Ca2+ channel selectivity at a single locus for high-affinity Ca2+ interactions

Structural biology of voltage-gated calcium channels

Voltage-gated calcium (Cav) channels mediate Ca2+ influx in response to membrane depolarization, playing critical roles in diverse physiological processes. Dysfunction or aberrant regulation of Cav channels can lead to life-threatening consequences. Cav-targeting drugs have been clinically used to treat cardiovascular and neuronal disorders for several decades. This review aims to provide an account of recent developments in the structural dissection of Cav channels. High-resolution structures have significantly advanced our understanding of the working and disease mechanisms of Cav channels, shed light on the molecular basis for their modulation, and elucidated the modes of actions (MOAs) of representative drugs and toxins. The progress in structural studies of Cav channels lays the foundation for future drug discovery efforts targeting Cav channelopathies.

Voltage-gated Calcium Channels as Potential Therapeutic Targets in Migraine

Migraine is a complex and highly incapacitating neurological disorder that affects around 15% of the general population with greater incidence in women, often at the most productive age of life. Migraine physiopathology is still not fully understood, but it involves multiple mediators and events in the trigeminovascular system and the central nervous system. The identification of calcitonin gene-related peptide as a key mediator in migraine physiopathology has led to the development of effective and highly selective antimigraine therapies. However, this treatment is neither accessible nor effective for all migraine sufferers. Thus, a better understanding of migraine mechanisms and the identification of potential targets are still clearly warranted. Voltage-gated calcium channels (VGCCs) are widely distributed in the trigeminovascular system, and there is accumulating evidence of their contribution to the mechanisms associated with headache pain. Several drugs used in migraine abortive or prophylactic treatment target VGCCs, which probably contributes to their analgesic effect. This review aims to summarize the current evidence of VGGC contribution to migraine physiopathology and to discuss how current pharmacological options for migraine treatment interfere with VGGC function. PERSPECTIVE: Calcitonin gene-related peptide (CGRP) represents a major migraine mediator, but few studies have investigated the relationship between CGRP and VGCCs. CGRP release is calcium channel-dependent and VGGCs are key players in familial migraine. Further studies are needed to determine whether VGCCs are suitable molecular targets for treating migraine.

Probing ion binding in the selectivity filter of the Cav1.1 channel with molecular dynamics

Cav1.1 is the voltage-gated calcium channel essential for the contraction of skeletal muscles upon membrane potential changes. Structural determination of the Cav1.1 channel opens the avenue toward understanding of the structure-function relationship of voltage-gated calcium channels. Here, we show that there exist two Ca2+-binding sites, termed S1 and S2, within the selectivity filter of Cav1.1 through extensive molecular dynamics simulations on various initial ion arrangement configurations. The formation of both binding sites is associated with the four Glu residues (Glu292/614/1014/1323) that constitute the so-called EEEE locus. At the S1 site near the extracellular side, the Ca2+ ion is coordinated with the negatively charged carboxylic groups of these Glu residues and of the Asp615 residue either in a direct way or via an intermediate water molecule. At the S2 site, Ca2+ binding shows two distinct states: an upper state involving two out of the four Glu residues in the EEEE locus and a lower state involving only one Glu residue. In addition, there exist two recruitment sites for Ca2+ above the entrance of the filter. These findings promote the understanding of mechanism for ion permeation and selectivity in calcium channels.

Nanosecond Pulsed Electric Field (nsPEF): Opening the Biotechnological Pandora’s Box

Nanosecond Pulsed Electric Field (nsPEF) is an electrostimulation technique first developed in 1995; nsPEF requires the delivery of a series of pulses of high electric fields in the order of nanoseconds into biological tissues or cells. They primary effects in cells is the formation of membrane nanopores and the activation of ionic channels, leading to an incremental increase in cytoplasmic Ca2+ concentration, which triggers a signaling cascade producing a variety of effects: from apoptosis up to cell differentiation and proliferation. Further, nsPEF may affect organelles, making nsPEF a unique tool to manipulate and study cells. This technique is exploited in a broad spectrum of applications, such as: sterilization in the food industry, seed germination, anti-parasitic effects, wound healing, increased immune response, activation of neurons and myocites, cell proliferation, cellular phenotype manipulation, modulation of gene expression, and as a novel cancer treatment. This review thoroughly explores both nsPEF’s history and applications, with emphasis on the cellular effects from a biophysics perspective, highlighting the role of ionic channels as a mechanistic driver of the increase in cytoplasmic Ca2+ concentration.

Theoretical investigation on the effect of terahertz wave on Ca2+ transport in the calcium channel

The question of whether terahertz (THz) waves can interact with ions in channels of nerve cells and cause a further reaction has attracted much attention. To answer this question, we investigate the spontaneous radiation generated by Ca²⁺ moving in calcium channels and the effect of THz radiation on the transport of Ca²⁺ by solving the mathematical physical model through Brownian dynamics (BD) simulations. It is obtained that the moving Ca²⁺ in a calcium channel can generate electromagnetic radiation, the corresponding spectrum of which is concentrated in the THz range. Meanwhile, both the ion number in the channel and the background temperature are proved to have significant effects on the spontaneous emission spectra. The studies also show that external THz radiation can accelerate Ca²⁺ transport through the ion channel. These results are expected to provide a theoretical basis for the future treatment of THz waves in the neurological field.

Animal Venom Peptides Cause Antinociceptive Effects by Voltage-gated Calcium Channels Activity Blockage

Pain is a complex phenomenon that is usually unpleasant and aversive. It can range widely in intensity, quality, and duration and has diverse pathophysiologic mechanisms and meanings. Voltage-gated sodium and calcium channels are essential to transmitting painful stimuli from the periphery until the dorsal horn of the spinal cord. Thus, blocking voltage-gated calcium channels (VGCCs) can effectively control pain refractory to treatments currently used in the clinic, such as cancer and neuropathic pain. VGCCs blockers isolated of cobra Naja naja kaouthia (α-cobratoxin), spider Agelenopsis aperta (ω-Agatoxin IVA), spider Phoneutria nigriventer (PhTx3.3, PhTx3.4, PhTx3.5, PhTx3.6), spider Hysterocrates gigas (SNX-482), cone snails Conus geographus (GVIA), Conus magus (MVIIA or ziconotide), Conus catus (CVID, CVIE and CVIF), Conus striatus (SO- 3), Conus fulmen (FVIA), Conus moncuri (MoVIA and MoVIB), Conus regularis (RsXXIVA), Conus eburneus (Eu1.6), Conus victoriae (Vc1.1.), Conus regius (RgIA), and spider Ornithoctonus huwena (huwentoxin-I and huwentoxin-XVI) venoms caused antinociceptive effects in different acute and chronic pain models. Currently, ziconotide is the only clinical used N-type VGCCs blocker peptide for chronic intractable pain. However, ziconotide causes different adverse effects, and the intrathecal route of administration also impairs its use in a more significant number of patients. In this sense, peptides isolated from animal venoms or their synthetic forms that act by modulating or blocking VGCCs channels seem to be a relevant prototype for developing new analgesics efficacious and well tolerated by patients.

