Identification of benz(othi)azepine-binding regions within L-type calcium channel alpha1 subunits

Voltage gated sodium and calcium channels: Discovery, structure, function, and Pharmacology

Voltage-gated sodium channels initiate action potentials in nerve and muscle, and voltage-gated calcium channels couple depolarization of the plasma membrane to intracellular events such as secretion, contraction, synaptic transmission, and gene expression. In this Review and Perspective article, I summarize early work that led to identification, purification, functional reconstitution, and determination of the amino acid sequence of the protein subunits of sodium and calcium channels and showed that their pore-forming subunits are closely related. Decades of study by antibody mapping, site-directed mutagenesis, and electrophysiological recording led to detailed two-dimensional structure-function maps of the amino acid residues involved in voltage-dependent activation and inactivation, ion permeation and selectivity, and pharmacological modulation. Most recently, high-resolution three-dimensional structure determination by X-ray crystallography and cryogenic electron microscopy has revealed the structural basis for sodium and calcium channel function and pharmacological modulation at the atomic level. These studies now define the chemical basis for electrical signaling and provide templates for future development of new therapeutic agents for a range of neurological and cardiovascular diseases.

T1143 essential for CaV1.2 inhibition by diltiazem

Careful analysis of previously published reports and some new insights into the structure activity studies revealed an important role of Threonine 1143 in drug binding. Substituting T1143 by alanine and other residues significantly reduced channel inhibition by qDil and Dil. Mutation T1143A did not affect channel activation or inactivation while almost completely diminishing channel block by Dil or qDil. These findings support the view that T1143 serves as drug binding determinant. Other mutations in this position than T1143A (T1143L/Y/S/N/C/V/E) diminished channel inhibition by qDil but additionally affected channel activation and inactivation and may therefore affect channel block allosterically. Collectively, our data suggest that T1143 is an essential diltiazem binding determinant.

Molecular Basis for Ligand Modulation of a Mammalian Voltage-Gated Ca2+ Channel

The L-type voltage-gated Ca²⁺ (Ca_v) channels are modulated by various compounds exemplified by 1,4-dihydropyridines (DHP), benzothiazepines (BTZ), and phenylalkylamines (PAA), many of which have been used for characterizing channel properties and for treatment of hypertension and other disorders. Here, we report the cryoelectron microscopy (cryo-EM) structures of Ca_v1.1 in complex with archetypal antagonistic drugs, nifedipine, diltiazem, and verapamil, at resolutions of 2.9 Å, 3.0 Å, and 2.7 Å, respectively, and with a DHP agonist Bay K 8644 at 2.8 Å. Diltiazem and verapamil traverse the central cavity of the pore domain, directly blocking ion permeation. Although nifedipine and Bay K 8644 occupy the same fenestration site at the interface of repeats III and IV, the coordination details support previous functional observations that Bay K 8644 is less favored in the inactivated state. These structures elucidate the modes of action of different Ca_v ligands and establish a framework for structure-guided drug discovery.

Structure and Pharmacology of Voltage-Gated Sodium and Calcium Channels

Voltage-gated sodium and calcium channels are evolutionarily related transmembrane signaling proteins that initiate action potentials, neurotransmission, excitation-contraction coupling, and other physiological processes. Genetic or acquired dysfunction of these proteins causes numerous diseases, termed channelopathies, and sodium and calcium channels are the molecular targets for several major classes of drugs. Recent advances in the structural biology of these proteins using X-ray crystallography and cryo-electron microscopy have given new insights into the molecular basis for their function and pharmacology. Here we review this recent literature and integrate findings on sodium and calcium channels to reveal the structural basis for their voltage-dependent activation, fast and slow inactivation, ion conductance and selectivity, and complex pharmacology at the atomic level. We conclude with the theme that new understanding of the diseases and therapeutics of these channels will be derived from application of the emerging structural principles from these recent structural analyses.

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.

