Essential role for the CRAC activation domain in store-dependent oligomerization of STIM1

A protease-independent function for SPPL3 in NFAT activation

The signal peptide peptidase (SPP)-related intramembrane aspartyl proteases are a homologous group of polytopic membrane proteins, some of which function in innate or adaptive immunity by cleaving proteins involved in antigen presentation or intracellular signaling. Signal peptide peptidase-like 3 (SPPL3) is a poorly characterized endoplasmic reticulum (ER)-localized member of this family, with no validated cellular substrates. We report here the isolation of SPPL3 in a screen for activators of NFAT, a transcription factor that controls lymphocyte development and function. We find that SPPL3 is required downstream of T cell receptor engagement for maximal Ca(2+) influx and NFAT activation. Surprisingly, the proteolytic activity of SPPL3 is not required for its role in this pathway. SPPL3 enhances the signal-induced association of stromal interaction molecule 1 (STIM1) and Orai1 and is even required for the full activity of constitutively active STIM1 variants that bind Orai1 independently of ER Ca(2+) release. SPPL3 associates with STIM1 through at least two independent domains, the transmembrane region and the CRAC activation domain (CAD), and can promote the association of the STIM1 CAD with Orai1. Our results assign a function in lymphocyte signaling to SPPL3 and highlight the emerging importance of nonproteolytic functions for members of the intramembrane aspartyl protease family.

A coiled-coil clamp controls both conformation and clustering of stromal interaction molecule 1 (STIM1)

Store-operated Ca²⁺ entry, essential for the adaptive immunity, is initiated by the endoplasmic reticulum (ER) Ca²⁺ sensor STIM1. Ca²⁺ entry occurs through the plasma membrane resident Ca²⁺ channel Orai1 that directly interacts with the C-terminal STIM1 domain, named SOAR/CAD. Depletion of the ER Ca²⁺ store controls this STIM1/Orai1 interaction via transition to an extended STIM1 C-terminal conformation, exposure of the SOAR/CAD domain, and STIM1/Orai1 co-clustering. Here we developed a novel approach termed FRET-derived Interaction in a Restricted Environment (FIRE) in an attempt to dissect the interplay of coiled-coil (CC) interactions in controlling STIM1 quiescent as well as active conformation and cluster formation. We present evidence of a sequential activation mechanism in the STIM1 cytosolic domains where the interaction between CC1 and CC3 segment regulates both SOAR/CAD exposure and CC3-mediated higher-order oligomerization as well as cluster formation. These dual levels of STIM1 auto-inhibition provide efficient control over the coupling to and activation of Orai1 channels.

Potent functional uncoupling between STIM1 and Orai1 by dimeric 2-aminodiphenyl borinate analogs

The coupling of ER Ca(2+)-sensing STIM proteins and PM Orai Ca(2+) entry channels generates "store-operated" Ca(2+) signals crucial in controlling responses in many cell types. The dimeric derivative of 2-aminoethoxydiphenyl borinate (2-APB), DPB162-AE, blocks functional coupling between STIM1 and Orai1 with an IC50 (200 nM) 100-fold lower than 2-APB. Unlike 2-APB, DPB162-AE does not affect L-type or TRPC channels or Ca(2+) pumps at maximal STIM1-Orai1 blocking levels. DPB162-AE blocks STIM1-induced Orai1 or Orai2, but does not block Orai3 or STIM2-mediated effects. We narrowed the DPB162-AE site of action to the STIM-Orai activating region (SOAR) of STIM1. DPB162-AE does not prevent the SOAR-Orai1 interaction but potently blocks SOAR-mediated Orai1 channel activation, yet its action is not as an Orai1 channel pore blocker. Using the SOAR-F394H mutant which prevents both physical and functional coupling to Orai1, we reveal DPB162-AE rapidly restores SOAR-Orai binding but only slowly restores Orai1 channel-mediated Ca(2+) entry. With the same SOAR mutant, 2-APB induces rapid physical and functional coupling to Orai1, but channel activation is transient. We infer that the actions of both 2-APB and DPB162-AE are directed toward the STIM1-Orai1 coupling interface. Compared to 2-APB, DPB162-AE is a much more potent and specific STIM1/Orai1 functional uncoupler. DPB162-AE provides an important pharmacological tool and a useful mechanistic probe for the function and coupling between STIM1 and Orai1 channels.