Pore mutation N617D in the skeletal muscle DHPR blocks Ca2+ influx due to atypical high-affinity Ca2+ binding

Skeletal muscle excitation-contraction (EC) coupling roots in Ca²⁺-influx-independent inter-channel signaling between the sarcolemmal dihydropyridine receptor (DHPR) and the ryanodine receptor (RyR1) in the sarcoplasmic reticulum. Although DHPR Ca²⁺ influx is irrelevant for EC coupling, its putative role in other muscle-physiological and developmental pathways was recently examined using two distinct genetically engineered mouse models carrying Ca²⁺ non-conducting DHPRs: DHPR(N617D) (Dayal et al., 2017) and DHPR(E1014K) (Lee et al., 2015). Surprisingly, despite complete block of DHPR Ca²⁺-conductance, histological, biochemical, and physiological results obtained from these two models were contradictory. Here, we characterize the permeability and selectivity properties and henceforth the mechanism of Ca²⁺ non-conductance of DHPR(N617). Our results reveal that only mutant DHPR(N617D) with atypical high-affinity Ca²⁺ pore-binding is tight for physiologically relevant monovalent cations like Na⁺ and K⁺. Consequently, we propose a molecular model of cooperativity between two ion selectivity rings formed by negatively charged residues in the DHPR pore region.

Cav2.3 channel function and Zn2+-induced modulation: potential mechanisms and (patho)physiological relevance

Voltage-gated calcium channels (VGCCs) are critical for Ca²⁺ influx into all types of excitable cells, but their exact function is still poorly understood. Recent reconstruction of homology models for all human VGCCs at atomic resolution provides the opportunity for a structure-based discussion of VGCC function and novel insights into the mechanisms underlying Ca²⁺ selective flux through these channels. In the present review, we use these data as a basis to examine the structure, function, and Zn²⁺-induced modulation of Ca_v2.3 VGCCs, which mediate native R-type currents and belong to the most enigmatic members of the family. Their unique sensitivity to Zn²⁺ and the existence of multiple mechanisms of Zn²⁺ action strongly argue for a role of these channels in the modulatory action of endogenous loosely bound Zn²⁺, pools of which have been detected in a number of neuronal, endocrine, and reproductive tissues. Following a description of the different mechanisms by which Zn²⁺ has been shown or is thought to alter the function of these channels, we discuss their potential (patho)physiological relevance, taking into account what is known about the magnitude and function of extracellular Zn²⁺ signals in different tissues. While still far from complete, the picture that emerges is one where Ca_v2.3 channel expression parallels the occurrence of loosely bound Zn²⁺ pools in different tissues and where these channels may serve to translate physiological Zn²⁺ signals into changes of electrical activity and/or intracellular Ca²⁺ levels.

Functional impact of a congenital stationary night blindness type 2 mutation depends on subunit composition of Cav1.4 Ca2+ channels

Voltage-gated Ca_v1 and Ca_v2 Ca²⁺ channels are comprised of a pore-forming α₁ subunit (Ca_v1.1-1.4, Ca_v2.1-2.3) and auxiliary β (β_1-4) and α₂δ (α₂δ-1-4) subunits. The properties of these channels vary with distinct combinations of Ca_v subunits and alternative splicing of the encoding transcripts. Therefore, the impact of disease-causing mutations affecting these channels may depend on the identities of Ca_v subunits and splice variants. Here, we analyzed the effects of a congenital stationary night blindness type 2 (CSNB2)-causing mutation, I745T (IT), in Ca_v1.4 channels typical of those in human retina: Ca_v1.4 splice variants with or without exon 47 (Ca_v1.4+ex47 and Ca_v1.4Δex47, respectively), and the auxiliary subunits, β_2X13 and α₂δ-4. We find that IT caused both Ca_v1.4 splice variants to activate at significantly more negative voltages and with slower deactivation kinetics than the corresponding WT channels. These effects of the IT mutation, along with unexpected alterations in ion selectivity, were generally larger in channels lacking exon 47. The weaker ion selectivity caused by IT led to hyperpolarizing shifts in the reversal potential and large outward currents that were evident in channels containing the auxiliary subunits β_2X13 and α₂δ-4 but not in those with β_2A and α₂δ-1. We conclude that the IT mutation stabilizes channel opening and alters ion selectivity of Ca_v1.4 in a manner that is strengthened by exclusion of exon 47 and inclusion of β_2X13 and α₂δ-4. Our results reveal complex actions of IT in modifying the properties of Ca_v1.4 channels, which may influence the pathological consequences of this mutation in retinal photoreceptors.

Disruption of the Key Ca2+ Binding Site in the Selectivity Filter of Neuronal Voltage-Gated Calcium Channels Inhibits Channel Trafficking

Voltage-gated calcium channels are exquisitely Ca²⁺ selective, conferred primarily by four conserved pore-loop glutamate residues contributing to the selectivity filter. There has been little previous work directly measuring whether the trafficking of calcium channels requires their ability to bind Ca²⁺ in the selectivity filter or to conduct Ca²⁺. Here, we examine trafficking of neuronal Ca_V2.1 and 2.2 channels with mutations in their selectivity filter and find reduced trafficking to the cell surface in cell lines. Furthermore, in hippocampal neurons, there is reduced trafficking to the somatic plasma membrane, into neurites, and to presynaptic terminals. However, the Ca_V2.2 selectivity filter mutants are still influenced by auxiliary α₂δ subunits and, albeit to a reduced extent, by β subunits, indicating the channels are not grossly misfolded. Our results indicate that Ca²⁺ binding in the pore of Ca_V2 channels may promote their correct trafficking, in combination with auxiliary subunits. Furthermore, physiological studies utilizing selectivity filter mutant Ca_V channels should be interpreted with caution.