Structural Basis for Pharmacology of Voltage-Gated Sodium and Calcium Channels

Voltage-gated sodium channels initiate action potentials in nerve, muscle, and other electrically excitable cells. Voltage-gated calcium channels are activated by depolarization during action potentials, and calcium influx through them is the key second messenger of electrical signaling, initiating secretion, contraction, neurotransmission, gene transcription, and many other intracellular processes. Drugs that block sodium channels are used in local anesthesia and the treatment of epilepsy, bipolar disorder, chronic pain, and cardiac arrhythmia. Drugs that block calcium channels are used in the treatment of epilepsy, chronic pain, and cardiovascular disorders, including hypertension, angina pectoris, and cardiac arrhythmia. The principal pore-forming subunits of voltage-gated sodium and calcium channels are structurally related and likely to have evolved from ancestral voltage-gated sodium channels that are widely expressed in prokaryotes. Determination of the structure of a bacterial ancestor of voltage-gated sodium and calcium channels at high resolution now provides a three-dimensional view of the binding sites for drugs acting on sodium and calcium channels. In this minireview, we outline the different classes of sodium and calcium channel drugs, review studies that have identified amino acid residues that are required for their binding and therapeutic actions, and illustrate how the analogs of those key amino acid residues may form drug-binding sites in three-dimensional models derived from bacterial channels.

Identification and analysis of cation channel homologues in human pathogenic fungi

Fungi are major causes of human, animal and plant disease. Human fungal infections can be fatal, but there are limited options for therapy, and resistance to commonly used anti-fungal drugs is widespread. The genomes of many fungi have recently been sequenced, allowing identification of proteins that may become targets for novel therapies. We examined the genomes of human fungal pathogens for genes encoding homologues of cation channels, which are prominent drug targets. Many of the fungal genomes examined contain genes encoding homologues of potassium (K⁺), calcium (Ca²⁺) and transient receptor potential (Trp) channels, but not sodium (Na⁺) channels or ligand-gated channels. Some fungal genomes contain multiple genes encoding homologues of K⁺ and Trp channel subunits, and genes encoding novel homologues of voltage-gated K_v channel subunits are found in Cryptococcus spp. Only a single gene encoding a homologue of a plasma membrane Ca²⁺ channel was identified in the genome of each pathogenic fungus examined. These homologues are similar to the Cch1 Ca²⁺ channel of Saccharomyces cerevisiae. The genomes of Aspergillus spp. and Cryptococcus spp., but not those of S. cerevisiae or the other pathogenic fungi examined, also encode homologues of the mitochondrial Ca²⁺ uniporter (MCU). In contrast to humans, which express many K⁺, Ca²⁺ and Trp channels, the genomes of pathogenic fungi encode only very small numbers of K⁺, Ca²⁺ and Trp channel homologues. Furthermore, the sequences of fungal K⁺, Ca²⁺, Trp and MCU channels differ from those of human channels in regions that suggest differences in regulation and susceptibility to drugs.

The antifungal activity of the Penicillium chrysogenum protein PAF disrupts calcium homeostasis in Neurospora crassa

