Cryo-EM structure of the calcium release-activated calcium channel Orai in an open conformation

Insights into the dynamics of the Ca2+ release-activated Ca2+ channel pore-forming complex Orai1

An important calcium (Ca2+) entry pathway into the cell is the Ca2+ release-activated Ca2+ (CRAC) channel, which controls a series of downstream signaling events such as gene transcription, secretion and proliferation. It is composed of a Ca2+ sensor in the endoplasmic reticulum (ER), the stromal interaction molecule (STIM), and the Ca2+ ion channel Orai in the plasma membrane (PM). Their activation is initiated by receptor-ligand binding at the PM, which triggers a signaling cascade within the cell that ultimately causes store depletion. The decrease in ER-luminal Ca2+ is sensed by STIM1, which undergoes structural rearrangements that lead to coupling with Orai1 and its activation. In this review, we highlight the current understanding of the Orai1 pore opening mechanism. In this context, we also point out the questions that remain unanswered and how these can be addressed by the currently emerging genetic code expansion (GCE) technology. GCE enables the incorporation of non-canonical amino acids with novel properties, such as light-sensitivity, and has the potential to provide novel insights into the structure/function relationship of CRAC channels at a single amino acid level in the living cell.

Activation mechanisms and structural dynamics of STIM proteins

The family of stromal interaction molecules (STIM) includes two widely expressed single-pass endoplasmic reticulum (ER) transmembrane proteins and additional splice variants that act as precise ER-luminal Ca2+ sensors. STIM proteins mainly function as one of the two essential components of the so-called Ca2+ release-activated Ca2+ (CRAC) channel. The second CRAC channel component is constituted by pore-forming Orai proteins in the plasma membrane. STIM and Orai physically interact with each other to enable CRAC channel opening, which is a critical prerequisite for various downstream signalling pathways such as gene transcription or proliferation. Their activation commonly requires the emptying of the intracellular ER Ca2+ store. Using their Ca2+ sensing capabilities, STIM proteins confer this Ca2+ content-dependent signal to Orai, thereby linking Ca2+ store depletion to CRAC channel opening. Here we review the conformational dynamics occurring along the entire STIM protein upon store depletion, involving the transition from the quiescent, compactly folded structure into an active, extended state, modulation by a variety of accessory components in the cell as well as the impairment of individual steps of the STIM activation cascade associated with disease.

Regulatory mechanisms controlling store-operated calcium entry

Calcium influx through plasma membrane ion channels is crucial for many events in cellular physiology. Cell surface stimuli lead to the production of inositol 1,4,5-trisphosphate (IP3), which binds to IP3 receptors (IP3R) in the endoplasmic reticulum (ER) to release calcium pools from the ER lumen. This leads to the depletion of ER calcium pools, which has been termed store depletion. Store depletion leads to the dissociation of calcium ions from the EF-hand motif of the ER calcium sensor Stromal Interaction Molecule 1 (STIM1). This leads to a conformational change in STIM1, which helps it to interact with the plasma membrane (PM) at ER:PM junctions. At these ER:PM junctions, STIM1 binds to and activates a calcium channel known as Orai1 to form calcium release-activated calcium (CRAC) channels. Activation of Orai1 leads to calcium influx, known as store-operated calcium entry (SOCE). In addition to Orai1 and STIM1, the homologs of Orai1 and STIM1, such as Orai2/3 and STIM2, also play a crucial role in calcium homeostasis. The influx of calcium through the Orai channel activates a calcium current that has been termed the CRAC current. CRAC channels form multimers and cluster together in large macromolecular assemblies termed "puncta". How CRAC channels form puncta has been contentious since their discovery. In this review, we will outline the history of SOCE, the molecular players involved in this process, as well as the models that have been proposed to explain this critical mechanism in cellular physiology.

