Ligand-induced accelerated dissociation of (+)-cis-diltiazem from L-type Ca2+ channels is simply explained by competition for individual attachment points

IgE, IgE Receptors and Anti-IgE Biologics: Protein Structures and Mechanisms of Action

The evolution of IgE in mammals added an extra layer of immune protection at body surfaces to provide a rapid and local response against antigens from the environment. The IgE immune response employs potent expulsive and inflammatory forces against local antigen provocation, at the risk of damaging host tissues and causing allergic disease. Two well-known IgE receptors, the high-affinity FcεRI and low-affinity CD23, mediate the activities of IgE. Unlike other known antibody receptors, CD23 also regulates IgE expression, maintaining IgE homeostasis. This mechanism evolved by adapting the function of the complement receptor CD21. Recent insights into the dynamic character of IgE structure, its resultant capacity for allosteric modulation, and the potential for ligand-induced dissociation have revealed previously unappreciated mechanisms for regulation of IgE and IgE complexes. We describe recent research, highlighting structural studies of the IgE network of proteins to analyze the uniquely versatile activities of IgE and anti-IgE biologics.

Tuning IgE: IgE-Associating Molecules and Their Effects on IgE-Dependent Mast Cell Reactions

The recent emergence of anti-immunoglobulin E (IgE) drugs and their candidates for humans has endorsed the significance of IgE-dependent pathways in allergic disorders. IgE is distributed locally in the tissues or systemically to confer a sensory mechanism in a domain of adaptive immunity to the otherwise innate type of effector cells, namely, mast cells and basophils. Bound on the high-affinity IgE receptor FcεRI, IgE enables fast memory responses against revisiting threats of venoms, parasites, and bacteria. However, the dysregulation of IgE-dependent reactions leads to potentially life-threatening allergic diseases, such as asthma and anaphylaxis. Therefore, reactivity of the IgE sensor is fine-tuned by various IgE-associating molecules. In this review, we discuss the mechanistic basis for how IgE-dependent mast cell activation is regulated by the IgE-associating molecules, including the newly developed therapeutic candidates.

Structural Basis for Diltiazem Block of a Voltage-Gated Ca2+ Channel

Diltiazem is a widely prescribed Ca²⁺ antagonist drug for cardiac arrhythmia, hypertension, and angina pectoris. Using the ancestral Ca_V channel construct Ca_VAb as a molecular model for X-ray crystallographic analysis, we show here that diltiazem targets the central cavity of the voltage-gated Ca²⁺ channel underneath its selectivity filter and physically blocks ion conduction. The diltiazem-binding site overlaps with the receptor site for phenylalkylamine Ca²⁺ antagonist drugs such as verapamil. The dihydropyridine Ca²⁺ channel blocker amlodipine binds at a distinct site and allosterically modulates the binding sites for diltiazem and Ca²⁺ Our studies resolve two distinct binding poses for diltiazem in the absence and presence of amlodipine. The binding pose in the presence of amlodipine may mimic a high-affinity binding configuration induced by voltage-dependent inactivation, which is favored by dihydropyridine binding. In this binding pose, the tertiary amino group of diltiazem projects upward into the inner end of the ion selectivity filter, interacts with ion coordination Site 3 formed by the backbone carbonyls of T175, and alters binding of Ca²⁺ to ion coordination Sites 1 and 2. Altogether, our results define the receptor site for diltiazem and elucidate the mechanisms for pore block and allosteric modulation by other Ca²⁺ channel-blocking drugs at the atomic level. SIGNIFICANCE STATEMENT: Calcium antagonist drugs that block voltage-gated calcium channels in heart and vascular smooth muscle are widely used in the treatment of cardiovascular diseases. Our results reveal the chemical details of diltiazem binding in a blocking position in the pore of a model calcium channel and show that binding of another calcium antagonist drug alters binding of diltiazem and calcium. This structural information defines the mechanism of drug action at the atomic level and provides a molecular template for future drug discovery.