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Selectivity filter mutations in Ca_V2 channels block inward Ba²⁺ currents

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Surprisingly, these mutations severely reduce trafficking of the Ca_V2 channels

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Pore mutant N-type channels show reduced expression in presynaptic terminals

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Pore mutant channels still require β and α₂δ and thus are not grossly misfolded

Ionic Coulomb blockade and the determinants of selectivity in the NaChBac bacterial sodium channel

Mutation-induced transformations of conductivity and selectivity in NaChBac bacterial channels are studied experimentally and interpreted within the framework of ionic Coulomb blockade (ICB), while also taking account of resonant quantised dehydration (QD) and site protonation. Site-directed mutagenesis and whole-cell patch-clamp experiments are used to investigate how the fixed charge Q_f at the selectivity filter (SF) affects both valence selectivity and same-charge selectivity. The new ICB/QD model predicts that increasing ∣Q_f∣ should lead to a shift in selectivity sequences toward larger ion sizes, in agreement with the present experiments and with earlier work. Comparison of the model with experimental data leads to the introduction of an effective charge Q_f^∗ at the SF, which was found to differ between Aspartate and Glutamate charged rings, and also to depend on position within the SF. It is suggested that protonation of the residues within the restricted space of the SF is important in significantly reducing the effective charge of the EEEE ring. Values of Q_f^∗ derived from experiments on divalent blockade agree well with expectations based on the ICB/QD model and have led to the first demonstration of ICB oscillations in Ca²⁺ conduction as a function of the fixed charge. Preliminary studies of the dependence of Ca²⁺ conduction on pH are qualitatively consistent with the predictions of the model.

Cryo-EM structures of apo and antagonist-bound human Cav3.1

Among the ten subtypes of mammalian voltage-gated calcium (Cav) channels, Cav3.1-Cav3.3 constitute the T-type, or the low-voltage-activated, subfamily, the abnormal activities of which are associated with epilepsy, psychiatric disorders and pain1-5. Here we report the cryo-electron microscopy structures of human Cav3.1 alone and in complex with a highly Cav3-selective blocker, Z9446,7, at resolutions of 3.3 Å and 3.1 Å, respectively. The arch-shaped Z944 molecule reclines in the central cavity of the pore domain, with the wide end inserting into the fenestration on the interface between repeats II and III, and the narrow end hanging above the intracellular gate like a plug. The structures provide the framework for comparative investigation of the distinct channel properties of different Cav subfamilies.

Structural basis for ion selectivity and engineering in channelrhodopsins

Channelrhodopsins have become an integral part of modern neuroscience approaches due to their ability to control neuronal activity in targeted cell populations. The recent determination of several channelrhodopsin X-ray structures now enables us to study their function with unprecedented molecular precision. We will discuss how these insights can guide the engineering of the ion conducting pathway to increase its selectivity for Cl^-, Ca²⁺, and K⁺ ions and improve the overall conductance. Engineering such channelrhodopsins would further increase their utility in neuroscience research and beyond by controlling a wider range of physiological events. To thoroughly address this issue, we compare channelrhodopsin structures with structural features of voltage and ligand-gated K⁺, Cl^- and Ca²⁺ channels and discuss how these could be implemented in channelrhodopsins.

How CaV1.2-bound verapamil blocks Ca2+ influx into cardiomyocyte: Atomic level views

The first clinically used antiarrhythmic, antianginal and anti-hypertensive phenylalkylamine, verapamil's cardiovascular activity is inextricably linked to its ability to antagonize Ca²⁺ overload via blocking Ca_V1.2, a cardiac L-type Ca²⁺ channel of undisputed physiological and pharmacological importance in cardiovascular disorders such as myocardial ischemia-reperfusion injury. From a structural point of view, however, the action mechanism of verapamil is still elusive. Therefore, incorporating previous findings for verapamil and Ca_V1.2, this review article puts forward two experimental data-derived and -supported 3D structure models for Ca_V1.2's α₁ subunit and its verapamil-bound form. Furthermore, this article suggests three biophysical mechanisms, namely competitive binding, steric hindrance and electrostatic repulsion, towards an atomic level understanding of how verapamil blocks the L-type Ca²⁺ current mediated by Ca_V1.2 in reality, which can be useful for the design and development of next-generation Ca²⁺ antagonists to provide safer and more effective treatment of cardiovascular diseases.

Different roles for aspartates and glutamates for cation permeation in bacterial sodium channels

A key driving force for ion channel selectivity is represented by the negative charge of the Selectivity Filter carried by aspartate (D) and glutamate (E) residues. However, the structural effects and specific properties of D and E residues have not been extensively studied. In order to investigate this issue we studied the mutants of NaChBac channel with all possible combinations of D and E in the charged rings in position 191 and 192. Electrophysiological measurements showed significant Ca²⁺ currents only when position 191 was occupied by E. Equilibrium Molecular Dynamics simulations revealed the existence of two binding sites, corresponding to the charged rings and another one, more internal, at the level of L190. The simulations showed that the ion in the innermost site can interact with the residue in position 191 only when this is glutamate. Based on the MD simulations, we suggest that a D in position 191 leads to a high affinity Ca²⁺ block site resulting from a significant drop in the free energy of binding for an ion moving between the binding sites; in contrast, the free energy change is more gradual when an E residue occupies position 191, resulting in Ca²⁺ permeability. This scenario is consistent with the model of ion channel selectivity through stepwise changes in binding affinity proposed by Dang and McCleskey. Our study also highlights the importance of the structure of the selectivity filter which should contribute to the development of more detailed physical models for ion channel selectivity.

A bilobal model of Ca2+-dependent inactivation to probe the physiology of L-type Ca2+ channels

L-type calcium channels undergo Ca²⁺-dependent inactivation (CDI) in order to precisely control the entry of Ca²⁺ into cells such as cardiomyocytes. Limpitikul et al. develop a bilobal model of CDI and use it to understand the pathogenesis of arrhythmias associated with mutations in CaM.