Reconstitution of Calcium Channel Protein Orai3 into Liposomes for Functional Studies

Store-operated calcium entry (SOCE) is the main mechanism for the Ca2+ influx in non-excitable cells. The two major components of SOCE are stromal interaction molecule 1 (STIM1) in the endoplasmic reticulum and Ca2+ release-activated Ca2+ channel (CRAC) Orai on the plasma membrane. SOCE requires interaction between STIM1 and Orai. Mammals have three Orai homologs: Orai1, Orai2, and Orai3. Although Orai1 has been widely studied and proven to essential for numerous cellular processes, Orai3 has also attracted a significant attention recently. The gating and activation mechanisms of Orai3 have yet to be fully elucidated. Here, we expressed, purified, and reconstituted Orai3 protein into liposomes and investigated its orientation and oligomeric state in the resulting proteoliposomes. STIM1 interacted with the Orai3-containing proteoliposomes and mediated calcium release from the them, suggesting that the Orai3 channel was functional and that recombinant STIM1 could directly open the Orai3 channel in vitro. The developed in vitro calcium release system could be used to study the structure, function, and pharmacology of Orai3 channel.

A pathogenic human Orai1 mutation unmasks STIM1-independent rapid inactivation of Orai1 channels

Ca²⁺ release-activated Ca²⁺ (CRAC) channels are activated by direct physical interactions between Orai1, the channel protein, and STIM1, the endoplasmic reticulum Ca²⁺ sensor. A hallmark of CRAC channels is fast Ca²⁺-dependent inactivation (CDI) which provides negative feedback to limit Ca²⁺ entry through CRAC channels. Although STIM1 is thought to be essential for CDI, its molecular mechanism remains largely unknown. Here, we examined a poorly understood gain-of-function (GOF) human Orai1 disease mutation, L138F, that causes tubular aggregate myopathy. Through pairwise mutational analysis, we determine that large amino acid substitutions at either L138 or the neighboring T92 locus located on the pore helix evoke highly Ca²⁺-selective currents in the absence of STIM1. We find that the GOF phenotype of the L138 pathogenic mutation arises due to steric clash between L138 and T92. Surprisingly, strongly activating L138 and T92 mutations showed CDI in the absence of STIM1, contradicting prevailing views that STIM1 is required for CDI. CDI of constitutively open T92W and L138F mutants showed enhanced intracellular Ca²⁺ sensitivity, which was normalized by re-adding STIM1 to the cells. Truncation of the Orai1 C-terminus reduced T92W CDI, indicating a key role for the Orai1 C-terminus for CDI. Overall, these results identify the molecular basis of a disease phenotype with broad implications for activation and inactivation of Orai1 channels.

ORAI Calcium Channels: Regulation, Function, Pharmacology, and Therapeutic Targets

The changes in intracellular free calcium (Ca2+) levels are one of the most widely regulators of cell function; therefore, calcium as a universal intracellular mediator is involved in very important human diseases and disorders. In many cells, Ca2+ inflow is mediated by store-operated calcium channels, and it is recognized that the store-operated calcium entry (SOCE) is mediated by the two partners: the pore-forming proteins Orai (Orai1-3) and the calcium store sensor, stromal interaction molecule (STIM1-2). Importantly, the Orai/STIM channels are involved in crucial cell signalling processes such as growth factors, neurotransmitters, and cytokines via interaction with protein tyrosine kinase coupled receptors and G protein-coupled receptors. Therefore, in recent years, the issue of Orai/STIM channels as a drug target in human diseases has received considerable attention. This review summarizes and highlights our current knowledge of the Orai/STIM channels in human diseases and disorders, including immunodeficiency, myopathy, tubular aggregate, Stormorken syndrome, York platelet syndrome, cardiovascular and metabolic disorders, and cancers, as well as suggesting these channels as drug targets for pharmacological therapeutic intervention. Moreover, this work will also focus on the pharmacological modulators of Orai/STIM channel complexes. Together, our thoughtful of the biology and physiology of the Orai/STIM channels have grown remarkably during the past three decades, and the next important milestone in the field of store-operated calcium entry will be to identify potent and selective small molecules as a therapeutic agent with the purpose to target human diseases and disorders for patient benefit.