Substrate-Induced Facilitated Dissociation of the Competitive Inhibitor from the Active Site of O-Acetyl Serine Sulfhydrylase Reveals a Competitive-Allostery Mechanism

By classical competitive antagonism, a substrate and competitive inhibitor must bind mutually exclusively to the active site. The competitive inhibition of O-acetyl serine sulfhydrylase (OASS) by the C-terminus of serine acetyltransferase (SAT) presents a paradox, because the C-terminus of SAT binds to the active site of OASS with an affinity that is 4-6 log-fold (10⁴-10⁶) greater than that of the substrate. Therefore, we employed multiple approaches to understand how the substrate gains access to the OASS active site under physiological conditions. Single-molecule and ensemble approaches showed that the active site-bound high-affinity competitive inhibitor is actively dissociated by the substrate, which is not consistent with classical views of competitive antagonism. We employed fast-flow kinetic approaches to demonstrate that substrate-mediated dissociation of full length SAT-OASS (cysteine regulatory complex) follows a noncanonical "facilitated dissociation" mechanism. To understand the mechanism by which the substrate induces inhibitor dissociation, we resolved the crystal structures of enzyme·inhibitor·substrate ternary complexes. Crystal structures reveal a competitive allosteric binding mechanism in which the substrate intrudes into the inhibitor-bound active site and disengages the inhibitor before occupying the site vacated by the inhibitor. In summary, here we reveal a new type of competitive allosteric binding mechanism by which one of the competitive antagonists facilitates the dissociation of the other. Together, our results indicate that "competitive allostery" is the general feature of noncanonical "facilitated/accelerated dissociation" mechanisms. Further understanding of the mechanistic framework of "competitive allosteric" mechanism may allow us to design a new family of "competitive allosteric drugs/small molecules" that will have improved selectivity and specificity as compared to their competitive and allosteric counterparts.

HLA-DM and HLA-DO, key regulators of MHC-II processing and presentation

Peptide loading of class II MHC molecules in endosomal compartments is regulated by HLA-DM. HLA-DO modulates HLA-DM function, with consequences for the spectrum of MHC-bound epitopes presented at the cell surface for interaction with T cells. Here, we summarize and discuss recent progress in investigating the molecular mechanisms of action of HLA-DM and HLA-DO and in understanding their roles in immune responses. Key findings are the long-awaited structures of HLA-DM in complex with its class II substrate and with HLA-DO, and observation of a novel phenotype--autoimmunity combined with immunodeficiency--in mice lacking HLA-DO. We also highlight several areas where gaps persist in our knowledge about this pair of proteins and their molecular biology and immunobiology.

Accelerated disassembly of IgE-receptor complexes by a disruptive macromolecular inhibitor

IgE antibodies bind the high-affinity IgE Fc receptor (FcεRI), found primarily on mast cells and basophils, and trigger inflammatory cascades of the allergic response. Inhibitors of IgE-FcεRI binding have been identified and an anti-IgE therapeutic antibody (omalizumab) is used to treat severe allergic asthma. However, preformed IgE-FcεRI complexes that prime cells before allergen exposure dissociate extremely slowly and cannot be disrupted by strictly competitive inhibitors. IgE-Fc conformational flexibility indicated that inhibition could be mediated by allosteric or other non-classical mechanisms. Here we demonstrate that an engineered protein inhibitor, DARPin E2_79 (refs 9, 10, 11), acts through a non-classical inhibition mechanism, not only blocking IgE-FcεRI interactions, but actively stimulating the dissociation of preformed ligand-receptor complexes. The structure of the E2_79-IgE-Fc(3-4) complex predicts the presence of two non-equivalent E2_79 sites in the asymmetric IgE-FcεRI complex, with site 1 distant from the receptor and site 2 exhibiting partial steric overlap. Although the structure is indicative of an allosteric inhibition mechanism, mutational studies and quantitative kinetic modelling indicate that E2_79 acts through a facilitated dissociation mechanism at site 2 alone. These results demonstrate that high-affinity IgE-FcεRI complexes can be actively dissociated to block the allergic response and suggest that protein-protein complexes may be more generally amenable to active disruption by macromolecular inhibitors.

