have yet emerged. Previously published biochemical and biophysical data link this binding with function. This Article is Part of a Special Issue Entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.     

Theory of hapten binding to IgM: the question of repulsive interactions between binding sites.

Various workers in their studies of the binding of haptens to IgM have observed that at low hapten concentration IgM has an apparent valence of five or near five, while at high hapten concentration IgM has a valence of ten. A possible explanation for this is that there is an interaction between binding sites on the same F(ab')2 region of the IgM molecule. In this paper the theory for such an interaction is presented and an expression for the apparent valence is derived. It is shown that the apparent valence depends on both the interaction between binding sites on the IgM molecule and on the width of the affinity distribution which characterizes the antiserum. A broad affinity distribution can give an apparent valence of five even when there is no interaction between sites, i.e., even when the ten binding sites on the IgM molecule are identical and independent. The general properties of a Scatchard plot are also discussed. When there is no interaction between sites it is shown that the average affinity and the variance of the affinity distribution can be obtained from a Scatchard plot. To illustrate the theory, an antiserum with affinities characterized by a normal distribution is considered and a simple method is presented for determining alpha, the parameter which measures the width of the normal distribution.     

Ca binding to the human red cell membrane: Characterization of membrane preparations and binding sites

Summary Inside out and right side out vesicles were used to study the sidedness of Ca binding to the human red cell membrane. It was shown that these vesicles exhibited only a limited permeability to Ca, enabling the independent characterization of Ca binding to the extracellular and cytoplasmic membrane surfaces. Ca binding was studied in 10 mM Tris HCl at pH 7.4, 22±2°C and was shown to be complete in under 5 min. Scatchard plots were made from Ca binding data obtained at free Ca concentrations in the range of 10^–6 to 10^–3M. Under these conditions inside out vesicles exhibit two independent binding sites for Ca with association constants of 1×10⁵ and 6×10³ M^–1, and right side out vesicles exhibit three independent binding sites with association constants of 2×10⁵, 1.4×10⁴ and 3×10²M^–1. Upon the addition of 0.1M KCl a third, high affinity site was found on inside out vesicles with an association constant of 3×10⁵, (in 0.1 M KCl). Ca binding to inside out vesicles increased nearly linearly with pH in the, range of pH 4 to pH 11, while binding to right side out vesicles remained practically unchanged in the range of pH 7 to pH 9. Progressive increase of the ionic strength of the medium by the addition of K, Mg or Tris decreased Ca binding to inside out vesicles as did the addition of ATP. Comparison of a series of cation competitors for Ca binding sites on inside out vesicles at 0.003 mM Ca showed that La was the most effective competitor of all while Cd was the most effective divalent cation competitor of those tested. Our findings suggest that the effects of low concentrations of Ca at the inner surface of the red cell membrane are mediated primarily through Ca binding to site 1 (and, possibly site 2) of inside out vesicles of which there are approximately 1.6×10⁵ per equivalent cell.     

Influence of membrane components in the binding of proteins to membrane surfaces

We have quantified the enhancement of membrane binding of activated and deactivated Galpha(s) and Galpha(q) subunits, Gbetagamma subunits, and phospholipase Cbeta(2) by lipid rafts and by the presence of membrane-associated protein partners. Membrane binding studies show that lipid rafts do not affect the intrinsic membrane affinity of Galpha(q)(GDP) and Galpha(s)(GDP), supporting the idea that these proteins partition evenly between the domains. Visualization of lipid rafts on monolayers by use of a probe that does not enter raft domains shows that neither activated nor deactivated Galpha(q)(GDP) subunits distribute evenly between the raft and nonraft domains, contrary to previous suggestions. Membrane binding of deactivated Galpha(q) and Galpha(s)(GDP) became weaker when Gbetagamma subunits were present, in contrast with the behavior predicted by thermodynamics. However, activated Galpha subunits and phospholipase Cbeta(2) were recruited to membrane surfaces by protein partners by predicted amounts. Our studies suggest that the anomalous behavior seen for deactivated Galpha subunits in the presence of Gbetagamma subunits may be due to conformational changes in the N-terminus and/or occlusion of a portion of its membrane interaction region by Gbetagamma. Even though membrane recruitment was clearly observed for one protein partner, the presence of a second partner of lower affinity did not further promote membrane binding. For these proteins, the formation of larger protein complexes with very high membrane affinities is unlikely.     

Ca binding to the human red cell membrane: characterization of membrane preparations and binding sites.

Inside out and right side out vesicles were used to study the sidedness of Ca binding to the human red cell membrane. It was shown that these vesicles exhibited only a limited permeability to Ca, enabling the independent characterization of Ca binding to the extracellular and cytoplasmic membrane surfaces...     

Detection of liposome membrane viscosity perturbations with ratiometric molecular rotors

Molecular rotors are a form of fluorescent intramolecular charge-transfer complexes that can undergo intramolecular twisting motion upon photoexcitation. Twisted-state formation leads to non-radiative relaxation that competes with fluorescence emission. In bulk solutions, these molecules exhibit a viscosity-dependent quantum yield. On the molecular scale, the fluorescence emission is a function of the local free volume, which in turn is related to the local micro-viscosity. Membrane viscosity, and the inverse; fluidity, are characteristic terms used to describe the ease of movement withing the membrane. Often, changes in membrane viscosity govern intracellular processes and are indicative of a disease state. Molecular rotors have been used to investigate viscosity changes in liposomes and cells, but accuracy is affected by local concentration gradients and sample optical properties. We have developed self-calibrating ratiometric molecular rotors to overcome this challenge and integrated the new molecules into a DLPC liposome model exposed to the membrane-fluidizing agent propanol. We show that the ratiometric emission intensity linearly decreases with the propanol exposure and that the ratiometric intensity is widely independent of the total liposome concentration. Conversely, dye concentration inside liposomes influences the sensitivity of the system. We suggest that the new self-calibrating dyes can be used for real-time viscosity sensing in liposome systems with the advantages of lifetime measurements, but with low-cost steady-state instrumentation.     