CRAC and SK Channels: Their Molecular Mechanisms Associated with Cancer Cell Development

Cancer represents a major health burden worldwide. Several molecular targets have been discovered alongside treatments with positive clinical outcomes. However, the reoccurrence of cancer due to therapy resistance remains the primary cause of mortality. Endeavors in pinpointing new markers as molecular targets in cancer therapy are highly desired. The significance of the co-regulation of Ca²⁺-permeating and Ca²⁺-regulated ion channels in cancer cell development, proliferation, and migration make them promising molecular targets in cancer therapy. In particular, the co-regulation of the Orai1 and SK3 channels has been well-studied in breast and colon cancer cells, where it finally leads to an invasion-metastasis cascade. Nevertheless, many questions remain unanswered, such as which key molecular components determine and regulate their interplay. To provide a solid foundation for a better understanding of this ion channel co-regulation in cancer, we first shed light on the physiological role of Ca²⁺ and how this ion is linked to carcinogenesis. Then, we highlight the structure/function relationship of Orai1 and SK3, both individually and in concert, their role in the development of different types of cancer, and aspects that are not yet known in this context.

Discovery of novel gating checkpoints in the Orai1 calcium channel by systematic analysis of constitutively active mutants of its paralogs and orthologs

In humans, there are three paralogs of the Orai Ca2+ channel that form the core of the store-operated calcium entry (SOCE) machinery. While the STIM-mediated gating mechanism of Orai channels is still under active investigation, several artificial and natural variants are known to cause constitutive activity of the human Orai1 channel. Surprisingly, little is known about the conservation of the gating checkpoints among the different human Orai paralogs and orthologs in other species. In our work, we show that the mutation corresponding to the activating mutation H134A in transmembrane helix 2 (TM2) of human Orai1 also activates Orai2 and Orai3, likely via a similar mechanism. However, this cross-paralog conservation does not apply to the "ANSGA" nexus mutations in TM4 of human Orai1, which is reported to mimic the STIM1-activated state of the channel. In investigating the mechanistic background of these differences, we identified two positions, H171 and F246 in human Orai1, that are not conserved among paralogs and that seem to be crucial for the channel activation triggered by the "ANSGA" mutations in Orai1. However, mutations of the same residues still allow gating of Orai1 by STIM1, suggesting that the ANSGA mutant of Orai1 may not be a surrogate for the STIM1-activated state of the Orai1 channel. Our results shed new light on these important gating checkpoints and show that the gating mechanism of Orai channels is affected by multiple factors that are not necessarily conserved among orai homologs, such as the TM4-TM3 coupling.

STIM and Orai Mediated Regulation of Calcium Signaling in Age-Related Diseases

Tight spatiotemporal regulation of intracellular Ca²⁺ plays a critical role in regulating diverse cellular functions including cell survival, metabolism, and transcription. As a result, eukaryotic cells have developed a wide variety of mechanisms for controlling Ca²⁺ influx and efflux across the plasma membrane as well as Ca²⁺ release and uptake from intracellular stores. The STIM and Orai protein families comprising of STIM1, STIM2, Orai1, Orai2, and Orai3, are evolutionarily highly conserved proteins that are core components of all mammalian Ca²⁺ signaling systems. STIM1 and Orai1 are considered key players in the regulation of Store Operated Calcium Entry (SOCE), where release of Ca²⁺ from intracellular stores such as the Endoplasmic/Sarcoplasmic reticulum (ER/SR) triggers Ca²⁺ influx across the plasma membrane. SOCE, which has been widely characterized in non-excitable cells, plays a central role in Ca²⁺-dependent transcriptional regulation. In addition to their role in Ca²⁺ signaling, STIM1 and Orai1 have been shown to contribute to the regulation of metabolism and mitochondrial function. STIM and Orai proteins are also subject to redox modifications, which influence their activities. Considering their ubiquitous expression, there has been increasing interest in the roles of STIM and Orai proteins in excitable cells such as neurons and myocytes. While controversy remains as to the importance of SOCE in excitable cells, STIM1 and Orai1 are essential for cellular homeostasis and their disruption is linked to various diseases associated with aging such as cardiovascular disease and neurodegeneration. The recent identification of splice variants for most STIM and Orai isoforms while complicating our understanding of their function, may also provide insight into some of the current contradictions on their roles. Therefore, the goal of this review is to describe our current understanding of the molecular regulation of STIM and Orai proteins and their roles in normal physiology and diseases of aging, with a particular focus on heart disease and neurodegeneration.