Structural and functional mechanisms of CRAC channel regulation

In many animal cells, stimulation of cell surface receptors coupled to G proteins or tyrosine kinases mobilizes Ca(2+) influx through store-operated Ca(2+)-release-activated Ca(2+) (CRAC) channels. The ensuing Ca(2+) entry regulates a wide variety of effector cell responses including transcription, motility, and proliferation. The physiological importance of CRAC channels for human health is underscored by studies indicating that mutations in CRAC channel genes produce a spectrum of devastating diseases including chronic inflammation, muscle weakness, and a severe combined immunodeficiency syndrome. Moreover, from a basic science perspective, CRAC channels exhibit a unique biophysical fingerprint characterized by exquisite Ca(2+) selectivity, store-operated gating, and distinct pore properties and therefore serve as fascinating model ion channels for understanding the biophysical mechanisms of Ca(2+) selectivity and channel opening. Studies in the last two decades have revealed the cellular and molecular choreography of the CRAC channel activation process, and it is now established that opening of CRAC channels is governed through direct interactions between the pore-forming Orai proteins and the endoplasmic reticulum Ca(2+) sensors STIM1 and STIM2. In this review, we summarize the functional and structural mechanisms of CRAC channel regulation, focusing on recent advances in our understanding of the conformational and structural dynamics of CRAC channel gating.

Redox regulation of store-operated Ca2+ entry

SIGNIFICANCE

Store-operated Ca2+ entry (SOCE) is a ubiquitous Ca2+ signaling mechanism triggered by Ca2+ depletion of the endoplasmic reticulum (ER) and by a variety of cellular stresses. Reactive oxygen species (ROS) are often concomitantly produced in response to these stresses, however, the relationship between redox signaling and SOCE is not completely understood. Various cardiovascular, neurological, and immune diseases are associated with alterations in both Ca2+ signaling and ROS production, and thus understanding this relationship has therapeutic implications.

RECENT ADVANCES

Several reactive cysteine modifications in stromal interaction molecule (STIM) and Orai proteins comprising the core SOCE machinery were recently shown to modulate SOCE in a redox-dependent manner. Moreover, STIM1 and Orai1 expression levels may reciprocally regulate and be affected by responses to oxidative stress. ER proteins involved in oxidative protein folding have gained increased recognition as important sources of ROS, and the recent discovery of their accumulation in contact sites between the ER and mitochondria provides a further link between ROS production and intracellular Ca2+ handling.

CRITICAL ISSUES AND FUTURE DIRECTIONS

Future research should aim to establish the complete set of SOCE controlling molecules, to determine their redox-sensitive residues, and to understand how intracellular Ca2+ stores dynamically respond to different types of stress. Mapping the precise nature and functional consequence of key redox-sensitive components of the pre- and post-translational control of SOCE machinery and of proteins regulating ER calcium content will be pivotal in advancing our understanding of the complex cross-talk between redox and Ca2+ signaling.

Single-molecule analysis of diffusion and trapping of STIM1 and Orai1 at endoplasmic reticulum-plasma membrane junctions

STIM1 and Orai1 move to endoplasmic reticulum–plasma membrane (ER-PM) junctions to trigger store-operated Ca²⁺ entry. Single-molecule tracking reveals pure diffusion of STIM1 in the ER, subdiffusion of Orai1 in the PM, and trapping at junctions via STIM-Orai binding. STIM-Orai binding is loose, generating free proteins that can exchange freely with extrajunctional pools.

Following endoplasmic reticulum (ER) Ca²⁺ depletion, STIM1 and Orai1 complexes assemble autonomously at ER–plasma membrane (PM) junctions to trigger store-operated Ca²⁺ influx. One hypothesis to explain this process is a diffusion trap in which activated STIM1 diffusing in the ER becomes trapped at junctions through interactions with the PM, and STIM1 then traps Orai1 in the PM through binding of its calcium release-activated calcium activation domain. We tested this model by analyzing STIM1 and Orai1 diffusion using single-particle tracking, photoactivation of protein ensembles, and Monte Carlo simulations. In resting cells, STIM1 diffusion is Brownian, while Orai1 is slightly subdiffusive. After store depletion, both proteins slow to the same speeds, consistent with complex formation, and are confined to a corral similar in size to ER–PM junctions. While the escape probability at high STIM:Orai expression ratios is <1%, it is significantly increased by reducing the affinity of STIM1 for Orai1 or by expressing the two proteins at comparable levels. Our results provide direct evidence that STIM-Orai complexes are trapped by their physical connections across the junctional gap, but also reveal that the complexes are surprisingly dynamic, suggesting that readily reversible binding reactions generate free STIM1 and Orai1, which engage in constant diffusional exchange with extrajunctional pools.