The antifungal protein PAF from Penicillium chrysogenum exhibits growth-inhibitory activity against a broad range of filamentous fungi. Evidence from this study suggests that disruption of Ca(2+) signaling/homeostasis plays an important role in the mechanistic basis of PAF as a growth inhibitor. Supplementation of the growth medium with high Ca(2+) concentrations counteracted PAF toxicity toward PAF-sensitive molds. By using a transgenic Neurospora crassa strain expressing codon-optimized aequorin, PAF was found to cause a significant increase in the resting level of cytosolic free Ca(2+) ([Ca(2+)](c)). The Ca(2+) signatures in response to stimulation by mechanical perturbation or hypo-osmotic shock were significantly changed in the presence of PAF. BAPTA [bis-(aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid], a Ca(2+) selective chelator, ameliorated the PAF toxicity in growth inhibition assays and counteracted PAF induced perturbation of Ca(2+) homeostasis. These results indicate that extracellular Ca(2+) was the major source of these PAF-induced effects. The L-type Ca(2+) channel blocker diltiazem disrupted Ca(2+) homeostasis in a similar manner to PAF. Diltiazem in combination with PAF acted additively in enhancing growth inhibition and accentuating the change in Ca(2+) signatures in response to external stimuli. Notably, both PAF and diltiazem increased the [Ca(2+)](c) resting level. However, experiments with an aequorin-expressing Deltacch-1 deletion strain of N. crassa indicated that the L-type Ca(2+) channel CCH-1 was not responsible for the observed PAF-induced elevation of the [Ca(2+)](c) resting level. This study is the first demonstration of the perturbation of fungal Ca(2+) homeostasis by an antifungal protein from a filamentous ascomycete and provides important new insights into the mode of action of PAF.

Inhibition of L-type Ca2+ channel by mitochondrial Na+-Ca2+ exchange inhibitor CGP-37157 in rat atrial myocytes

7-chloro-5-(2-chlorophenyl)-1,5-dihydro-4,1-benzothiazepine-2(3H)-one (CGP-37157) inhibits mitochondrial Na(+)-Ca(2+) exchange. It is often used as an experimental tool for studying the role of the mitochondrial Na(+)-Ca(2+) exchanger in Ca(2+) signaling. Because the selectivity of CGP-37157 in adult cardiomyocytes has not been confirmed, we tested whether CGP-37157 affects the L-type Ca(2+) channel using a whole-cell patch-clamp in adult rat atrial myocytes. We found that CGP-37157 suppressed L-type Ca(2+) current (I(Ca)) with IC(50) of approximately 0.27 microM, without altering the voltage dependence of the current-voltage relationships. CGP-37157 inhibited the Ba(2+) current (I(Ba)) through the Ca(2+) channel with a similar dose-response. The inhibitory effects of CGP-37157 on I(Ca) or I(Ba) were resistant to the intracellular Ca(2+) buffering. Intracellular application of CGP-37157 did not significantly alter I(Ca). The combination of CGP-37157 with known Ca(2+) channel inhibitor diltiazem yielded antagonism consistent with additivity of response. Our data suggest that CGP-37157 directly suppresses the L-type Ca(2+) channel in intact adult cardiomyocytes.

Ca2+ currents in cardiac myocytes: Old story, new insights

Calcium is a ubiquitous second messenger which plays key roles in numerous physiological functions. In cardiac myocytes, Ca2+ crosses the plasma membrane via specialized voltage-gated Ca2+ channels which have two main functions: (i) carrying depolarizing current by allowing positively charged Ca2+ ions to move into the cell; (ii) triggering Ca2+ release from the sarcoplasmic reticulum. Recently, it has been suggested than Ca2+ channels also participate in excitation-transcription coupling. The purpose of this review is to discuss the physiological roles of Ca2+ currents in cardiac myocytes. Next, we describe local regulation of Ca2+ channels by cyclic nucleotides. We also provide an overview of recent studies investigating the structure-function relationship of Ca2+ channels in cardiac myocytes using heterologous system expression and transgenic mice, with descriptions of the recently discovered Ca2+ channels alpha(1D) and alpha(1E). We finally discuss the potential involvement of Ca2+ currents in cardiac pathologies, such as diseases with autoimmune components, and cardiac remodeling.