Transient binding patches: a plausible concept for drug binding

Dose–response curves for inhibitors (drugs) generally are analyzed by means of four-parameter fits, yielding IC₅₀, background, amplitude, and Hill coefficient. Hill coefficients ≠1 contradict 1:1 competition. If binding of substrates to proteins is a stepwise process where initial binding to initial locations (patches) leads to strong binding on defined sites, then drugs (non-endogenous inhibitors) may bind to those presumably larger patches and need not follow a 1:1 stoichiometry for specific inhibition. This concept was translated into three computable models and successfully fitted to 1,282 phosphatase dose–response curves. The models only required four parameters, namely, the equilibrium dissociation constant K_D(1) of the first inhibitor binding step, background, amplitude, and a compound interaction factor to quantify the interaction of inhibitors on those patches. Binding of one established inhibitor to the vaccinia virus VH1-related (VHR) phosphatase was directly measured with microcalorimetry, confirming multiple inhibitor binding with equilibrium constants obtained from corresponding inhibition curves.

Direct labelling of the human P2X7 receptor and identification of positive and negative cooperativity of binding

BACKGROUND AND PURPOSE

The P2X(7) receptor exhibits complex pharmacological properties. In this study, binding of a [(3)H]-labelled P2X(7) receptor antagonist to human P2X(7) receptors has been examined to further understand ligand interactions with this receptor.

EXPERIMENTAL APPROACH

The P2X(7) receptor antagonist, N-[2-({2-[(2-hydroxyethyl)amino]ethyl}amino)-5-quinolinyl]-2-tricyclo[3.3.1.1(3,7)]dec-1-ylacetamide (compound-17), was radiolabelled with tritium and binding studies were performed using membranes prepared from U-2 OS or HEK293 cells expressing human recombinant P2X(7) receptors.

KEY RESULTS

Binding of [(3)H]-compound-17 was higher in membranes prepared from cells expressing P2X(7) receptors than from control cells and was inhibited by ATP suggesting labelled sites represented human P2X(7) receptors. Binding was reversible, saturable and modulated by P2X(7) receptor ligands (Brilliant Blue G, KN62, ATP, decavanadate). Furthermore, ATP potency was reduced in the presence of divalent cations or NaCl. Radioligand binding exhibited both positive and negative cooperativity. Positive cooperativity was evident from bell shaped Scatchard plots, reduction in radioligand dissociation rate by unlabelled compound-17 and enhancement of radioligand binding by KN62 and unlabelled compound-17. ATP and decavanadate inhibited binding in a negative cooperative manner as they enhanced radioligand dissociation.

CONCLUSIONS

These data demonstrate that human P2X(7) receptors can be directly labelled and provide novel insights into receptor function. The positive cooperativity observed suggests that binding of compound-17 to one subunit in the P2X(7) receptor complex enhances subsequent binding to other P2X(7) subunits in the same complex. The negative cooperative effects of ATP suggest that ATP and compound-17 bind at separate, interacting, sites on the P2X(7) receptor.

A disubstituted succinamide is a potent sodium channel blocker with efficacy in a rat pain model

Sodium channel blockers are used clinically to treat a number of neuropathic pain conditions, but more potent and selective agents should improve on the therapeutic index of currently used drugs. In a high-throughput functional assay, a novel sodium channel (Na(V)) blocker, N-[[2'-(aminosulfonyl)biphenyl-4-yl]methyl]-N'-(2,2'-bithien-5-ylmethyl)succinamide (BPBTS), was discovered. BPBTS is 2 orders of magnitude more potent than anticonvulsant and antiarrhythmic sodium channel blockers currently used to treat neuropathic pain. Resembling block by these agents, block of Na(V)1.2, Na(V)1.5, and Na(V)1.7 by BPBTS was found to be voltage- and use-dependent. BPBTS appeared to bind preferentially to open and inactivated states and caused a dose-dependent hyperpolarizing shift in the steady-state availability curves for all sodium channel subtypes tested. The affinity of BPBTS for the resting and inactivated states of Na(V)1.2 was 1.2 and 0.14 microM, respectively. BPBTS blocked Na(V)1.7 and Na(V)1.2 with similar potency, whereas block of Na(V)1.5 was slightly more potent. The slow tetrodotoxin-resistant Na(+) current in small-diameter DRG neurons was also potently blocked by BPBTS. [(3)H]BPBTS bound with high affinity to a single class of sites present in rat brain synaptosomal membranes (K(d) = 6.1 nM), and in membranes derived from HEK cells stably expressing Na(V)1.5 (K(d) = 0.9 nM). BPBTS dose-dependently attenuated nociceptive behavior in the formalin test, a rat model of tonic pain. On the basis of these findings, BPBTS represents a structurally novel and potent sodium channel blocker that may be used as a template for the development of analgesic agents.