STIM1/Orai1 coiled-coil interplay in the regulation of store-operated calcium entry

Orai1 calcium channels in the plasma membrane are activated by stromal interaction molecule-1 (STIM1), an endoplasmic reticulum calcium sensor, to mediate store-operated calcium entry (SOCE). The cytosolic region of STIM1 contains a long putative coiled-coil (CC)1 segment and shorter CC2 and CC3 domains. Here we present solution nuclear magnetic resonance structures of a trypsin-resistant CC1–CC2 fragment in the apo and Orai1-bound states. Each CC1–CC2 subunit forms a U-shaped structure that homodimerizes through antiparallel interactions between equivalent α-helices. The CC2:CC2′ helix pair clamps two identical acidic Orai1 C-terminal helices at opposite ends of a hydrophobic/basic STIM–Orai association pocket. STIM1 mutants disrupting CC1:CC1′ interactions attenuate, while variants promoting CC1 stability spontaneously activate Orai1 currents. CC2 mutations cause remarkable variability in Orai1 activation because of a dual function in binding Orai1 and autoinhibiting STIM1 oligomerization via interactions with CC3. We conclude that SOCE is activated through dynamic interplay between STIM1 and Orai1 helices.

When endoplasmic reticulum calcium levels are low, STIM1 binds to and opens Orai1 channels in the plasma membrane to replenish calcium stores. Stathopulos et al. present solution structures of the STIM1 coiled-coil domain in the presence and absence of Orai1, revealing the structural basis for this interaction.

Atomic force microscopy (AFM) imaging suggests that stromal interaction molecule 1 (STIM1) binds to Orai1 with sixfold symmetry

Depletion of Ca(2+) from the endoplasmic reticulum (ER) lumen triggers the opening of Ca(2+) release-activated Ca(2+) (CRAC) channels at the plasma membrane. CRAC channels are activated by stromal interaction molecule 1 (STIM1), an ER resident protein that senses Ca(2+) store depletion and interacts with Orai1, the pore-forming subunit of the channel. The subunit stoichiometry of the CRAC channel is controversial. Here we provide evidence, using atomic force microscopy (AFM) imaging, that Orai1 assembles as a hexamer, and that STIM1 binds to Orai1 with sixfold symmetry. STIM1 associates with Orai1 in the form of monomers, dimers, and multimeric string-like structures that form links between the Orai1 hexamers. Our results provide new insights into the nature of the interactions between STIM1 and Orai1.

Molecular regulation of the pore component of CRAC channels, Orai1

Store-operated Ca(2+) entry (SOCE) is a fundamental mechanism ubiquitously employed by cells to elevate intracellular Ca(2+) concentrations ([Ca(2+)]i). Increased intracellular Ca(2+) ions act as a second messenger that can stimulate a variety of downstream signaling pathways affecting proliferation, secretion, differentiation, and death of cells. In immune cells, immune receptor stimulation induces endoplasmic reticulum Ca(2+) store depletion that subsequently activates Ca(2+)-release-activated-Ca(2+) (CRAC) channels, a prototype of store-operated Ca(2+) (SOC) channels. Identification of Orai1 as the pore subunit of CRAC channels has provided the much-needed molecular tool to dissect the mechanism of activation and regulation of these channels. In this review, we discuss the recent advances in understanding the regulatory mechanisms and posttranslational modifications that regulate diverse aspects of CRAC channel function.

Distinct Orai-coupling domains in STIM1 and STIM2 define the Orai-activating site

STIM1 and STIM2 are widely expressed endoplasmic reticulum (ER) Ca(2+) sensor proteins able to translocate within the ER membrane to physically couple with and gate plasma membrane Orai Ca(2+) channels. Although they are structurally similar, we reveal critical differences in the function of the short STIM-Orai-activating regions (SOAR) of STIM1 and STIM2. We narrow these differences in Orai1 gating to a strategically exposed phenylalanine residue (Phe-394) in SOAR1, which in SOAR2 is substituted by a leucine residue. Remarkably, in full-length STIM1, replacement of Phe-394 with the dimensionally similar but polar histidine head group prevents both Orai1 binding and gating, creating an Orai1 non-agonist. Thus, this residue is critical in tuning the efficacy of Orai activation. While STIM1 is a full Orai1-agonist, leucine-replacement of this crucial residue in STIM2 endows it with partial agonist properties, which may be critical for limiting Orai1 activation stemming from its enhanced sensitivity to store-depletion.