On the Connections between TRPM Channels and SOCE

Plasma membrane protein channels provide a passageway for ions to access the intracellular milieu. Rapid entry of calcium ions into cells is controlled mostly by ion channels, while Ca2+-ATPases and Ca2+ exchangers ensure that cytosolic Ca2+ levels ([Ca2+]cyt) are maintained at low (~100 nM) concentrations. Some channels, such as the Ca2+-release-activated Ca2+ (CRAC) channels and voltage-dependent Ca2+ channels (CACNAs), are highly Ca2+-selective, while others, including the Transient Receptor Potential Melastatin (TRPM) family, have broader selectivity and are mostly permeable to monovalent and divalent cations. Activation of CRAC channels involves the coupling between ORAI1-3 channels with the endoplasmic reticulum (ER) located Ca2+ store sensor, Stromal Interaction Molecules 1-2 (STIM1/2), a pathway also termed store-operated Ca2+ entry (SOCE). The TRPM family is formed by 8 members (TRPM1-8) permeable to Mg2+, Ca2+, Zn2+ and Na+ cations, and is activated by multiple stimuli. Recent studies indicated that SOCE and TRPM structure-function are interlinked in some instances, although the molecular details of this interaction are only emerging. Here we review the role of TRPM and SOCE in Ca2+ handling and highlight the available evidence for this interaction.

The Role of Lipids in CRAC Channel Function

The composition and dynamics of the lipid membrane define the physical properties of the bilayer and consequently affect the function of the incorporated membrane transporters, which also applies for the prominent Ca²⁺ release-activated Ca²⁺ ion channel (CRAC). This channel is activated by receptor-induced Ca²⁺ store depletion of the endoplasmic reticulum (ER) and consists of two transmembrane proteins, STIM1 and Orai1. STIM1 is anchored in the ER membrane and senses changes in the ER luminal Ca²⁺ concentration. Orai1 is the Ca²⁺-selective, pore-forming CRAC channel component located in the plasma membrane (PM). Ca²⁺ store-depletion of the ER triggers activation of STIM1 proteins, which subsequently leads to a conformational change and oligomerization of STIM1 and its coupling to as well as activation of Orai1 channels at the ER-PM contact sites. Although STIM1 and Orai1 are sufficient for CRAC channel activation, their efficient activation and deactivation is fine-tuned by a variety of lipids and lipid- and/or ER-PM junction-dependent accessory proteins. The underlying mechanisms for lipid-mediated CRAC channel modulation as well as the still open questions, are presented in this review.

S-acylation by ZDHHC20 targets ORAI1 channels to lipid rafts for efficient Ca2+ signaling by Jurkat T cell receptors at the immune synapse

Efficient immune responses require Ca²⁺ fluxes across ORAI1 channels during engagement of T cell receptors (TCR) at the immune synapse (IS) between T cells and antigen presenting cells. Here, we show that ZDHHC20-mediated S-acylation of the ORAI1 channel at residue Cys143 promotes TCR recruitment and signaling at the IS. Cys143 mutations reduced ORAI1 currents and store-operated Ca²⁺ entry in HEK-293 cells and nearly abrogated long-lasting Ca²⁺ elevations, NFATC1 translocation, and IL-2 secretion evoked by TCR engagement in Jurkat T cells. The acylation-deficient channel remained in cholesterol-poor domains upon enforced ZDHHC20 expression and was recruited less efficiently to the IS along with actin and TCR. Our results establish S-acylation as a critical regulator of ORAI1 channel trafficking and function at the IS and reveal that ORAI1 S-acylation enhances TCR recruitment to the synapse.