High affinity interaction of mibefradil with voltage-gated calcium and sodium channels

Mibefradil is a novel Ca(2+) antagonist which blocks both high-voltage activated and low voltage-activated Ca(2+) channels. Although L-type Ca(2+) channel block was demonstrated in functional experiments its molecular interaction with the channel has not yet been studied. We therefore investigated the binding of [(3)H]-mibefradil and a series of mibefradil analogues to L-type Ca(2+) channels in different tissues. [(3)H]-Mibefradil labelled a single class of high affinity sites on skeletal muscle L-type Ca(2+) channels (K(D) of 2.5+/-0.4 nM, B(max)=56.4+/-2.3 pmol mg(-1) of protein). Mibefradil (and a series of analogues) partially inhibited (+)-[(3)H]-isradipine binding to skeletal muscle membranes but stimulated binding to brain L-type Ca(2+) channels and alpha1C-subunits expressed in tsA201 cells indicating a tissue-specific, non-competitive interaction between the dihydropyridine and mibefradil binding domain. [(3)H]-Mibefradil also labelled a heterogenous population of high affinity sites in rabbit brain which was inhibited by a series of nonspecific Ca(2+) and Na(+)-channel blockers. Mibefradil and its analogue RO40-6040 had high affinity for neuronal voltage-gated Na(+)-channels as confirmed in binding (apparent K(i) values of 17 and 1.0 nM, respectively) and functional experiments (40% use-dependent inhibition of Na(+)-channel current by 1 microM mibefradil in GH3 cells). Our data demonstrate that mibefradil binds to voltage-gated L-type Ca(2+) channels with very high affinity and is also a potent blocker of voltage-gated neuronal Na(+)-channels. More lipophilic mibefradil analogues may possess neuroprotective properties like other nonselective Ca(2+)-/Na(+)-channel blockers.

Binding of nucleotides to guanylate kinase, p21(ras), and nucleoside-diphosphate kinase studied by nano-electrospray mass spectrometry

The binding of nucleotides to three different nucleotide-binding proteins and to a control protein was studied by means of nano-electrospray mass spectrometry applied to aqueous nondenaturing solutions. The method leads to unambiguous identification of enzyme complexes with substrates and products but does not allow the determination of dissociation constants or even stoichiometries relevant to the binding in solution. For guanylate kinase (EC 2.7.4. 8), the transfer of HPO(3) between nucleotides was observed whenever a ternary complex with adenylate or guanylate nucleotides was formed. Guanosine 5'-tetraphosphate was generated after prolonged incubation with GDP or GTP. Mg(2+) binding was considerably enhanced in functional high affinity complexes, such as observed between guanylate kinase and its bisubstrate inhibitor P(1)-(5'-guanosyl)-P(5)-(5'-adenosyl) pentaphosphate or with the tight nucleotide-binding protein p21(ras) and GDP. Nucleoside-diphosphate kinase (EC 2.7.4.6) itself was phosphorylated in accordance to its known ping-pong mechanism. All nucleotide-binding proteins were shown to bind sulfate (SO(4)(2-)) with presumably high affinity and slow exchange rate. The binding of phosphate (PO(4)(3-)) could be inferred indirectly from competition with SO(4)(2-).