Structural aspects of calcium-release activated calcium channel function

Store-operated calcium (Ca(2+)) entry is the process by which molecules located on the endo/sarcoplasmic reticulum (ER/SR) respond to decreased luminal Ca(2+) levels by signaling Ca(2+) release activated Ca(2+) channels (CRAC) channels to open on the plasma membrane (PM). This activation of PM CRAC channels provides a sustained cytosolic Ca(2+) elevation associated with myriad physiological processes. The identities of the molecules which mediate SOCE include stromal interaction molecules (STIMs), functioning as the ER/SR luminal Ca(2+) sensors, and Orai proteins, forming the PM CRAC channels. This review examines the current available high-resolution structural information on these CRAC molecular components with particular focus on the solution structures of the luminal STIM Ca(2+) sensing domains, the crystal structures of cytosolic STIM fragments, a closed Orai hexameric crystal structure and a structure of an Orai1 N-terminal fragment in complex with calmodulin. The accessible structural data are discussed in terms of potential mechanisms of action and cohesiveness with functional observations.

Oligomerization and Ca2+/calmodulin control binding of the ER Ca2+-sensors STIM1 and STIM2 to plasma membrane lipids

Ca²⁺ (calcium) homoeostasis and signalling rely on physical contacts between Ca²⁺ sensors in the ER (endoplasmic reticulum) and Ca²⁺ channels in the PM (plasma membrane). STIM1 (stromal interaction molecule 1) and STIM2 Ca²⁺ sensors oligomerize upon Ca²⁺ depletion in the ER lumen, contact phosphoinositides at the PM via their cytosolic lysine (K)-rich domains, and activate Ca²⁺ channels. Differential sensitivities of STIM1 and STIM2 towards ER luminal Ca²⁺ have been studied but responses towards elevated cytosolic Ca²⁺ concentration and the mechanism of lipid binding remain unclear. We found that tetramerization of the STIM1 K-rich domain is necessary for efficient binding to PI(4,5)P₂-containing PM-like liposomes consistent with an oligomerization-driven STIM1 activation. In contrast, dimerization of STIM2 K-rich domain was sufficient for lipid binding. Furthermore, the K-rich domain of STIM2, but not of STIM1, forms an amphipathic α-helix. These distinct features of the STIM2 K-rich domain cause an increased affinity for PI(4,5)P₂, consistent with the lower activation threshold of STIM2 and a function as regulator of basal Ca²⁺ levels. Concomitant with higher affinity for PM lipids, binding of CaM (calmodulin) inhibited the interaction of the STIM2 K-rich domain with liposomes in a Ca²⁺ and PI(4,5)P₂ concentration-dependent manner. Therefore we suggest that elevated cytosolic Ca²⁺ concentration down-regulates STIM2-mediated ER–PM contacts via CaM binding.

17β-Estradiol inhibits phosphorylation of stromal interaction molecule 1 (STIM1) protein: implication for store-operated calcium entry and chronic lung diseases

Sex plays a significant role in the development of lung diseases including asthma, cancer, chronic bronchitis, and cystic fibrosis. In cystic fibrosis, 17β-estradiol (E2) may inhibit store-operated Ca(2+) entry (SOCE) to impinge upon airway secretions, leaving females at greater risk of contracting lung infections. Stromal interaction molecule 1 (STIM1)-mediated SOCE is essential for cell homeostasis and regulates numerous processes including cell proliferation, smooth muscle contraction, and secretion. E2 can signal nongenomically to modulate Ca(2+) signaling, but little is known of the underlying mechanisms. We found that E2 exposure inhibited STIM1 translocation in airway epithelia, preventing SOCE. This correlated with a decrease in STIM1-STIM1 FRET and STIM1 mobility in E2-exposed HEK293T cells co-expressing estrogen receptor α. We also examined the role of STIM1 phosphorylation in E2-mediated inhibition of STIM1 mobility. STIM1 is basally phosphorylated at serine 575, which is required for SOCE. Exposure to E2 significantly decreased STIM1 serine phosphorylation. Mutating serine 575 to an alanine blocked STIM1 phosphorylation, reduced basal STIM1 mobility, and rendered STIM1 insensitive to E2. These data indicate that E2 can signal nongenomically by inhibiting basal phosphorylation of STIM1, leading to a reduction in SOCE.