Molecular pharmacology of high voltage-activated calcium channels

Voltage-gated calcium channels are key sources of calcium entry into the cytosol of many excitable tissues. A number of different types of calcium channels have been identified and shown to mediate specialized cellular functions. Because of their fundamental nature, they are important targets for therapeutic intervention in disorders such as hypertension, pain, stroke, and epilepsy. Calcium channel antagonists fall into one of the following three groups: small inorganic ions, large peptide blockers, and small organic molecules. Inorganic ions nonselectively inhibit calcium entry by physical pore occlusion and are of little therapeutic value. Calcium-channel-blocking peptides isolated from various predatory animals such as spiders and cone snails are often highly selective blockers of individual types of calcium channels, either by preventing calcium flux through the pore or by antagonizing channel activation. There are many structure-activity-relation classes of small organic molecules that interact with various sites on the calcium channel protein, with actions ranging from selective high affinity block to relatively nondiscriminatory action on multiple calcium channel isoforms. Detailed interactions with the calcium channel protein are well understood for the dihydropyridine and phenylalkylamine drug classes, whereas we are only beginning to understand the molecular actions of some of the more recently discovered calcium channel blockers. Here, we provide a comprehensive review of pharmacology of high voltage-activated calcium channels.

Probing the pore of the auditory hair cell mechanotransducer channel in turtle

Hair cell mechano-electric transducer (MET) channels play a pivotal role in auditory and vestibular signal detection, yet few data exist regarding their molecular nature. Present work characterizes the MET channel pore, a region whose properties are thought to be intrinsically determined. Two approaches were used. First, the channel was probed with antagonists of candidate channel subtypes including: cyclic nucleotide-gated channels, transient receptor potential channels and gap-junctional channels. Eight new antagonists were identified. Most of the effective antagonists had a partially charged amine group predicted to penetrate the channel pore, antagonizing current flow, while the remainder of the molecule prevented further permeation of the compound through the pore. This blocking mechanism was tested using curare to demonstrate the open channel nature of the block and by identifying methylene blue as a permeant channel blocker. The second approach estimated dimensions of the channel pore with simple amine compounds. The narrowest diameter of the pore was calculated as 12.5 +/- 0.8 A and the location of a binding site approximately 45% of the way through the membrane electric field was calculated. Channel length was estimated as approximately 31 A and the width of the pore mouth at < 17 A. Each effective antagonist had a minimal diameter, measured about the penetrating amine, of less than the pore diameter, with a direct correlation between IC(50) and minimal diameter. The IC(50) was also directly related to the length of the amine side chains, further validating the proposed pore blocking mechanism. Data provided by these two approaches support a hypothesis regarding channel permeation and block that incorporates molecular dimensions and ion interactions within the pore.

Molecular determinants of diltiazem block in domains IIIS6 and IVS6 of L-type Ca(2+) channels

The benzothiazepine diltiazem blocks ionic current through L-type Ca(2+) channels, as do the dihydropyridines (DHPs) and phenylalkylamines (PAs), but it has unique properties that distinguish it from these other drug classes. Wild-type L-type channels containing alpha(1CII) subunits, wild-type P/Q-type channels containing alpha(1A) subunits, and mutants of both channel types were transiently expressed in tsA-201 cells with beta(1B) and alpha(2)delta subunits. Whole-cell, voltage-clamp recordings showed that diltiazem blocks L-type Ca(2+) channels approximately 5-fold more potently than it does P/Q-type channels. Diltiazem blocked a mutant P/Q-type channel containing nine amino acid changes that made it highly sensitive to DHPs, with the same potency as L-type channels. Thus, amino acids specific to the L-type channel that confer DHP sensitivity in an alpha(1A) background also increase sensitivity to diltiazem. Analysis of single amino acid mutations in domains IIIS6 and IVS6 of alpha(1CII) subunits confirmed the role of these L-type-specific amino acid residues in diltiazem block, and also indicated that Y1152 of alpha(1CII), an amino acid critical to both DHP and PA block, does not play a role in diltiazem block. Furthermore, T1039 and Y1043 in domain IIIS5, which are both critical for DHP block, are not involved in block by diltiazem. Conversely, three amino acid residues (I1150, M1160, and I1460) contribute to diltiazem block but have not been shown to affect DHP or PA block. Thus, binding of diltiazem to L-type Ca(2+) channels requires residues that overlap those that are critical for DHP and PA block as well as residues unique to diltiazem.