L-type calcium channels (LTCCs) are critical elements of normal cardiac function, playing a major role in orchestrating cardiac electrical activity and initiating downstream signaling processes. LTCCs thus use feedback mechanisms to precisely control calcium (Ca²⁺) entry into cells. Of these, Ca²⁺-dependent inactivation (CDI) is significant because it shapes cardiac action potential duration and is essential for normal cardiac rhythm. This important form of regulation is mediated by a resident Ca²⁺ sensor, calmodulin (CaM), which is comprised of two lobes that are each capable of responding to spatially distinct Ca²⁺ sources. Disruption of CaM-mediated CDI leads to severe forms of long-QT syndrome (LQTS) and life-threatening arrhythmias. Thus, a model capable of capturing the nuances of CaM-mediated CDI would facilitate increased understanding of cardiac (patho)physiology. However, one critical barrier to achieving a detailed kinetic model of CDI has been the lack of quantitative data characterizing CDI as a function of Ca²⁺. This data deficit stems from the experimental challenge of uncoupling the effect of channel gating on Ca²⁺ entry. To overcome this obstacle, we use photo-uncaging of Ca²⁺ to deliver a measurable Ca²⁺ input to CaM/LTCCs, while simultaneously recording CDI. Moreover, we use engineered CaMs with Ca²⁺ binding restricted to a single lobe, to isolate the kinetic response of each lobe. These high-resolution measurements enable us to build mathematical models for each lobe of CaM, which we use as building blocks for a full-scale bilobal model of CDI. Finally, we use this model to probe the pathogenesis of LQTS associated with mutations in CaM (calmodulinopathies). Each of these models accurately recapitulates the kinetics and steady-state properties of CDI in both physiological and pathological states, thus offering powerful new insights into the mechanistic alterations underlying cardiac arrhythmias.

Calcium channel gating

Tuned calcium entry through voltage-gated calcium channels is a key requirement for many cellular functions. This is ensured by channel gates which open during membrane depolarizations and seal the pore at rest. The gating process is determined by distinct sub-processes: movement of voltage-sensing domains (charged S4 segments) as well as opening and closure of S6 gates. Neutralization of S4 charges revealed that pore opening of CaV1.2 is triggered by a "gate releasing" movement of all four S4 segments with activation of IS4 (and IIIS4) being a rate-limiting stage. Segment IS4 additionally plays a crucial role in channel inactivation. Remarkably, S4 segments carrying only a single charged residue efficiently participate in gating. However, the complete set of S4 charges is required for stabilization of the open state. Voltage clamp fluorometry, the cryo-EM structure of a mammalian calcium channel, biophysical and pharmacological studies, and mathematical simulations have all contributed to a novel interpretation of the role of voltage sensors in channel opening, closure, and inactivation. We illustrate the role of the different methodologies in gating studies and discuss the key molecular events leading CaV channels to open and to close.

A pore-localizing CACNA1C-E1115K missense mutation, identified in a patient with idiopathic QT prolongation, bradycardia, and autism spectrum disorder, converts the L-type calcium channel into a hybrid nonselective monovalent cation channel

BACKGROUND

Gain-of-function variants in the CACNA1C-encoded L-type calcium channel (LTCC, Ca_v1.2) cause type 8 long QT syndrome (LQT8). The pore region contains highly conserved glutamic acid (E) residues that collectively form the LTCC's selectivity filter. Here, we identified and characterized a pore-localizing missense variant, E1115K, that yielded a novel perturbation in the LTCC.

OBJECTIVE

The purpose of this study was to determine whether CACNA1C-E1115K alters the LTCC's selectivity and is the substrate for the patient's LQTS.

METHODS

The proband was a 14-year-old male with idiopathic QT prolongation and bradycardia. Genetic testing revealed a missense variant, CACNA1C-E1115K. The whole-cell patch clamp technique was used to measure CACNA1C-WT and -E1115K currents when heterologously expressed in TSA201 cells.

RESULTS

The CACNA1C-E1115K channel exhibited no inward calcium current. Instead, robust cardiac transient outward potassium current (I_to)-like outward currents that were blocked significantly by nifedipine were measured when 2 mM/0.1 mM extracellular/intracellular CaCl2 or 4 mM/141 mM extracellular/intracellular KCl was applied. Furthermore, when 140 mM extracellular NaCl was applied, the CACNA1C-E1115K channel revealed both robust inward persistent Na⁺ currents with slower inactivation and outward currents, which were also nifedipine sensitive. In contrast, CACNA1C-WT revealed only a small inward persistent Na⁺ current without a robust outward current.

CONCLUSION

This CACNA1C-E1115K variant destroyed the LTCC's calcium selectivity and instead converted the mutant channel into a channel with a marked increase in sodium-mediated inward currents and potassium-mediated outward currents. Despite the anticipated 50% reduction in LTCC, the creation of a new population of channels with accentuated inward and outward currents represents the likely pathogenic substrates for the patient's LQTS and arrhythmia phenotype.

β-Subunit of the voltage-gated Ca2+ channel Cav1.2 drives signaling to the nucleus via H-Ras

Depolarization-induced signaling to the nucleus by the L-type voltage-gated calcium channel Cav1.2 is widely assumed to proceed by elevating intracellular calcium. The apparent lack of quantitative correlation between Ca²⁺ influx and gene activation suggests an alternative activation pathway. Here, we demonstrate that membrane depolarization of HEK293 cells transfected with α₁1.2/β2b/α2δ subunits (Cav1.2) triggers c-Fos and MeCP2 activation via the Ras/ERK/CREB pathway. Nuclear signaling is lost either by absence of the intracellular β2 subunit or by transfecting the cells with the channel mutant α₁1.2^W440A/β2b/α2δ, a mutation that disrupts the interaction between α₁1.2 and β2 subunits. Pulldown assays in neuronal SH-SY5Y cells and in vitro binding of recombinant H-Ras and β2 confirmed the importance of the intracellular β2 subunit for depolarization-induced gene activation. Using a Ca²⁺-impermeable mutant channel α₁1.2^L745P/β2b/α2δ or disrupting Ca²⁺/calmodulin binding to the channel using the channel mutant α₁1.2^I1624A/β2b/α2δ, we demonstrate that depolarization-induced c-Fos and MeCP2 activation does not depend on Ca²⁺ transport by the channel. Thus, in contrast to the paradigm that elevated intracellular Ca²⁺ drives nuclear signaling, we show that Cav1.2-triggered c-Fos or MeCP2 is dependent on extracellular Ca²⁺ and Ca²⁺ occupancy of the open channel pore, but is Ca²⁺-influx independent. An indispensable β-subunit interaction with H-Ras, which is triggered by conformational changes at α₁1.2 independently of Ca²⁺ flux, brings to light a master regulatory role of β2 in transcriptional activation via the ERK/CREB pathway. This mode of H-Ras activation could have broad implications for understanding the coupling of membrane depolarization to the rapid induction of gene transcription.