The inhibitory helix controls the intramolecular conformational switching of the C-terminus of STIM1

Store-operated Ca(2+) entry (SOCE) is a critical Ca(2+) signaling pathway in many cell types. After sensing Ca(2+) store depletion in the endoplasmic reticulum (ER) lumen, STIM1 (STromal Interaction Molecule 1) oligomerizes and then interacts with and activates the Orai1 calcium channel. Our previous research has demonstrated that the inhibitory helix (IH) adjacent to the first coiled-coil region (CC1) of STIM1 may keep the whole C-terminus of STIM1 in an inactive state. However, the specific conformational change of CC1-IH that drives the transition of STIM1 from the resting state to the active state remains elusive. Herein, we report the structural analysis of CC1-IH, which revealed that the entire CC1-IH molecule forms a very long helix. Structural and biochemical analyses indicated that IH, and not the CC1 region, contributes to the oligomerization of STIM1. Small-angle X-ray scattering (SAXS) analysis suggested that the C-terminus of STIM1 including the IH region displays a collapsed conformation, whereas the construct without the IH region has an extended conformation. These two conformations may correspond to the conformational states of the C-terminus of STIM1 before and after activation. Taken together, our results provide direct biochemical evidence that the IH region controls the conformational switching of the C-terminus of STIM1.

STIM1 negatively regulates Ca²⁺ release from the sarcoplasmic reticulum in skeletal myotubes

STIM1 (stromal interaction molecule 1) mediates SOCE (store-operated Ca²⁺ entry) in skeletal muscle. However, the direct role(s) of STIM1 in skeletal muscle, such as Ca²⁺ release from the SR (sarcoplasmic reticulum) for muscle contraction, have not been identified. The times required for the maximal expression of endogenous STIM1 or Orai1, or for the appearance of puncta during the differentiation of mouse primary skeletal myoblasts to myotubes, were all different, and the formation of puncta was detected with no stimulus during differentiation, suggesting that, in skeletal muscle, the formation of puncta is a part of the differentiation. Wild-type STIM1 and two STIM1 mutants (Triple mutant, missing Ca²⁺-sensing residues but possessing the intact C-terminus; and E136X, missing the C-terminus) were overexpressed in the myotubes. The wild-type STIM1 increased SOCE, whereas neither mutant had an effect on SOCE. It was interesting that increases in the formation of puncta were observed in the Triple mutant as well as in wild-type STIM1, suggesting that SOCE-irrelevant puncta could exist in skeletal muscle. On the other hand, overexpression of wild-type or Triple mutant, but not E136X, attenuated Ca²⁺ releases from the SR in response to KCl [evoking ECC (excitation-contraction coupling) via activating DHPR (dihydropyridine receptor)] in a dominant-negative manner. The attenuation was removed by STIM1 knockdown, and STIM1 was co-immunoprecipitated with DHRP in a Ca²⁺-independent manner. These results suggest that STIM1 negatively regulates Ca²⁺ release from the SR through the direct interaction of the STIM1 C-terminus with DHPR, and that STIM1 is involved in both ECC and SOCE in skeletal muscle.

Exploring the unique features of the ARC channel, a store-independent Orai channel

The discovery of the Orai proteins, and the identification of STIM1 as the molecule that regulates them, was based on their role in the agonist-activated store-operated entry of calcium via the CRAC channels. However, these same proteins are also essential components of the ARC channels responsible for a similar agonist-activated, but store-independent, arachidonic acid-regulated entry of calcium. The fact that these 2 biophysically similar calcium entry pathways frequently co-exist in the same cells suggests that they must each possess different features that allow them to function in distinct ways to regulate specific cellular activities. This review begins to address this question by describing recent findings characterizing the unique features of the ARC channels--their molecular composition, STIM1-dependent activation, and physiological activities--and the importance of defining such features for the accurate therapeutic targeting of these 2 Orai channel subtypes.

The STIM1/Orai signaling machinery

Ca(2+) influx via store-operated Ca(2+) release activated Ca(2+) (CRAC) channels represents a main signaling pathway for T-cell activation as well as mast-cell degranulation. The ER-located Ca(2+)-sensor, STIM1 and the Ca(2+)-selective ion pore, Orai1 in the membrane are sufficient to fully reconstitute CRAC currents. Their identification, but even more the recent structural resolution of both proteins by X-ray crystallography has substantially advanced the understanding of the activation mechanism of CRAC channels. In this review, we provide a detailed description of the STIM1/Orai1 signaling pathway thereby focusing on the critical domains mediating both, intra- as well as intermolecular interactions and on the ion permeation pathway. Based on the results of functional studies as well as the recently published crystal structures, we portray a mechanistic view of the steps in the CRAC channel signaling cascade ranging from STIM1 oligomerization over STIM1-Orai1 coupling to the ultimate Orai1 channel activation and permeation.