A region in IVS5 of the human cardiac L-type calcium channel is required for the use-dependent block by phenylalkylamines and benzothiazepines

Mutations in motif IVS5 and IVS6 of the human cardiac calcium channel were made using homologous residues from the rat brain sodium channel 2a. [3H]PN200-110 and allosteric binding assays revealed that the dihydropyridine and benzothiazepine receptor sites maintained normal coupling in the chimeric mutant channels. Whole cell voltage clamp recording from Xenopus oocytes showed a dramatically slowed inactivation and a complete loss of use-dependent block for mutations in the cytoplasmic connecting link to IVS5 (HHT-5371) and in IVS5 transmembrane segment (HHT-5411) with both diltiazem and verapamil. However, the use-dependent block by isradipine was retained by these two mutants. For mutants HHT-5411 and HHT-5371, the residual current appeared associated with a loss of voltage dependence in the rate of inactivation indicating a destabilization of the inactivated state. Furthermore, both HHT-5371 and -5411 recovered from inactivation significantly faster after drug block than that of the wild type channel. Our data demonstrate that accelerated recovery of HHT-5371 and HHT-5411 decreased accumulation of these channels in inactivation during pulse trains and suggest a close link between inactivation gating of the channel and use-dependent block by phenylalkylamines and benzothiazepines and provide evidence of a role for the transmembrane and cytoplasmic regions of IVS5 in the use-dependent block by diltiazem and verapamil.

Sequence differences between alpha1C and alpha1S Ca2+ channel subunits reveal structural determinants of a guarded and modulated benzothiazepine receptor

The molecular basis of the Ca2+ channel block by (+)-cis-diltiazem was studied in class A/L-type chimeras and mutant alpha1C-a Ca2+ channels. Chimeras consisted of either rabbit heart (alpha1C-a) or carp skeletal muscle (alpha1S) sequence in transmembrane segments IIIS6, IVS6, and adjacent S5-S6 linkers. Only chimeras containing sequences from alpha1C-a were efficiently blocked by (+)-cis-diltiazem, whereas the phenylalkylamine (-)-gallopamil efficiently blocked both constructs. Carp skeletal muscle and rabbit heart Ca2+ channel alpha1 subunits differ with respect to two nonconserved amino acids in segments IVS6. Transfer of a single leucine (Leu1383, located at the extracellular mouth of the pore) from IVS6 alpha1C-a to IVS6 of alpha1S significantly increased the (+)-cis-diltiazem sensitivity of the corresponding mutant L1383I. An analysis of the role of the two heterologous amino acids in a L-type alpha1 subunit revealed that corresponding amino acids in position 1487 (outer channel mouth) determine recovery of resting Ca2+ channels from block by (+)-cis-diltiazem. The second heterologous amino acid in position 1504 of segment IVS6 (inner channel mouth) was identified as crucial inactivation determinant of L-type Ca2+ channels. This residue simultaneously modulates drug binding during membrane depolarization. Our study provides the first evidence for a guarded and modulated benzothiazepine receptor on L-type channels.

Molecular mechanism of diltiazem interaction with L-type Ca2+ channels

Benzothiazepine Ca2+ antagonists (such as (+)-cis-diltiazem) interact with transmembrane segments IIIS6 and IVS6 in the alpha1 subunit of L-type Ca2+ channels. We investigated the contribution of individual IIIS6 amino acid residues for diltiazem sensitivity by employing alanine scanning mutagenesis in a benzothiazepine-sensitive alpha1 subunit chimera (ALDIL) expressed in Xenopus laevis oocytes. The most dramatic decrease of block by 100 microM diltiazem (ALDIL 45 +/- 4.8% inhibition) during trains of 100-ms pulses (0.1 Hz, -80 mV holding potential) was found after mutation of adjacent IIIS6 residues Phe1164(21 +/- 3%) and Val1165 (8.5 +/- 1.4%). Diltiazem delayed current recovery by promoting a slowly recovering current component. This effect was similar in ALDIL and F1164A but largely prevented in V1165A. Both mutations slowed inactivation kinetics during a pulse. The reduced diltiazem block can therefore be explained by slowing of inactivation kinetics (F1164A and V1165A) and accelerated recovery from drug block (V1165A). The bulkier diltiazem derivative benziazem still efficiently blocked V1165A. From these functional and from additional radioligand binding studies with the dihydropyridine (+)-[3H]isradipine we propose a model in which Val1165 controls dissociation of the bound diltiazem molecule, and where bulky substituents on the basic nitrogen of diltiazem protrude toward the adjacent dihydropyridine binding domain.