Unveiling the Gating Mechanism of CRAC Channel: A Computational Study

CRAC channel is ubiquitous and its importance in the regulation of the immune system is testified by the severe immunodeficiencies caused by its mutations. In this work we took advantage of the availability of open and closed structures of this channel to run for the first time simulations of the whole gating process reaching the relevant time-scale with an enhanced sampling technique, Targeted Molecular Dynamics. Our simulations highlighted a complex allosteric propagation of the conformational change from peripheral helices, where the activator STIM1 binds, to the central pore helices. In agreement with mutagenesis data, our simulations revealed the key role of residue H206 whose displacement creates an empty space behind the hydrophobic region of the pore, thus releasing a steric brake and allowing the opening of the channel. Conversely, the process of pore closing culminates with the formation of a bubble that occludes the pore even in the absence of steric block. This mechanism, known as "hydrophobic gating", has been observed in an increasing number of biological ion channels and also in artificial nanopores. Our study therefore shows promise not only to better understand the molecular origin of diseases caused by disrupted calcium signaling, but also to clarify the mode of action of hydrophobically gated ion channels, possibly even suggesting strategies for the biomimetic design of synthetic nanopores.

Deciphering Molecular Mechanisms and Intervening in Physiological and Pathophysiological Processes of Ca2+ Signaling Mechanisms Using Optogenetic Tools

Calcium ion channels are involved in numerous biological functions such as lymphocyte activation, muscle contraction, neurotransmission, excitation, hormone secretion, gene expression, cell migration, memory, and aging. Therefore, their dysfunction can lead to a wide range of cellular abnormalities and, subsequently, to diseases. To date various conventional techniques have provided valuable insights into the roles of Ca2+ signaling. However, their limited spatiotemporal resolution and lack of reversibility pose significant obstacles in the detailed understanding of the structure-function relationship of ion channels. These drawbacks could be partially overcome by the use of optogenetics, which allows for the remote and well-defined manipulation of Ca2+-signaling. Here, we review the various optogenetic tools that have been used to achieve precise control over different Ca2+-permeable ion channels and receptors and associated downstream signaling cascades. We highlight the achievements of optogenetics as well as the still-open questions regarding the resolution of ion channel working mechanisms. In addition, we summarize the successes of optogenetics in manipulating many Ca2+-dependent biological processes both in vitro and in vivo. In summary, optogenetics has significantly advanced our understanding of Ca2+ signaling proteins and the used tools provide an essential basis for potential future therapeutic application.

The Impact of Mutation L138F/L210F on the Orai Channel: A Molecular Dynamics Simulation Study

The calcium release-activated calcium channel, composed of the Orai channel and the STIM protein, plays a crucial role in maintaining the Ca²⁺ concentration in cells. Previous studies showed that the L138F mutation in the human Orai1 creates a constitutively open channel independent of STIM, causing severe myopathy, but how the L138F mutation activates Orai1 is still unclear. Here, based on the crystal structure of Drosophila melanogaster Orai (dOrai), molecular dynamics simulations for the wild-type (WT) and the L210F (corresponding to L138F in the human Orai1) mutant were conducted to investigate their structural and dynamical properties. The results showed that the L210F dOrai mutant tends to have a more hydrated hydrophobic region (V174 to F171), as well as more dilated basic region (K163 to R155) and selectivity filter (E178). Sodium ions were located deeper in the mutant than in the wild-type. Further analysis revealed two local but essential conformational changes that may be the key to the activation. A rotation of F210, a previously unobserved feature, was found to result in the opening of the K163 gate through hydrophobic interactions. At the same time, a counter-clockwise rotation of F171 occurred more frequently in the mutant, resulting in a wider hydrophobic gate with more hydration. Ultimately, the opening of the two gates may facilitate the opening of the Orai channel independent of STIM.

Structural Insights into Ca2+ Permeation through Orai Channels

Orai channels belong to the calcium release-activated calcium (CRAC) channel family. Orai channels are responsible for the influx of extracellular Ca²⁺ that is triggered by Ca²⁺ depletion from the endoplasmic reticulum (ER); this function is essential for many types of non-excitable cells. Extensive structural and functional studies have advanced the knowledge of the molecular mechanism by which Orai channels are activated. However, the gating mechanism that allows Ca²⁺ permeation through Orai channels is less well explained. Here, we reviewed and summarized the existing structural studies of Orai channels. We detailed the structural features of Orai channels, described structural comparisons of their closed and open states, and finally proposed a "push-pull" model of Ca²⁺ permeation.