Structure of a eukaryotic voltage-gated sodium channel at near-atomic resolution

Voltage-gated sodium (Na_v) channels are responsible for the initiation and propagation of action potentials. They are associated with a variety of channelopathies and are targeted by multiple pharmaceutical drugs and natural toxins. Here, we report the cryogenic electron microscopy structure of a putative Na_v channel from American cockroach (designated Na_vPaS) at 3.8 angstrom resolution. The voltage-sensing domains (VSDs) of the four repeats exhibit distinct conformations. The entrance to the asymmetric selectivity filter vestibule is guarded by heavily glycosylated and disulfide bond-stabilized extracellular loops. On the cytoplasmic side, a conserved amino-terminal domain is placed below VSD_I, and a carboxy-terminal domain binds to the III-IV linker. The structure of Na_vPaS establishes an important foundation for understanding function and disease mechanism of Na_v and related voltage-gated calcium channels.

The STIM-Orai Pathway: Orai, the Pore-Forming Subunit of the CRAC Channel

This chapter focuses on the Orai proteins, Orai1-Orai3, with special emphasis on Orai1, in humans and other mammals, and on the definitive evidence that Orai is the pore subunit of the CRAC channel. It begins by reviewing briefly the defining characteristics of the CRAC channel, then discusses the studies that implicated Orai as part of the store-operated Ca²⁺ entry pathway and as the CRAC channel pore subunit, and finally examines ongoing work that is providing insights into CRAC channel structure and gating.

Structure of the voltage-gated calcium channel Ca(v)1.1 at 3.6 Å resolution

The voltage-gated calcium (Ca_v) channels convert membrane electrical signals to intracellular Ca²⁺-mediated events. Among the ten subtypes of Ca_v channel in mammals, Ca_v1.1 is specified for the excitation-contraction coupling of skeletal muscles. Here we present the cryo-electron microscopy structure of the rabbit Ca_v1.1 complex at a nominal resolution of 3.6 Å. The inner gate of the ion-conducting α1-subunit is closed and all four voltage-sensing domains adopt an 'up' conformation, suggesting a potentially inactivated state. The extended extracellular loops of the pore domain, which are stabilized by multiple disulfide bonds, form a windowed dome above the selectivity filter. One side of the dome provides the docking site for the α2δ-1-subunit, while the other side may attract cations through its negative surface potential. The intracellular I-II and III-IV linker helices interact with the β_1a-subunit and the carboxy-terminal domain of α1, respectively. Classification of the particles yielded two additional reconstructions that reveal pronounced displacement of β_1a and adjacent elements in α1. The atomic model of the Ca_v1.1 complex establishes a foundation for mechanistic understanding of excitation-contraction coupling and provides a three-dimensional template for molecular interpretations of the functions and disease mechanisms of Ca_v and Na_v channels.

Cd(2+) sensitivity and permeability of a low voltage-activated Ca(2+) channel with CatSper-like selectivity filter

CatSper is a sperm-specific Ca(2+) channel that plays an essential role in the male fertility. However, its biophysical properties have been poorly characterized mainly due to its deficient heterologous expression. As other voltage-gated Ca(2+) channels (CaVs), CatSper possesses a conserved Ca(2+)-selective filter motif ([T/S]x[D/E]xW) in the pore region. Interestingly, CatSper conserves four aspartic acids (DDDD) as the negatively charged residues in this motif while high voltage-activated CaVs have four glutamic acids (EEEE) and low voltage-activated CaVs possess two glutamic acids and two aspartic acids (EEDD). Previous studies based on site-directed mutagenesis of L- and T-type channels showed that the number of D seems to have a negative correlation with their cadmium (Cd(2+)) sensitivity. These results suggest that CatSper (DDDD) would have low sensitivity to Cd(2+). To explore Cd(2+)-sensitivity and -permeability of CatSper, we performed two types of experiments: 1) Electrophysiological analysis of heterologously expressed human CaV3.1 channel and three pore mutants (DEDD, EDDD and DDDD), 2) Cd(2+) imaging of human spermatozoa with FluoZin-1. Electrophysiological studies showed a significant increase in Cd(2+) and manganese (Mn(2+)) currents through the CaV3.1 mutants as well as a reduction in the inhibitory effect of Cd(2+) on the Ca(2+) current. In fluorescence imaging with human sperm, we observed an increase in Cd(2+) influx potentiated by progesterone, a potent activator of CatSper. These results support our hypothesis, namely that Cd(2+)-sensitivity and -permeability are related to the absolute number of D in the Ca(2+)-selective filter independently to the type of the Cav channels.

Large Ca²⁺-dependent facilitation of Ca(V)2.1 channels revealed by Ca²⁺ photo-uncaging

KEY POINTS

CaV 2.1 channels constitute a dominant Ca(2+) entry pathway into brain neurons, triggering downstream Ca(2+) -dependent processes such as neurotransmitter release. CaV 2.1 is itself modulated by Ca(2+) , resulting in activity-dependent enhancement of channel opening termed Ca(2+) -dependent facilitation (CDF). Real-time Ca(2+) imaging and Ca(2+) uncaging here reveal that CDF turns out to be strikingly faster, more Ca(2+) sensitive, and larger than anticipated on previous grounds. Robust resolution of the quantitative profile of CDF enables deduction of a realistic biophysical model for this process. These results suggest that CaV 2.1 CDF would figure most prominently in short-term synaptic plasticity and cerebellar Purkinje cell rhythmicity.