Molecular basis of drug interaction with L-type Ca2+ channels

Different types of voltage-gated Ca2+ channels exist in the plasma membrane of electrically excitable cells. By controlling depolarization-induced Ca2+ entry into cells they serve important physiological functions, such as excitation-contraction coupling, neurotransmitter and hormone secretion, and neuronal plasticity. Their function is fine-tuned by a variety of modulators, such as enzymes and G-proteins. Block of so-called L-type Ca2+ channels by drugs is exploited as a therapeutic principle to treat cardiovascular disorders, such as hypertension. More recently, block of so-called non-L-type Ca2+ channels was found to exert therapeutic effects in the treatment of severe pain and ischemic stroke. As the subunits of different Ca2+ channel types have been cloned, the modulatory sites for enzymes, G-proteins, and drugs can now be determined using molecular engineering and heterologous expression. Here we summarize recent work that has allowed us to determine the sites of action of L-type Ca2+ channel modulators. Together with previous biochemical, electrophysiological, and drug binding data these results provide exciting insight into the molecular pharmacology of this voltage-gated Ca2+ channel family.

Structural models of the transmembrane region of voltage-gated and other K+ channels in open, closed, and inactivated conformations

A large collaborative, multidisciplinary effort involving many research laboratories continues which uses indirect methods of molecular biology and membrane biophysics to analyze the three-dimensional structures and functional mechanisms of K+ channels. This work also extends to the distant relatives of these channels, including the voltage-gated Na+ and Ca2+ channels. The role that our group plays in this process is to combine the information gained from experimental studies with molecular modeling techniques to generate atomic-scale structural models of these proteins. The modeling process involves three stages which are summarized as: (I) prediction of the channel sequence transmembrane topology, including the functionality and secondary structure of the segments; (II) prediction of the relative positions of the transmembrane segments, and (III) filling in all atoms of the amino acid residues, with conformations for energetically stabilized interactions. Both physiochemical and evolutionary principles (including sequence homology analysis) are used to guide the development. In addition to testing the steric and energetic feasibilities of different structural hypotheses, the models provide guidance for the design of new experiments. Structural modeling also serves to "fill in the gaps" of experimental data, such as predicting additional residue interactions and conformational changes responsible for functional processes. The modeling process is currently at the stage that experimental studies have definitely confirmed most of our earlier predictions about the transmembrane topology and functionality of different segments. Additionally, this report describes the detailed, three-dimensional models we have developed for the entire transmembrane region and important functional sites of the voltage-gated Shaker K+ channel in the open, closed, and inactivated conformations (including the ion-selective pore and voltage-sensor regions). As part of this effort, we also describe how our development of structural models for many of the other major K+ channel families aids in determining common structural motifs. As an example, we also present a detailed model of the smaller, bacterial K+ channel from Streptomyces lividans. Finally, we discuss strategies for using newly developed experimental methods for determining the structures and analyzing the functions of these channel proteins.