Isoform-Specific Properties of Orai Homologues in Activation, Downstream Signaling, Physiology and Pathophysiology

Ca2+ ion channels are critical in a variety of physiological events, including cell growth, differentiation, gene transcription and apoptosis. One such essential entry pathway for calcium into the cell is the Ca2+ release-activated Ca2+ (CRAC) channel. It consists of the Ca2+ sensing protein, stromal interaction molecule 1 (STIM1) located in the endoplasmic reticulum (ER) and a Ca2+ ion channel Orai in the plasma membrane. The Orai channel family includes three homologues Orai1, Orai2 and Orai3. While Orai1 is the "classical" Ca2+ ion channel within the CRAC channel complex and plays a universal role in the human body, there is increasing evidence that Orai2 and Orai3 are important in specific physiological and pathophysiological processes. This makes them an attractive target in drug discovery, but requires a detailed understanding of the three Orai channels and, in particular, their differences. Orai channel activation is initiated via Ca2+ store depletion, which is sensed by STIM1 proteins, and induces their conformational change and oligomerization. Upon STIM1 coupling, Orai channels activate to allow Ca2+ permeation into the cell. While this activation mechanism is comparable among the isoforms, they differ by a number of functional and structural properties due to non-conserved regions in their sequences. In this review, we summarize the knowledge as well as open questions in our current understanding of the three isoforms in terms of their structure/function relationship, downstream signaling and physiology as well as pathophysiology.

The fluoride permeation pathway and anion recognition in Fluc family fluoride channels

Fluc family fluoride channels protect microbes against ambient environmental fluoride by undermining the cytoplasmic accumulation of this toxic halide. These proteins are structurally idiosyncratic, and thus the permeation pathway and mechanism have no analogy in other known ion channels. Although fluoride-binding sites were identified in previous structural studies, it was not evident how these ions access aqueous solution, and the molecular determinants of anion recognition and selectivity have not been elucidated. Using x-ray crystallography, planar bilayer electrophysiology, and liposome-based assays, we identified additional binding sites along the permeation pathway. We used this information to develop an oriented system for planar lipid bilayer electrophysiology and observed anion block at one of these sites, revealing insights into the mechanism of anion recognition. We propose a permeation mechanism involving alternating occupancy of anion-binding sites that are fully assembled only as the substrate approaches.

Electrophysiological Properties from Computations at a Single Voltage: Testing Theory with Stochastic Simulations

We use stochastic simulations to investigate the performance of two recently developed methods for calculating the free energy profiles of ion channels and their electrophysiological properties, such as current–voltage dependence and reversal potential, from molecular dynamics simulations at a single applied voltage. These methods require neither knowledge of the diffusivity nor simulations at multiple voltages, which greatly reduces the computational effort required to probe the electrophysiological properties of ion channels. They can be used to determine the free energy profiles from either forward or backward one-sided properties of ions in the channel, such as ion fluxes, density profiles, committor probabilities, or from their two-sided combination. By generating large sets of stochastic trajectories, which are individually designed to mimic the molecular dynamics crossing statistics of models of channels of trichotoxin, p7 from hepatitis C and a bacterial homolog of the pentameric ligand-gated ion channel, GLIC, we find that the free energy profiles obtained from stochastic simulations corresponding to molecular dynamics simulations of even a modest length are burdened with statistical errors of only 0.3 kcal/mol. Even with many crossing events, applying two-sided formulas substantially reduces statistical errors compared to one-sided formulas. With a properly chosen reference voltage, the current–voltage curves can be reproduced with good accuracy from simulations at a single voltage in a range extending for over 200 mV. If possible, the reference voltages should be chosen not simply to drive a large current in one direction, but to observe crossing events in both directions.