Molecular pharmacology of store-operated CRAC channels

Calcium influx through store-operated Ca(2+) release-activated Ca(2+) channels (CRAC channels) is a well-defined mechanism of generating cellular Ca(2+) elevations that regulates many functions including gene expression, exocytosis and cell proliferation. The identifications of the ER Ca(2+) sensing proteins, STIM1-2 and the CRAC channel proteins, Orai1-3, have led to improved understanding of the physiological roles and the activation mechanism of CRAC channels. Defects in CRAC channel function are associated with serious human diseases such as immunodeficiency and auto-immunity. In this review, we discuss several pharmacological modulators of CRAC channels, focusing specifically on the molecular mechanism of drug action and their utility in illuminating the mechanism of CRAC channel operation and their physiological roles in different cells.

STIM1/Orai1-mediated SOCE: current perspectives and potential roles in cardiac function and pathology

Store-operated Ca²⁺ entry (SOCE) is critical for Ca²⁺ signaling in nonexcitable cells; however, its role in the regulation of cardiomyocyte Ca²⁺ homeostasis has only recently been investigated. The increased understanding of the role of stromal interaction molecule 1 (STIM1) in regulating SOCE combined with recent studies demonstrating the presence of STIM1 in cardiomyocytes provides support that this pathway co-exists in the heart with the more widely recognized Ca²⁺ handling pathways associated with excitation-contraction coupling. There is now substantial evidence that STIM1-mediated SOCE plays a key role in mediating cardiomyocyte hypertrophy, both in vitro and in vivo, and there is growing support for the contribution of SOCE to Ca²⁺ overload associated with ischemia/reperfusion injury. Here, we provide an overview of our current understanding of the molecular regulation of SOCE and discuss the evidence supporting the role of STIM1/Orai1-mediated SOCE in regulating cardiomyocyte function.

Mechanisms of STIM1 activation of store-independent leukotriene C4-regulated Ca2+ channels

We recently showed, in primary vascular smooth muscle cells (VSMCs), that the platelet-derived growth factor activates canonical store-operated Ca(2+) entry and Ca(2+) release-activated Ca(2+) currents encoded by Orai1 and STIM1 genes. However, thrombin activates store-independent Ca(2+) selective channels contributed by both Orai3 and Orai1. These store-independent Orai3/Orai1 channels are gated by cytosolic leukotriene C4 (LTC4) and require STIM1 downstream LTC4 action. However, the source of LTC4 and the signaling mechanisms of STIM1 in the activation of this LTC4-regulated Ca(2+) (LRC) channel are unknown. Here, we show that upon thrombin stimulation, LTC4 is produced through the sequential activities of phospholipase C, diacylglycerol lipase, 5-lipo-oxygenease, and leukotriene C4 synthase. We show that the endoplasmic reticulum-resident STIM1 is necessary and sufficient for LRC channel activation by thrombin. STIM1 does not form sustained puncta and does not colocalize with Orai1 either under basal conditions or in response to thrombin. However, STIM1 is precoupled to Orai3 and Orai3/Orai1 channels under basal conditions as shown using Forster resonance energy transfer (FRET) imaging. The second coiled-coil domain of STIM1 is required for coupling to either Orai3 or Orai3/Orai1 channels and for LRC channel activation. We conclude that STIM1 employs distinct mechanisms in the activation of store-dependent and store-independent Ca(2+) entry pathways.

Initial activation of STIM1, the regulator of store-operated calcium entry

Physiological Ca²⁺ signalling in T lymphocytes and other cells depends on the STIM-ORAI pathway of store-operated Ca²⁺ entry. STIM1 and STIM2 are Ca²⁺ sensors located in the endoplasmic reticulum (ER) membrane, with ER-luminal domains that monitor cellular Ca²⁺ stores and cytoplasmic domains that gate ORAI channels in the plasma membrane. The STIM ER-luminal domain dimerizes or oligomerizes upon dissociation of Ca²⁺, but the mechanism transmitting activation to the STIM cytoplasmic domain has not been defined. Here we demonstrate, using Tb³⁺–acceptor energy transfer, that dimerization of STIM1 ER-luminal domains can initiate an extensive conformational change in murine STIM1 cytoplasmic domains. The conformational change, triggered by apposition of the predicted coiled-coil 1 (CC1) regions, releases the ORAI-activating domains from their interaction with the CC1 regions and allows physical extension of the STIM1 cytoplasmic domain across the gap between ER and plasma membrane to communicate with ORAI channels.