Photochemical identification of transmembrane segment IVS6 as the binding region of semotiadil, a new modulator for the L-type voltage-dependent Ca2+ channel

To identify the binding domain of a new Ca2+ antagonist semotiadil on L-type Ca2+ channels from skeletal muscle, photolabeling was carried out by using an azidophenyl derivative of [3H]semotiadil. Photoincorporation was observed in several polypeptides of membrane triad preparations; the only specific photoincorporation was in the alpha1 subunit of the Ca2+ channel. After solubilization and purification, the photolabeled alpha1 subunit was subjected to proteolytic and CNBr cleavage followed by antibody mapping. Specific labeling was associated solely with the region of transmembrane segment S6 in repeat IV. Quantitative immunoprecipitation was found in the tryptic and the Lys-C/Glu-C fragments of 6.6 and 6.1 kDa, respectively. Further CNBr cleavage of the Lys-C digests produced two smaller fragments of 3.4 and 1.8 kDa that were included in the tryptic and Lys-C/Glu-C fragments. The smallest labeled fragments were: Tyr1350-Met1366 and Leu1367-Met1381 containing IVS6, a possible pore-forming region. The data suggest that semotiadil binds to a region that is overlapped with but not identical to those for phenylalkylamines, dihydropyridines and benzothiazepines. The present study also provides evidence that region IV represents an important component of a binding pocket for Ca2+ antagonists.

Nine L-type amino acid residues confer full 1,4-dihydropyridine sensitivity to the neuronal calcium channel alpha1A subunit. Role of L-type Met1188

Pharmacological modulation by 1,4-dihydropyridines is a central feature of L-type calcium channels. Recently, eight L-type amino acid residues in transmembrane segments IIIS5, IIIS6, and IVS6 of the calcium channel alpha1 subunit were identified to substantially contribute to 1,4-dihydropyridine sensitivity. To determine whether these eight L-type residues (Thr1066, Gln1070, Ile1180, Ile1183, Tyr1490, Met1491, Ile1497, and Ile1498; alpha1C-a numbering) are sufficient to form a high affinity 1,4-dihydropyridine binding site in a non-L-type calcium channel, we transferred them to the 1, 4-dihydropyridine-insensitive alpha1A subunit using site-directed mutagenesis. 1,4-Dihydropyridine agonist and antagonist modulation of barium inward currents mediated by the mutant alpha1A subunits, coexpressed with alpha2delta and beta1a subunits in Xenopus laevis oocytes, was investigated with the two-microelectrode voltage clamp technique. The resulting mutant alpha1A-DHPi displayed low sensitivity for 1,4-dihydropyridines. Analysis of the 1,4-dihydropyridine binding region of an ancestral L-type alpha1 subunit previously cloned from Musca domestica body wall muscle led to the identification of Met1188 (alpha1C-a numbering) as an additional critical constituent of the L-type 1,4-dihydropyridine binding domain. The introduction of this residue into alpha1A-DHPi restored full sensitivity for 1,4-dihydropyridines. It also transferred functional properties considered hallmarks of 1, 4-dihydropyridine agonist and antagonist effects (i.e. stereoselectivity, voltage dependence of drug modulation, and agonist-induced shift in the voltage-dependence of activation). Our gain-of-function mutants provide an excellent model for future studies of the structure-activity relationship of 1, 4-dihydropyridines to obtain critical structural information for the development of drugs for neuronal, non-L-type calcium channels.

Molecular studies on the voltage dependence of dihydropyridine action on L-type Ca2+ channels. Critical involvement of tyrosine residues in motif IIIS6 and IVS6