Endoplasmic Reticulum-Plasma Membrane Contact Sites as an Organizing Principle for Compartmentalized Calcium and cAMP Signaling

In eukaryotic cells, ultimate specificity in activation and action-for example, by means of second messengers-of the myriad of signaling cascades is primordial. In fact, versatile and ubiquitous second messengers, such as calcium (Ca2+) and cyclic adenosine monophosphate (cAMP), regulate multiple-sometimes opposite-cellular functions in a specific spatiotemporal manner. Cells achieve this through segregation of the initiators and modulators to specific plasma membrane (PM) subdomains, such as lipid rafts and caveolae, as well as by dynamic close contacts between the endoplasmic reticulum (ER) membrane and other intracellular organelles, including the PM. Especially, these membrane contact sites (MCSs) are currently receiving a lot of attention as their large influence on cell signaling regulation and cell physiology is increasingly appreciated. Depletion of ER Ca2+ stores activates ER membrane STIM proteins, which activate PM-residing Orai and TRPC Ca2+ channels at ER-PM contact sites. Within the MCS, Ca2+ fluxes relay to cAMP signaling through highly interconnected networks. However, the precise mechanisms of MCS formation and the influence of their dynamic lipid environment on their functional maintenance are not completely understood. The current review aims to provide an overview of our current understanding and to identify open questions of the field.

Interrogating permeation and gating of Orai channels using chemical modification of cysteine residues

Chemical modification of ion channels using the substituted cysteine accessibility method has a rich and successful history in elucidating the structural basis of ion channel function. In this approach, cysteine residues are introduced in regions of interest into the protein and their accessibility to water soluble thiol-reactive reagents is determined by monitoring ion channel activity. Because a wide range of these reagents are available with differing size, charge, and membrane solubility, the physio-chemical environment of the introduced cysteine residue and therefore the protein domain of interest can be probed with great precision. The approach has been widely employed for determining the secondary structure of specific ion channel domains, the location and nature of the channel gate, and the conformational rearrangements in the channel pore that underlie the opening/closing of the pore. In this chapter, we describe the use of these and related approaches to probe the functional architecture and gating of store-operated Orai1 channels.

Transmembrane Domain 3 (TM3) Governs Orai1 and Orai3 Pore Opening in an Isoform-Specific Manner

STIM1-mediated activation of calcium selective Orai channels is fundamental for life. The three Orai channel isoforms, Orai1-3, together with their multiple ways of interplay, ensure their highly versatile role in a variety of cellular functions and tissues in both, health and disease. While all three isoforms are activated in a store-operated manner by STIM1, they differ in diverse biophysical and structural properties. In the present study, we provide profound evidence that non-conserved residues in TM3 control together with the cytosolic loop2 region the maintenance of the closed state and the configuration of an opening-permissive channel conformation of Orai1 and Orai3 in an isoform-specific manner. Indeed, analogous amino acid substitutions of these non-conserved residues led to distinct extents of gain- (GoF) or loss-of-function (LoF). Moreover, we showed that enhanced overall hydrophobicity along TM3 correlates with an increase in GoF mutant currents. Conclusively, while the overall activation mechanisms of Orai channels appear comparable, there are considerable variations in gating checkpoints crucial for pore opening. The elucidation of regions responsible for isoform-specific functional differences provides valuable targets for drug development selective for one of the three Orai homologs.

An open pore structure of the Orai channel, finally

Store-operated Orai channels are a primary mechanism for mobilizing Ca²⁺ signals in both non-excitable cells and excitable cells. The structure of the open channel, vital for understanding the mechanism of channel opening, is incompletely understood. We highlight a new study that unveils the structure of a constitutively active Orai mutant and takes us closer towards understanding the molecular basis of Orai channel activation.

Relevance of Membrane Contact Sites in Cancer Progression

Membrane contact sites (MCS) are typically defined as areas of proximity between heterologous or homologous membranes characterized by specific proteins. The study of MCS is considered as an emergent field that shows how crucial organelle interactions are in cell physiology. MCS regulate a myriad of physiological processes such as apoptosis, calcium, and lipid signaling, just to name a few. The membranal interactions between the endoplasmic reticulum (ER)–mitochondria, the ER–plasma membrane, and the vesicular traffic have received special attention in recent years, particularly in cancer research, in which it has been proposed that MCS regulate tumor metabolism and fate, contributing to their progression. However, as the therapeutic or diagnostic potential of MCS has not been fully revisited, in this review, we provide recent information on MCS relevance on calcium and lipid signaling in cancer cells and on its role in tumor progression. We also describe some proteins associated with MCS, like CERT, STIM1, VDAC, and Orai, that impact on cancer progression and that could be a possible diagnostic marker. Overall, these information might contribute to the understanding of the complex biology of cancer cells.