Molecular basis of activation of the arachidonate-regulated Ca2+ (ARC) channel, a store-independent Orai channel, by plasma membrane STIM1

Currently, Orai proteins are known to encode two distinct agonist-activated, highly calcium-selective channels: the store-operated Ca(2+) release-activated Ca(2+) (CRAC) channels, and the store-independent, arachidonic acid-activated ARC channels. Surprisingly, whilst the trigger for activation of these channels is entirely different, both depend on stromal interacting molecule 1 (STIM1). However, whilst STIM1 in the endoplasmic reticulum membrane is the critical sensor for the depletion of this calcium store that triggers CRAC channel activation, it is the pool of STIM1 constitutively resident in the plasma membrane that is essential for activation of the ARC channels. Here, using a variety of approaches, we show that the key domains within the cytosolic part of STIM1 identified as critical for the activation of CRAC channels are also key for activation of the ARC channels. However, examination of the actual steps involved in such activation reveal marked differences between these two Orai channel types. Specifically, loss of calcium from the EF-hand of STIM1 that forms the key initiation point for activation of the CRAC channels has no effect on ARC channel activity. Secondly, in marked contrast to the dynamic and labile nature of interactions between STIM1 and the CRAC channels, STIM1 in the plasma membrane appears to be constitutively associated with the ARC channels. Finally, specific mutations in STIM1 that induce an extended, constitutively active, conformation for the CRAC channels actually prevent activation of the ARC channels by arachidonic acid. Based on these findings, we propose that the likely role of arachidonic acid lies in inducing the actual gating of the channel.

Intramolecular shielding maintains the ER Ca²⁺ sensor STIM1 in an inactive conformation

Store-operated calcium entry (SOCE) represents a major calcium influx pathway in non-excitable cells and is central to many physiological processes such as T cell activation and mast cell degranulation. SOCE is activated through intricate coordination between the Ca(2+) sensor on the ER membrane (stromal interaction molecule 1, STIM1) and the plasma membrane channel Orai1. When Ca(2+) stores are depleted, STIM1 oligomerizes and physically interacts with Orai1 through its SOAR/CAD domain, resulting in Orai1 gating and Ca(2+) influx. Here, we describe novel inter- and intramolecular FRET sensors in the context of the full-length membrane-anchored STIM1, and show that STIM1 undergoes a conformational change in response to store depletion to adopt a stretched 'open' conformation that exposes SOAR/CAD and allows it to interact with Orai1. Mutational analyses reveal that electrostatic interactions between the predicted first and third coiled-coil domains of STIM1 are not involved in maintaining the 'closed' inactive conformation. In addition, the results argue that an amphipathic α-helix between residues 317 and 336 in the so-called inhibitory domain is important to maintain STIM1 in a closed conformation at rest. Indeed, mutations that alter the amphipathic properties of this helix result in a STIM1 variant that is unable to respond to store depletion in terms of forming puncta, translocation to the cortical ER or activating Orai1.

Multifaceted roles of STIM proteins

Stromal interaction molecules (STIM1 and STIM2) are critical components of store-operated calcium entry. Sensing depletion of endoplasmic reticulum (ER) Ca(2+) stores, STIM couples with plasma membrane Orai channels, resulting in the influx of Ca(2+) across the PM into the cytosol. Although best recognized for their primary role as ER Ca(2+) sensors, increasing evidence suggests that STIM proteins have a broader variety of sensory capabilities than first envisaged, reacting to cell stressors such as oxidative stress, temperature, and hypoxia. Further, the array of partners for STIM proteins is now understood to range far beyond the Orai channel family. Here we discuss the implications of STIM's expanding role, both as a stress sensor and a general modulator of multiple physiological processes in the cell.

Orai channel pore properties and gating by STIM: implications from the Orai crystal structure

The Orai channels are unusual, yet prominent, calcium (Ca2+) signal mediators in most cell types. Orai proteins are structurally unique, having little sequence homology with other ion channels. They are also functionally unique with exceedingly high selectivity for Ca2+, mediating both short-term Ca2+ homeostasis and long-term Ca2+ signals important for transcriptional control. Operating in the plasma membrane (PM), Orai channel regulation is unprecedented among ion channels; channel gating occurs through an elaborate intermembrane coupling with stromal interaction molecule (STIM) proteins in the endoplasmic reticulum (ER). STIM proteins function as sensors of Ca2+ stored in the ER lumen and translocate into ER-PM junctions to tether and activate Orai channels when ER Ca2+ concentration decreases. Crystallization studies reveal an unexpected hexameric structure for the Orai channel and provide important insights into the pore architecture, the structural basis of its unusual cation selectivity, and how channel gating occurs through its coupling with STIM proteins.