Nuclear magnetic resonance and restrained molecular dynamics studies of the interaction of an epidermal growth factor-derived peptide with protein tyrosine phosphatase 1B

The epidermal growth factor-derived (EGFR988) fluorophosphonate peptide, DADE(F2Pmp)L, is a potent (30 pM) inhibitor of the protein tyrosine phosphatase PTP1B. Nuclear magnetic resonance (NMR) transferred nuclear Overhauser effect (nOe) experiments have been used to determine the conformation of DADE(F2Pmp)L while bound in the active site of PTP1B. When bound, the peptide adopts an extended beta-strand conformation. Molecular modeling and molecular dynamics simulations allowed the elucidation of the sources of many of the interactions leading to binding of this inhibitor. Electrostatic, hydrophobic, and hydrogen-bonding interactions were all found to contribute significantly to its binding. However, despite the overall tight binding of this inhibitor, the N-terminal and adjacent residue of the peptide were virtually unrestrained in their motion. The major contributions to binding arose from hydrophobic interactions at the leucine and at the aromatic center, hydrogen bonding to the pro-R fluorine of the fluorophosphonomethyl group, and electrostatic interactions involving the carboxylate functionalities of the aspartate and glutamate residues. These latter two residues were found to form tight contacts with surface recognition elements (arginine and lysine) situated near the active-site cleft.

The diversity of GABAA receptors. Pharmacological and electrophysiological properties of GABAA channel subtypes

The amino acid gamma-aminobutyric-acid (GABA) prevails in the CNS as an inhibitory neurotransmitter that mediates most of its effects through fast GABA-gated Cl(-)-channels (GABAAR). Molecular biology uncovered the complex subunit architecture of this receptor channel, in which a pentameric assembly derived from five of at least 17 mammalian subunits, grouped in the six classes alpha, beta, gamma, delta, sigma and epsilon, permits a vast number of putative receptor isoforms. The subunit composition of a particular receptor determines the specific effects of allosterical modulators of the GABAARs like benzodiazepines (BZs), barbiturates, steroids, some convulsants, polyvalent cations, and ethanol. To understand the physiology and diversity of GABAARs, the native isoforms have to be identified by their localization in the brain and by their pharmacology. In heterologous expression systems, channels require the presence of alpha, beta, and gamma subunits in order to mimic the full repertoire of native receptor responses to drugs, with the BZ pharmacology being determined by the particular alpha and gamma subunit variants. Little is known about the functional properties of the beta, delta, and epsilon subunit classes and only a few receptor subtype-specific substances like loreclezole and furosemide are known that enable the identification of defined receptor subtypes. We will summarize the pharmacology of putative receptor isoforms and emphasize the characteristics of functional channels. Knowledge of the complex pharmacology of GABAARs might eventually enable site-directed drug design to further our understanding of GABA-related disorders and of the complex interaction of excitatory and inhibitory mechanisms in neuronal processing.

Subtle changes in residue 77 of the gamma subunit of alpha1beta2gamma2 GABAA receptors drastically alter the affinity for ligands of the benzodiazepine binding site

Recombinant alpha1beta2gamma2 gamma-aminobutyric acid type A (GABAA) receptors were functionally expressed in Xenopus oocytes. Upon the mutation F77L, diazepam and Ro 15-1788 retained the ability to interact with the benzodiazepine binding site, but zolpidem lost this ability. To quantify these data, radioligand binding experiments were performed using membrane preparations of transiently transfected human embryonic kidney 293 cells. The amino acid gamma77, phenylalanine, was also mutated to tyrosine, tryptophan, and isoleucine. Although there was little effect on Ro 15-1788 binding upon mutation to tyrosine, the loss in affinity for diazepam was from 12 to 2,720 nM. The change to leucine, in contrast, resulted in little change in the diazepam affinity, whereas there was a strongly reduced affinity for zolpidem from 17 to 4,870 nM and for methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) from 1.9 to 1,780 nM, respectively. The change to tryptophan resulted in two-phasic displacement curves, and only about 50% of the [3H]flunitrazepam binding could be displaced by zolpidem, DMCM, and Ro 15-1788, respectively, whereas midazolam and diazepam still resulted in 100% displacement, indicating the presence of two sites upon expression of this mutant receptor. Functional expression in Xenopus oocytes showed that all mutant channels displayed a comparatively small change (<4.3-fold) in their apparent agonist affinity and that these channels could still be functionally modulated by ligands of the benzodiazepine binding site. We conclude that subtle changes in gammaF77 drastically affect benzodiazepine pharmacology and that this residue probably interacts directly with most ligands of the benzodiazepine binding site and therefore defines part of the benzodiazepine binding pocket.