The Orai Pore Opening Mechanism

Cell survival and normal cell function require a highly coordinated and precise regulation of basal cytosolic Ca²⁺ concentrations. The primary source of Ca²⁺ entry into the cell is mediated by the Ca²⁺ release-activated Ca²⁺ (CRAC) channel. Its action is stimulated in response to internal Ca²⁺ store depletion. The fundamental constituents of CRAC channels are the Ca²⁺ sensor, stromal interaction molecule 1 (STIM1) anchored in the endoplasmic reticulum, and a highly Ca²⁺-selective pore-forming subunit Orai1 in the plasma membrane. The precise nature of the Orai1 pore opening is currently a topic of intensive research. This review describes how Orai1 gating checkpoints in the middle and cytosolic extended transmembrane regions act together in a concerted manner to ensure an opening-permissive Orai1 channel conformation. In this context, we highlight the effects of the currently known multitude of Orai1 mutations, which led to the identification of a series of gating checkpoints and the determination of their role in diverse steps of the Orai1 activation cascade. The synergistic action of these gating checkpoints maintains an intact pore geometry, settles STIM1 coupling, and governs pore opening. We describe the current knowledge on Orai1 channel gating mechanisms and summarize still open questions of the STIM1-Orai1 machinery.

CRAC channel opening is determined by a series of Orai1 gating checkpoints in the transmembrane and cytosolic regions

The initial activation step in the gating of ubiquitously expressed Orai1 calcium (Ca2+) ion channels represents the activation of the Ca2+-sensor protein STIM1 upon Ca2+ store depletion of the endoplasmic reticulum. Previous studies using constitutively active Orai1 mutants gave rise to, but did not directly test, the hypothesis that STIM1-mediated Orai1 pore opening is accompanied by a global conformational change of all Orai transmembrane domain (TM) helices within the channel complex. We prove that a local conformational change spreads omnidirectionally within the Orai1 complex. Our results demonstrate that these locally induced global, opening-permissive TM motions are indispensable for pore opening and require clearance of a series of Orai1 gating checkpoints. We discovered these gating checkpoints in the middle and cytosolic extended TM domain regions. Our findings are based on a library of double point mutants that contain each one loss-of-function with one gain-of-function point mutation in a series of possible combinations. We demonstrated that an array of loss-of-function mutations are dominant over most gain-of-function mutations within the same as well as of an adjacent Orai subunit. We further identified inter- and intramolecular salt-bridge interactions of Orai subunits as a core element of an opening-permissive Orai channel architecture. Collectively, clearance and synergistic action of all these gating checkpoints are required to allow STIM1 coupling and Orai1 pore opening. Our results unravel novel insights in the preconditions of the unique fingerprint of CRAC channel activation, provide a valuable source for future structural resolutions, and help to understand the molecular basis of disease-causing mutations.

Molecular Choreography and Structure of Ca2+ Release-Activated Ca2+ (CRAC) and KCa2+ Channels and Their Relevance in Disease with Special Focus on Cancer

Ca2+ ions play a variety of roles in the human body as well as within a single cell. Cellular Ca2+ signal transduction processes are governed by Ca2+ sensing and Ca2+ transporting proteins. In this review, we discuss the Ca2+ and the Ca2+-sensing ion channels with particular focus on the structure-function relationship of the Ca2+ release-activated Ca2+ (CRAC) ion channel, the Ca2+-activated K+ (KCa2+) ion channels, and their modulation via other cellular components. Moreover, we highlight their roles in healthy signaling processes as well as in disease with a special focus on cancer. As KCa2+ channels are activated via elevations of intracellular Ca2+ levels, we summarize the current knowledge on the action mechanisms of the interplay of CRAC and KCa2+ ion channels and their role in cancer cell development.