Covalent binding of 1,N(6)-ethenoadenosine diphosphate to catalytic and noncatalytic sites of chloroplast ATP-synthase

The catalytic and noncatalytic sites of the chloroplast coupling factor (CF1) were selectively modified by incubation with the dialdehyde derivative of the fluorescent adenosine diphosphate analogue 1,N(6)-ethenoadenosine diphosphate. The modified CF1 was reconstituted with EDTA-treated chloroplast thylakoid membranes. The influence of light-induced transmembrane proton gradient and of phosphate ions on the fluorescence of 1,N(6)-ethenoadenosine diphosphate covalently bound to catalytic sites of reconstituted CF1 (ATP-synthase) was studied. Upon illumination of thylakoid membranes with saturating white light, the quenching of fluorescence of covalently bound 1,N(6)-ethenoadenosine diphosphate was observed. The quenching was reversed by the addition of inorganic phosphate to the reaction mixture in the dark. Repeated illumination induced the quenching once again: however, the addition of phosphate ions did not affect the fluorescence intensity now. When 1,N(6)-ethenoadenosine diphosphate was covalently bound to noncatalytic sites of ATP-synthase, no similar fluorescent changes were observed. The interrelation of the observed changes of 1,N(6)-ethenoadenosine diphosphate fluorescence and the mechanism of energy-dependent changes in the structure of the catalytic site of ATP-synthase is discussed.     

Calcium binding to the chloroplast andE. coli (CF0) F0 subunit III (c) of the ATP-synthase

Summary Subunit III and c, the 8 kDa components of the chloroplast CF₀, andE. coli H⁺ channel complexes respectively, were isolated and purified for the purpose of studying their Ca⁺⁺-binding properties. Purified subunit III or c as well as the unfractionated organic-solvent soluble preparation from chloroplasts were used in a⁴⁵Ca⁺⁺-ligand blot assay known to detect high affinity Ca⁺⁺-binding sites in proteins. Both subunit III and c showed strong⁴⁵Ca⁺⁺-binding. None of the other CF₀ subunits bound Ca⁺⁺ and of the CF₁ only a weak binding was detected in the region of the , subunits. The Ca⁺⁺-binding was inhibited after treating the proteins in solution by derivatizing aqueously exposed carboxyl groups with a water soluble carbodiimide plus a nucleophile, after de-formylation of the N-terminal methionine, or with a subsequent treatment with La³⁺. Dicyclohexylcarbodiimide treatment (no nucleophile was added) of thylakoid membranes, which derivatizes the hydrophobically located Glu 61 (Asp 61 inE. coli), did not inhibit the Ca⁺⁺-binding in either protein. The data indicate that for both proteins the carbonyl group of the formylated N-terminal Met-1 and probably the carboxyl group of the subunit III (or c) C-terminal provide some of seven essential oxygen ligands normally required for defining a Ca⁺⁺-binding site in proteins. Based on the accepted models for the hairpin conformation of the subunit III (c), it seems clear that the Ca⁺⁺-binding site can form on the lumenal side of the membrane in the functional CF₀ structures or on the periplasmic side of theE. coli membrane. A working hypothesis we are testing is that Ca⁺⁺-binding to the CF₀ (or F₀) can form an easily reversible gating site such as to enhance the probability for membrane-localized H⁺ gradients being coupled to ATP formation under moderate energization loads, but under excess energization the local H⁺ ion concentration may build up high enough to displace the bound Ca⁺⁺, resulting in delocalization of the H⁺ gradient. The latter situation seems, in chloroplasts at least, to function as a signal for over-energization; i.e., excess light absorption, a potential stress situation for plants. Lumenal acidification appears to be a trigger for initiating stress alleviation responses.On leave from the Institute of Soil Science and Photosynthesis, Russian Academy of Sciences, Puschchino, Russia.     

Features of organic nonelectrolyte binding to the erythrocyte membrane

The capacity of erythrocyte membranes for organic nonelectrolytes from different chemical groups of chemical compounds was studied by the spin probe method and scanning electron microscopy. Hydrophobic spin-labeled derivatives of gamma-carbolin and stearic acid and the screened phenol antioxidant fenozan-1 were used as nonelectrolytes. Based on the analysis of electron spin resonance spectra of the hydrophobic spin-labeled nonelectrolytes and electron micrographs of erythrocytes, differences in the capacity of distribution regions in the intramembrane space of the derivative of gamma-carbolin and fenozan-1, on the one hand, and the spin-labeled derivative of fatty acid, on the other hand, were found. The first group has at least two membrane distribution regions, whereas in the second case only one type of distribution was found. The influence of limited membrane capacity on the realization of biological activity of organic nonelectrolytes is discussed.     

Asymmetric binding of membrane proteins to GroEL

The interaction of GroEL with non-native soluble proteins has been studied intensively and structure-function relationships have been established in considerable detail. Recently, we found that GroEL is also able to bind membrane proteins in the absence of detergents and deliver them to liposomes in a biologically active state. Here, we report that three well-studied membrane proteins (bacteriorhodopsin, LacY, and the bacteriophage lambda holin) bind asymmetrically to tetradecameric GroEL. Each of the membrane proteins was visualized in one of the center cavities of GroEL using single particle analysis.