ABSTRACT

CaV 2.1 (P-type) voltage-gated Ca(2+) channels constitute a major source of neuronal Ca(2+) current, strongly influencing rhythmicity and triggering neurotransmitter release throughout the central nervous system. Fitting with such stature among Ca(2+) entry pathways, CaV 2.1 is itself feedback regulated by intracellular Ca(2+) , acting through calmodulin to facilitate channel opening. The precise neurophysiological role of this calcium-dependent facilitation (CDF) remains uncertain, however, in large measure because the very magnitude, Ca(2+) dependence and kinetics of CDF have resisted quantification by conventional means. Here, we utilize the photo-uncaging of Ca(2+) with CaV 2.1 channels fluxing Li(+) currents, so that voltage-dependent activation of channel gating is no longer conflated with Ca(2+) entry, and CDF is then driven solely by light-induced increases in Ca(2+) . By using this strategy, we now find that CDF can be unexpectedly large, enhancing currents by as much as twofold at physiological voltages. CDF is steeply Ca(2+) dependent, with a Hill coefficient of approximately two, a half-maximal effect reached by nearly 500 nm Ca(2+) , and Ca(2+) on/off kinetics in the order of milliseconds to tens of milliseconds. These properties were established for both native P-type currents in cerebellar Purkinje neurons, as well as their recombinant channel counterparts under heterologous expression. Such features suggest that CDF of CaV 2.1 channels may substantially enhance the regularity of rhythmic firing in cerebellar Purkinje neurons, where regularity is believed crucial for motor coordination. In addition, this degree of extensive CDF would be poised to exert large order-of-magnitude effects on short-term synaptic plasticity via rapid modulation of presynaptic Ca(2+) entry.

Arrhythmogenesis in Timothy Syndrome is associated with defects in Ca(2+)-dependent inactivation

Timothy Syndrome (TS) is a multisystem disorder, prominently featuring cardiac action potential prolongation with paroxysms of life-threatening arrhythmias. The underlying defect is a single de novo missense mutation in Ca_V1.2 channels, either G406R or G402S. Notably, these mutations are often viewed as equivalent, as they produce comparable defects in voltage-dependent inactivation and cause similar manifestations in patients. Yet, their effects on calcium-dependent inactivation (CDI) have remained uncertain. Here, we find a significant defect in CDI in TS channels, and uncover a remarkable divergence in the underlying mechanism for G406R versus G402S variants. Moreover, expression of these TS channels in cultured adult guinea pig myocytes, combined with a quantitative ventricular myocyte model, reveals a threshold behaviour in the induction of arrhythmias due to TS channel expression, suggesting an important therapeutic principle: a small shift in the complement of mutant versus wild-type channels may confer significant clinical improvement.

Timothy Syndrome (TS) is a multisystem disorder caused by two mutations leading to dysfunction of the Ca_V1.2 channel. Here, Dick et al. uncover a major and mechanistically divergent effect of both mutations on Ca²⁺/calmodulin-dependent inactivation of Ca_V1.2 channels, suggesting genetic variant-tailored therapy for TS treatment.

Structure of the voltage-gated calcium channel Cav1.1 complex

The voltage-gated calcium channel Ca(v)1.1 is engaged in the excitation-contraction coupling of skeletal muscles. The Ca(v)1.1 complex consists of the pore-forming subunit α1 and auxiliary subunits α2δ, β, and γ. We report the structure of the rabbit Ca(v)1.1 complex determined by single-particle cryo-electron microscopy. The four homologous repeats of the α1 subunit are arranged clockwise in the extracellular view. The γ subunit, whose structure resembles claudins, interacts with the voltage-sensing domain of repeat IV (VSD(IV)), whereas the cytosolic β subunit is located adjacent to VSD(II) of α1. The α2 subunit interacts with the extracellular loops of repeats I to III through its VWA and Cache1 domains. The structure reveals the architecture of a prototypical eukaryotic Ca(v) channel and provides a framework for understanding the function and disease mechanisms of Ca(v) and Na(v) channels.

Store-Operated Calcium Channels

Store-operated calcium channels (SOCs) are a major pathway for calcium signaling in virtually all metozoan cells and serve a wide variety of functions ranging from gene expression, motility, and secretion to tissue and organ development and the immune response. SOCs are activated by the depletion of Ca(2+) from the endoplasmic reticulum (ER), triggered physiologically through stimulation of a diverse set of surface receptors. Over 15 years after the first characterization of SOCs through electrophysiology, the identification of the STIM proteins as ER Ca(2+) sensors and the Orai proteins as store-operated channels has enabled rapid progress in understanding the unique mechanism of store-operate calcium entry (SOCE). Depletion of Ca(2+) from the ER causes STIM to accumulate at ER-plasma membrane (PM) junctions where it traps and activates Orai channels diffusing in the closely apposed PM. Mutagenesis studies combined with recent structural insights about STIM and Orai proteins are now beginning to reveal the molecular underpinnings of these choreographic events. This review describes the major experimental advances underlying our current understanding of how ER Ca(2+) depletion is coupled to the activation of SOCs. Particular emphasis is placed on the molecular mechanisms of STIM and Orai activation, Orai channel properties, modulation of STIM and Orai function, pharmacological inhibitors of SOCE, and the functions of STIM and Orai in physiology and disease.

Deciphering voltage-gated Na(+) and Ca(2+) channels by studying prokaryotic ancestors

Voltage-gated sodium channels (NaVs) and calcium channels (CaVs) are involved in electrical signaling, contraction, secretion, synaptic transmission, and other physiological processes activated in response to depolarization. Despite their physiological importance, the structures of these closely related proteins have remained elusive because of their size and complexity. Bacterial NaVs have structures analogous to a single domain of eukaryotic NaVs and CaVs and are their likely evolutionary ancestor. Here we review recent work that has led to new understanding of NaVs and CaVs through high-resolution structural studies of their prokaryotic ancestors. New insights into their voltage-dependent activation and inactivation, ion conductance, and ion selectivity provide realistic structural models for the function of these complex membrane proteins at the atomic level.

Fast kinetics of calcium signaling and sensor design

Fast calcium signaling is regulated by numerous calcium channels exhibiting high spatiotemporal profiles which are currently measured by fluorescent calcium sensors. There is still a strong need to improve the kinetics of genetically encoded calcium indicators (sensors) to capture calcium dynamics in the millisecond time frame. In this review, we summarize several major fast calcium signaling pathways and discuss the recent developments and application of genetically encoded calcium indicators to detect these pathways. A new class of genetically encoded calcium indicators designed with site-directed mutagenesis on the surface of beta-barrel fluorescent proteins to form a pentagonal bipyramidal-like calcium binding domain dramatically accelerates calcium binding kinetics. Furthermore, novel genetically encoded calcium indicators with significantly increased fluorescent lifetime change are advantageous in deep-field imaging with high light-scattering and notable morphology change.