Agonist binding to the Torpedo acetylcholine receptor. 2. Complexities revealed by association kinetics

The binding of suberyldicholine to membrane-bound Torpedo acetylcholine receptor has been monitored by fluorescence changes of covalently bound 5-iodoacetamidosalicylic acid (IAS). At equilibrium, suberyldicholine binds to two high-affinity binding sites (Kd approximately 20 nM). Kinetic experiments reveal that there is rapid formation of an initial complex (Kd approximately 2 microM) which undergoes sequential fast (k(app) approximately 1 s(-1)) and slow (k(app) approximately 0.05 s(-1)) conformational changes. These kinetics differ from those reported for other agonists [Blanchard, S. G., Dunn, S. M. J., & Raftery, M. A. (1982) Biochemistry 24, 6258-6264] in that, for suberyldicholine, there is no evidence for a second pathway involving the binding of an additional agonist molecule. These results, considered together with the observed dissociation kinetics (accompanying manuscript), suggest that each high-affinity site for acetylcholine is made up of two subsites, which suberyldicholine is able to bridge, thus occluding the binding of a second ligand. The kinetic mechanism for acetylcholine binding has been re-examined to accommodate the complexities of the [3H]-acetylcholine dissociation kinetics and the observation that, at equilibrium, no more than two occupied binding sites are detected [accompanying manuscript: Dunn, S. M. J., & Raftery, M. A. (1997) Biochemistry 36, 3846-3853]. It is suggested that, for each acetylcholine binding site, a second ligand is able to bind but that the ternary complex is transient since one of the two bound ligands again dissociates in the formation of the equilibrium mono-liganded complex. To further probe the physical nature of the two subsites, the binding of a series of bis-quaternary suberyldicholine analogues, (CH3)3N+CH2CH2OCO(CH2)n-COOCH2CH2N+(CH3)3, to IAS-labeled receptor preparations has been examined. Analogues in which n < 5 behave like acetylcholine, i.e., a second ligand binding pathway is observed, but longer ligands (n = 5-10) act like suberyldicholine and may be long enough to cross-link the sites.

Agonist binding to the Torpedo acetylcholine receptor. 1. Complexities revealed by dissociation kinetics

Examination of the kinetics of dissociation of [3H]acetylcholine and [3H]suberyldicholine from the membrane-bound acetylcholine receptor from Torpedo californica has revealed complexities in the high-affinity binding of nicotinic agonists. Each agonist binds to two high-affinity sites per receptor with an equilibrium dissociation constant of approximately 15 nM. When dissociation of [3H]acetylcholine from the receptor complex was triggered by dilution, dissociation occurred as a monophasic process with an apparent rate of 0.023 +/- 0.010 s(-1). However, when micromolar concentrations of unlabeled agonists (acetylcholine, carbamylcholine or suberyldicholine) were included in the dilution buffer this rate increased about 5-fold. This accelerating effect occurred even when the two high-affinity sites were initially saturated with the radioligand. This suggested the presence of an additional site (or subsite) for agonist with affinity in the micromolar range. However, at concentrations of 0-20 microM, no additional sites for [3H]acetylcholine were detected at equilibrium. To explain these results, we propose that each high-affinity site is made up of two subsites, A and B, which are mutually exclusive at equilibrium. With [3H]acetylcholine initially occupying site A, occupancy of site B by unlabeled ligand reduces the affinity for site A and accelerates the dissociation of the radioligand. Studies of dissociation of [3H]suberyldicholine, a large bis-quaternary agonist, provide some clue as to the possible physical nature of these subsites. Whereas its dissociation rate was similar to that of [3H]acetylcholine (0.028 +/- 0.012 s(-1)), this rate was only marginally, if at all, affected by the presence of unlabeled ligands. These results, in addition to those presented in the accompanying manuscript, lead to the proposal that [3H]suberyldicholine is able to cross-link the two subsites or at least sterically occlude the second site.