The interaction site(s) of dihydropyridine (DHP) antagonists and agonists have been identified by site-directed mutagenesis and localized on motifs IIIS5, IIIS6, and IVS6 of L-type voltage-gated calcium channels. In this study, we investigated the voltage-dependent action of DHPs with mutants of the IIIS6 and IVS6 segments of a cardiac calcium channel. Tyrosine residues in both motifs (Tyr1178 and Tyr1489) strongly contributed to the action of DHP agonists and antagonists. When these two sites were mutated, the communication between the voltage sensor and the DHP interaction site(s) was substantially impaired. In contrast, mutants of a nearby Ile (Ile1182) had much less influence on DHP agonist and antagonist interaction, and the voltage dependence of DHP antagonists was very similar to that of the wild type. The effect of a mutating of Ile1182, on agonist or antagonist action, however, depended strongly on the type of amino acid change. When Ile1182 was substituted with alanine, small changes were noted for DHP agonist and antagonist action. Changing this site into phenylalanine, however, significantly decreased the action of the DHP antagonist. These data show that Ile1182 can preferentially interact with DHP antagonists, but has a lesser contribution in agonist interaction. Thus, even though the agonist and antagonist interaction sites for DHPs with L-type calcium channels may overlap, some amino acids in this site may exhibit a preference for either DHP enantiomers.

Molecular determinants of high affinity phenylalkylamine block of L-type calcium channels in transmembrane segment IIIS6 and the pore region of the alpha1 subunit

Recent studies of the phenylalkylamine binding site in the alpha1C subunit of L-type Ca2+ channels have revealed three amino acid residues in transmembrane segment IVS6 that are critical for high affinity block and are unique to L-type channels. We have extended this analysis of the phenylalkylamine binding site to amino acid residues in transmembrane segment IIIS6 and the pore region. Twenty-two consecutive amino acid residues in segment IIIS6 were mutated to alanine and the conserved Glu residues in the pore region of each homologous domain were mutated to Gln. Mutant channels were expressed in tsA-201 cells along with the beta1b and alpha2delta auxiliary subunits. Assay for block of Ba2+ current by (-)-D888 at -60 mV revealed that mutation of five amino acid residues in segment IIIS6 and the pore region that are conserved between L-type and non-L-type channels (Tyr1152, Phe1164, Val1165, Glu1118, and Glu1419) and one L-type-specific amino acid (Ile1153) decreased affinity for (-)-D888 from 10-20-fold. Combination of the four mutations in segment IIIS6 increased the IC50 for block by (-)-D888 to approximately 9 microM, similar to the affinity of non-L-type Ca2+ channels for this drug. These results indicate that there are important determinants of phenylalkylamine binding in both the S6 segments and the pore regions of domains III and IV, some of which are conserved across the different classes of voltage-gated Ca2+ channels. A model of the phenylalkylamine receptor site at the interface between domains III and IV of the alpha1 subunit is presented.

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.

1,5-benzothiazepine binding domain is located on the extracellular side of the cardiac L-type Ca2+ channel

To determine whether 1,5-benzothiazepine Ca2+ channel blocker approaches its binding domain within the cardiac L-type Ca2+ channel from inside or outside of the membrane, we tested the effects of a novel potent 1,5-benzothiazepine derivative (DTZ323) and its quaternary ammonium derivative (DTZ417) on guinea pig ventricular myocytes by using the whole-cell patch-clamp technique. The extracellular application of DTZ417 suppressed the L-type Ca2+ channel currents (I[Ca(L)]) with an IC50 value of 1.2 +/- 0.02 microM, which was close to the IC50 value of diltiazem (0.63 +/- 0.01 microM). The suppression of I[Ca(L)] by DTZ417 was voltage and use dependent but lacked tonic block, which allowed us to investigate the onset of the effect on I[Ca(L)] by changing the holding potential (HP) from -90 to -50 mV in the presence of DTZ417. DTZ417 did not have significant effects on I[Ca(L)] at an HP of -90 mV. At -50 mV, DTZ417 (50 microM) applied from the extracellular side completely suppressed I[Ca(L)], whereas it had no effect from the intracellular side. DTZ323 (1 microM) also inhibited I[Ca(L)] only from the extracellular side, without any effects by the intracellular application of < or = 10 microM. However, a quaternary phenylalkylamine derivative, D890 (0.1 mM), acted only from the intracellular side. These results suggest that in contrast to the phenylalkylamine binding site, in cardiac myocytes the 1,5-benzothiazepine binding site is accessible from the extracellular side of the L-type Ca2+ channel.