Voltage-gated calcium channels: Determinants of channel function and modulation by inorganic cations

Voltage-gated calcium channels (VGCCs) represent a key link between electrical signals and non-electrical processes, such as contraction, secretion and transcription. Evolved to achieve high rates of Ca(2+)-selective flux, they possess an elaborate mechanism for selection of Ca(2+) over foreign ions. It has been convincingly linked to competitive binding in the pore, but the fundamental question of how this is reconcilable with high rates of Ca(2+) transfer remains unanswered. By virtue of their similarity to Ca(2+), polyvalent cations can interfere with the function of VGCCs and have proven instrumental in probing the mechanisms underlying selective permeation. Recent emergence of crystallographic data on a set of Ca(2+)-selective model channels provides a structural framework for permeation in VGCCs, and warrants a reconsideration of their diverse modulation by polyvalent cations, which can be roughly separated into three general mechanisms: (I) long-range interactions with charged regions on the surface, affecting the local potential sensed by the channel or influencing voltage-sensor movement by repulsive forces (electrostatic effects), (II) short-range interactions with sites in the ion-conducting pathway, leading to physical obstruction of the channel (pore block), and in some cases (III) short-range interactions with extracellular binding sites, leading to non-electrostatic modifications of channel gating (allosteric effects). These effects, together with the underlying molecular modifications, provide valuable insights into the function of VGCCs, and have important physiological and pathophysiological implications. Allosteric suppression of some of the pore-forming Cavα1-subunits (Cav2.3, Cav3.2) by Zn(2+) and Cu(2+) may play a major role for the regulation of excitability by endogenous transition metal ions. The fact that these ions can often traverse VGCCs can contribute to the detrimental intracellular accumulation of metal ions following excessive release of endogenous Cu(2+) and Zn(2+) or exposure to non-physiological toxic metal ions.

A study of quantum mechanical probabilities in the classical Hodgkin-Huxley model

The Hodgkin-Huxley (HH) model is a powerful model to explain different aspects of spike generation in excitable cells. However, the HH model was proposed in 1952 when the real structure of the ion channel was unknown. It is now common knowledge that in many ion-channel proteins the flow of ions through the pore is governed by a gate, comprising a so-called "selectivity filter" inside the ion channel, which can be controlled by electrical interactions. The selectivity filter (SF) is believed to be responsible for the selection and fast conduction of particular ions across the membrane of an excitable cell. Other (generally larger) parts of the molecule such as the pore-domain gate control the access of ions to the channel protein. In fact, two types of gates are considered here for ion channels: the "external gate", which is the voltage sensitive gate, and the "internal gate" which is the selectivity filter gate (SFG). Some quantum effects are expected in the SFG due to its small dimensions, which may play an important role in the operation of an ion channel. Here, we examine parameters in a generalized model of HH to see whether any parameter affects the spike generation. Our results indicate that the previously suggested semi-quantum-classical equation proposed by Bernroider and Summhammer (BS) agrees strongly with the HH equation under different conditions and may even provide a better explanation in some cases. We conclude that the BS model can refine the classical HH model substantially.

Selecting ions by size in a calcium channel: the ryanodine receptor case study

Many calcium channels can distinguish between ions of the same charge but different size. For example, when cations are in direct competition with each other, the ryanodine receptor (RyR) calcium channel preferentially conducts smaller cations such as Li(+) and Na(+) over larger ones such as K(+) and Cs(+). Here, we analyze the physical basis for this preference using a previously established model of RyR permeation and selectivity. Like other calcium channels, RyR has four aspartate residues in its GGGIGDE selectivity filter. These aspartates have their terminal carboxyl group in the pore lumen, which take up much of the available space for permeating ions. We find that small ions are preferred by RyR because they can fit into this crowded environment more easily.

Bacterial voltage-gated sodium channels (BacNa(V)s) from the soil, sea, and salt lakes enlighten molecular mechanisms of electrical signaling and pharmacology in the brain and heart

Voltage-gated sodium channels (Na(V)s) provide the initial electrical signal that drives action potential generation in many excitable cells of the brain, heart, and nervous system. For more than 60years, functional studies of Na(V)s have occupied a central place in physiological and biophysical investigation of the molecular basis of excitability. Recently, structural studies of members of a large family of bacterial voltage-gated sodium channels (BacNa(V)s) prevalent in soil, marine, and salt lake environments that bear many of the core features of eukaryotic Na(V)s have reframed ideas for voltage-gated channel function, ion selectivity, and pharmacology. Here, we analyze the recent advances, unanswered questions, and potential of BacNa(V)s as templates for drug development efforts.

Voltage control of Ca²⁺ permeation through N-type calcium (Ca(V)2.2) channels

Voltage dependence of permeation enhances Ca²⁺ influx through Ca_V2.2 channels relative to that of other ions at depolarized voltages.

Voltage-gated calcium (Ca_V) channels deliver Ca²⁺ to trigger cellular functions ranging from cardiac muscle contraction to neurotransmitter release. The mechanism by which these channels select for Ca²⁺ over other cations is thought to involve multiple Ca²⁺-binding sites within the pore. Although the Ca²⁺ affinity and cation preference of these sites have been extensively investigated, the effect of voltage on these sites has not received the same attention. We used a neuronal preparation enriched for N-type calcium (Ca_V2.2) channels to investigate the effect of voltage on Ca²⁺ flux. We found that the EC₅₀ for Ca²⁺ permeation increases from 13 mM at 0 mV to 240 mM at 60 mV, indicating that, during permeation, Ca²⁺ ions sense the electric field. These data were nicely reproduced using a three-binding-site step model. Using roscovitine to slow Ca_V2.2 channel deactivation, we extended these measurements to voltages <0 mV. Permeation was minimally affected at these hyperpolarized voltages, as was predicted by the model. As an independent test of voltage effects on permeation, we examined the Ca²⁺-Ba²⁺ anomalous mole fraction (MF) effect, which was both concentration and voltage dependent. However, the Ca²⁺-Ba²⁺ anomalous MF data could not be reproduced unless we added a fourth site to our model. Thus, Ca²⁺ permeation through Ca_V2.2 channels may require at least four Ca²⁺-binding sites. Finally, our results suggest that the high affinity of Ca²⁺ for the channel helps to enhance Ca²⁺ influx at depolarized voltages relative to other ions (e.g., Ba²⁺ or Na⁺), whereas the absence of voltage effects at negative potentials prevents Ca²⁺ from becoming a channel blocker. Both effects are needed to maximize Ca²⁺ influx over the voltages spanned by action potentials.