L-type calcium channels: binding domains for dihydropyridines and benzothiazepines are located in close proximity to each other

We investigated the binding of a fluorescent diltiazem analogue (3R,4S)-cis-1-[2-[[3-[[3-[4,4-difluoro-3a,4-dihydro-5,7-dimethyl-4-bo ra-3a,4a-diaza-s-indacen-3-yl]propionyl]amino]propyl]amin o]ethy]-1,3,4,5-tetrahydro-3-hydroxy-4-(4-methoxyphenyl)-6-(triflu oromethyl)-2H-1-benzazepin-2-one (DMBODIPY-BAZ) to L-type Ca2+ channels in the presence of different 1,4-dihydropyridines (DHPs) by using fluorescence resonance energy transfer (FRET) [Brauns, T., Cai, Z.-W., Kimball, S. D., Kang, H.-C., Haugland, R. P., Berger, W., Berjukov, S., Hering, S., Glossmann, H., & Striessnig, J. (1995) Biochemistry 34, 3461]. When channels are occupied with DMBODIPY-BAZ, a rapid fluorescence change occurred upon addition of different DHPs. The direction of this intensity modulation was found to be only dependent on the chemical composition of the dihydropyridine employed. DHPs containing a nitro group decreased, whereas others (e.g., isradipine) enhanced the fluorescence signal. In addition, all DHPs markedly decreased the association rate constant for DMBODIPY-BAZ without affecting equilibrium binding. Both observations together are best explained by a steric model where the DHP binding site is located in close proximity to the accession pathway of DMBODIPY-BAZ.

Yeast sterol C8-C7 isomerase: identification and characterization of a high-affinity binding site for enzyme inhibitors

The yeast gene ERG2 encodes a sterol C8-C7 isomerase and is essential for ergosterol synthesis and cell proliferation. Its striking homology with the so-called sigma1 receptor of guinea pig brain, a polyvalent steroid and drug binding protein, suggested that the yeast sterol C8-C7 isomerase (ERG2) carries a similar high affinity drug binding domain. Indeed the sigma ligands [3H]haloperidol (Kd = 0.3 nM) and [3H]ifenprodil (Kd = 1.4 nM) bound to a single population of sites in ERG2 wild type yeast microsomes (Bmax values of 77 and 61 pmol/mg of protein, respectively), whereas binding activity was absent in strains carrying ERG2 gene mutations or disruptions. [3H]Ifenprodil binding was inhibited by sterol isomerase inhibitors such as fenpropimorph (Ki = 0.05 nM), tridemorph (Ki = 0.09 nM), MDL28,815 (Ki = 0.44 nM), triparanol (Ki = 1.5 nM), and AY-9944 (Ki = 5.8 nM). [3H]Haloperidol specifically photoaffinity-labeled a protein with an apparent molecular weight of 27400, in agreement with the molecular mass of the sterol C8-C7 isomerase (24900 Da). 9E10 c-myc antibodies specifically immunoprecipitated the c-myc tagged protein after [3H]haloperidol photolabeling, unequivocally proving that the drug binding site is localized on the ERG2 gene product. Haloperidol, trifluperidol, and ifenprodil inhibited the growth of Saccharomyces cerevisiae and reduced the ergosterol content of cells grown in their presence. Our results demonstrate that the yeast sterol C8-C7 isomerase has a polyvalent high-affinity drug binding site similar to mammalian sigma receptors and that in yeast sigma ligands inhibit sterol biosynthesis.

Reversible labeling of a chemosensitizer binding domain of p-glycoprotein with a novel 1,4-dihydropyridine drug transport inhibitor