Neuronal voltage-gated calcium channels: structure, function, and dysfunction

Voltage-gated calcium channels are the primary mediators of depolarization-induced calcium entry into neurons. There is great diversity of calcium channel subtypes due to multiple genes that encode calcium channel α1 subunits, coassembly with a variety of ancillary calcium channel subunits, and alternative splicing. This allows these channels to fulfill highly specialized roles in specific neuronal subtypes and at particular subcellular loci. While calcium channels are of critical importance to brain function, their inappropriate expression or dysfunction gives rise to a variety of neurological disorders, including, pain, epilepsy, migraine, and ataxia. This Review discusses salient aspects of voltage-gated calcium channel function, physiology, and pathophysiology.

Outstanding questions regarding the permeation, selectivity, and regulation of the mitochondrial calcium uniporter

The recent discovery of genes encoding the mitochondrial calcium (Ca(2+)) uniporter has revealed new opportunities for studying how abnormal Ca(2+) signals cause disease. Ca(2+) transport across the mitochondrial inner membrane is highly regulated, and the uniporter is the channel that acts as a major portal for Ca(2+) influx. Low amounts of mitochondrial Ca(2+) can boost ATP synthesis, but excess amounts, such as following cytoplasmic Ca(2+) overload in heart failure, triggers mitochondrial failure and cell death. In fact, precisely because mitochondrial Ca(2+) transport is so tightly regulated, a fundamental understanding of how the uniporter functions is necessary. Two key uniporter features allow Ca(2+) influx without mitochondrial damage during normal physiology. First, the channel is significantly more selective than other known Ca(2+) channels. This prevents the permeation of other ions and uncoupling of the electrochemical gradient. Second, the uniporter becomes active at only high Ca(2+) concentrations, preventing a resting leak of cytoplasmic Ca(2+) itself. Now possessing the identities of the various proteins forming the uniporter, we can proceed with efforts to define the molecular determinants of permeation, selectivity and Ca(2+)-regulation.

Conduits of life's spark: a perspective on ion channel research since the birth of neuron

Heartbeats, muscle twitches, and lightning-fast thoughts are all manifestations of bioelectricity and rely on the activity of a class of membrane proteins known as ion channels. The basic function of an ion channel can be distilled into, "The hole opens. Ions go through. The hole closes." Studies of the fundamental mechanisms by which this process happens and the consequences of such activity in the setting of excitable cells remains the central focus of much of the field. One might wonder after so many years of detailed poking at such a seemingly simple process, is there anything left to learn?

Structural basis for Ca2+ selectivity of a voltage-gated calcium channel

Voltage-gated calcium (Ca_V) channels catalyze rapid, highly selective influx of Ca²⁺ into cells despite 70-fold higher extracellular concentration of Na⁺. How Ca_V channels solve this fundamental biophysical problem remains unclear. Here we report physiological and crystallographic analyses of a calcium selectivity filter constructed in the homotetrameric bacterial Na_V channel Na_VAb. Our results reveal interactions of hydrated Ca²⁺ with two high-affinity Ca²⁺-binding sites followed by a third lower-affinity site that would coordinate Ca²⁺ as it moves inward. At the selectivity filter entry, Site 1 is formed by four carboxyl side-chains, which play a critical role in determining Ca²⁺ selectivity. Four carboxyls plus four backbone carbonyls form Site 2, which is targeted by the blocking cations, Cd²⁺ and Mn²⁺, with single occupancy. The lower-affinity Site 3 is formed by four backbone carbonyls alone, which mediate exit into the central cavity. This pore architecture suggests a conduction pathway involving transitions between two main states with one or two hydrated Ca²⁺ ions bound in the selectivity filter and supports a “knock-off” mechanism of ion permeation through a stepwise-binding process. The multi-ion selectivity filter of our Ca_VAb model establishes a structural framework for understanding mechanisms of ion selectivity and conductance by vertebrate Ca_V channels.

Ca2+ channel nanodomains boost local Ca2+ amplitude

Local Ca(2+) signals through voltage-gated Ca(2+) channels (CaVs) drive synaptic transmission, neural plasticity, and cardiac contraction. Despite the importance of these events, the fundamental relationship between flux through a single CaV channel and the Ca(2+) signaling concentration within nanometers of its pore has resisted empirical determination, owing to limitations in the spatial resolution and specificity of fluorescence-based Ca(2+) measurements. Here, we exploited Ca(2+)-dependent inactivation of CaV channels as a nanometer-range Ca(2+) indicator specific to active channels. We observed an unexpected and dramatic boost in nanodomain Ca(2+) amplitude, ten-fold higher than predicted on theoretical grounds. Our results uncover a striking feature of CaV nanodomains, as diffusion-restricted environments that amplify small Ca(2+) fluxes into enormous local Ca(2+) concentrations. This Ca(2+) tuning by the physical composition of the nanodomain may represent an energy-efficient means of local amplification that maximizes information signaling capacity, while minimizing global Ca(2+) load.

The structure and regulation of magnesium selective ion channels

The magnesium ion (Mg(2+)) is the most abundant divalent cation within cells. In man, Mg(2+)-deficiency is associated with diseases affecting the heart, muscle, bone, immune, and nervous systems. Despite its impact on human health, little is known about the molecular mechanisms that regulate magnesium transport and storage. Complete structural information on eukaryotic Mg(2+)-transport proteins is currently lacking due to associated technical challenges. The prokaryotic MgtE and CorA magnesium transport systems have recently succumbed to structure determination by X-ray crystallography, providing first views of these ubiquitous and essential Mg(2+)-channels. MgtE and CorA are unique among known membrane protein structures, each revealing a novel protein fold containing distinct arrangements of ten transmembrane-spanning α-helices. Structural and functional analyses have established that Mg(2+)-selectivity in MgtE and CorA occurs through distinct mechanisms. Conserved acidic side-chains appear to form the selectivity filter in MgtE, whereas conserved asparagines coordinate hydrated Mg(2+)-ions within the selectivity filter of CorA. Common structural themes have also emerged whereby MgtE and CorA sense and respond to physiologically relevant, intracellular Mg(2+)-levels through dedicated regulatory domains. Within these domains, multiple primary and secondary Mg(2+)-binding sites serve to staple these ion channels into their respective closed conformations, implying that Mg(2+)-transport is well guarded and very tightly regulated. The MgtE and CorA proteins represent valuable structural templates to better understand the related eukaryotic SLC41 and Mrs2-Alr1 magnesium channels. Herein, we review the structure, function and regulation of MgtE and CorA and consider these unique proteins within the expanding universe of ion channel and transporter structural biology.