A photoreactive dihydropyridine (DHP), BZDC-DHP (2,6-dimethyl-4-(2-(trifluoromethyl)-phenyl)-1,4-dihydropyridine-3,5- dicarboxylic acid (2-[3-(4-benzoylphenyl)propionylamino]ethyl) ester ethyl ester), and its tritiated derivative were synthesized as novel probes for human p-glycoprotein (p-gp). (-)-[3H]BZDC-DHP specifically photolabeled p-gp in membranes of multidrug-resistant CCRF-ADR5000 cells. In reversible labeling experiments a saturable, vinblastine-sensitive and high-affinity (Kd = 16.3 nM, Bmax = 58 pmol/mg of protein, k(+1) = 0.031 nM-1 min-1, k(-1) = 0.172 min-1) binding component was present in CCRF-ADR5000 membranes but absent in the sensitive parent cell line. Binding was inhibited by cytotoxics and known chemosensitizers with a p-gp characteristic pharmacological profile. For eight chemosensitizers tested, the potency for binding inhibition correlated (r > 0.94) with the potency for drug transport inhibition (measured using rhodamine 123 accumulation). The DHP niguldipine and a structurally related pyrimidine stereoselectively stimulated reversible (-)-[3H]BZDC-DHP binding, suggesting that more than one DHP molecule can bind to p-gp at the same time. Our data demonstrate that DHPs label multiple chemosensitizer domains on p-gp, distinct from the vinblastine interaction site. (-)-[3H]BZDC-DHP represents a valuable tool to characterize the molecular organization of chemosensitizer binding domains on p-gp by both reversible binding and photoinduced covalent modification. It provides a novel simple screening assay for p-gp active drugs.

Benzothiazepine binding domain of purified L-type calcium channels: direct labeling using a novel fluorescent diltiazem analogue

We have synthesized a series of N-propylamino-substituted benzazepinones (NPSBs) as specific probes for the benzothiazepinone (BTZ) binding domain of muscle L-type calcium channels (LTCCs). NPSBs were identified which possess high affinity for the channel after purification. We synthesized a fluorescent NPSB, DMBODIPY-BAZ, as the first benz(othi)azepinone derivative known to reversibly label partially purified LTCCs. DMBODIPY-BAZ binds to the partially purified channel with high affinity (Kd = 25 nM, Bmax = 580 pmol/mg of protein). Fluorescence resonance energy transfer (FRET) occurred between tryptophan residues of the channel protein and the DMBODIPY fluorophore upon specific drug binding. FRET was exploited to allow highly time-resolved detection of specific drug binding kinetics. We found that the dissociation half-life (t1/2) of DMBODIPY-BAZ decreased with the concentration of an unlabeled competitor, which indicates ligand-induced accelerated dissociation. In contrast, t1/2 was concentration-dependently increased by the dihydropyridine (DHP) (+)-isradipine. These kinetic properties of DMBODIPY-BAZ indicate that a high-affinity BTZ binding domain also exists on purified LTCCs. NPSBs represent novel tools to provide further insight into the molecular pharmacology of the BTZ binding domain on LTCCs.

Complex molecular mechanism for dihydropyridine binding to L-type Ca(2+)-channels as revealed by fluorescence resonance energy transfer

We analyzed binding-induced changes in the fluorescence properties of the 1,4-dihydropyridine (DHP), DMBODIPY-DHP [(-)-1,4-dihydro-2,6-dimethyl-4-(2-trifluromethylphenyl)- 3,5-pyridinedicarboxylic acid 2-[4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-3- (s-indacene)propionylamino]ethylethyl ester)], to study the molecular mechanisms underlying the interaction of DHPs with the alpha 1-subunit of skeletal muscle L-type Ca2+ channels. The quantum yield of the fluorophore DMBODIPY was similar in solvents of different polarity. In contrast, the quantum yield of DMBODIPY-DHP was low in buffer but increased with solvent polarity and upon specific binding. This indicates the existence of binding-induced changes of intramolecular quenching of the fluorophore by the DHP moiety. Specific ligand binding also induced fluorescence resonance energy transfer (FRET) between one or more tryptophanes of the channel protein and the DMBODIPY-DHP fluorophore. The specific FRET signal was successfully used to directly measure DHP binding at high time resolution. It revealed complex association and dissociation kinetics of DMBODIPY-DHP although no site heterogeneity was detected in equilibrium experiments. We therefore fitted our data to a binding scheme considering one or more intermediate conformational states for the formation of the ligand-receptor complex. Such a step-wise binding mechanism explains previously observed differences in the binding site densities and the kinetic constants determined for different DHPs using conventional binding (for example filtration) assays.