Regulation of CRAC channels by protein interactions and post-translational modification

Store-operated Ca(2+) entry (SOCE) is a widespread mechanism to elevate the intracellular Ca(2+) concentrations and stimulate downstream signaling pathways affecting proliferation, secretion, differentiation and death in different cell types. In immune cells, immune receptor stimulation induces intracellular Ca(2+) store depletion that subsequently activates Ca(2+)-release-activated-Ca(2+) (CRAC) channels, a prototype of store-operated Ca(2+) (SOC) channels. CRAC channel opening leads to activation of diverse downstream signaling pathways affecting proliferation, differentiation, cytokine production and cell death. Recent identification of STIM1 as the endoplasmic reticulum Ca(2+) sensor and Orai1 as the pore subunit of CRAC channels has provided the much-needed molecular tools to dissect the mechanism of activation and regulation of CRAC channels. In this review, we discuss the recent advances in understanding the associating partners and posttranslational modifications of Orai1 and STIM1 proteins that regulate diverse aspects of CRAC channel function.

Subdiffusion of proteins and oligomers on membranes

Diffusion of proteins on lipid membranes plays a central role in cell signaling processes. From a mathematical perspective, most membrane diffusion processes are explained by the Saffman-Delbrück theory. However, recent studies have suggested a major limitation in the theoretical framework, the lack of complexity in the modeled lipid membrane. Lipid domains (sometimes termed membrane rafts) are known to slow protein diffusion, but there have been no quantitative theoretical examinations of how much diffusion is slowed in a general case. We provide an overall theoretical framework for confined-domain ("corralled") diffusion. Further, there have been multiple apparent contradictions of the basic conclusions of Saffman and Delbrück, each involving cases in which a single protein or an oligomer has multiple transmembrane regions passing through a lipid phase barrier. We present a set of corrections to the Saffman-Delbrück theory to account for these experimental observations. Our corrections are able to provide a quantitative explanation of numerous cellular signaling processes that have been considered beyond the scope of the Saffman-Delbrück theory, and may be extendable to other forms of subdiffusion.

Structure and function of endoplasmic reticulum STIM calcium sensors

Store-operated calcium (Ca(2+)) entry (SOCE) is a vital Ca(2+) signaling pathway in nonexcitable as well as electrically excitable cells, regulating countless physiological and pathophysiological pathways. Stromal interaction molecules (STIMs) are the principal regulating molecules of SOCE, sensing changes in sarco-/endoplasmic reticulum (S/ER) luminal Ca(2+) levels and directly interacting with the Orai channel subunits to orchestrate the opening of Ca(2+) release-activated Ca(2+) (CRAC) channels. Recent atomic resolution structures on human STIM1 and STIM2 have illuminated critical mechanisms of STIM function in SOCE; further, the first high-resolution structure of the Drosophila melanogaster Orai channel has revealed vital data on the atomic composition of the CRAC channel pore and the assembly of individual Orai subunits. This chapter focuses on the mechanistic information garnered from these high-resolution structures and the supporting biophysical, biochemical, and live cell work that has enhanced our understanding of the relationship between STIM and Orai structural features and CRAC channel function.

STIM-ORAI interactions that control the CRAC channel

The Ca(2+) sensor STIM1 and the Ca(2+) channel ORAI1 are the fundamental working machinery of the CRAC channel, a classical pathway for store-operated Ca(2+) entry. This chapter focuses on the protein-protein interactions of STIM and ORAI proteins that control the channel.

Calcium signaling in glioma cells--the role of nucleotide receptors

Calcium signaling is probably one of the evolutionary oldest and the most common way by which the signal can be transmitted from the cell environment to the cytoplasmic calcium binding effectors. Calcium signal is fast and due to diversity of calcium binding proteins it may have a very broad effect on cell behavior. Being a crucial player in neuronal transmission it is also very important for glia physiology. It is responsible for the cross-talk between neurons and astrocytes, for microglia activation and motility. Changes in calcium signaling are also crucial for the behavior of transformed glioma cells. The present Chapter summarizes molecular mechanisms of calcium signal formation present in glial cells with a strong emphasis on extracellular nucleotide-evoked signaling pathways. Some aspects of glioma C6 signaling such as the cross-talk between P2Y(1) and P2Y(12) nucleotide receptors in calcium signal generation will be discussed in-depth, to show complexity of machinery engaged in formation of this signal. Moreover, possible mechanisms of modulation of the calcium signal in diverse environments there will be presented herein. Finally, the possible role of calcium signal in glioma motility is also discussed. This is a very important issue, since glioma cells, contrary to the vast majority of neoplastic cells, cannot spread in the body with the bloodstream and, at least in early stages of tumor development, may expand only by means of sheer motility.

Store-operated Orai channels: structure and function

In many animal cells, store-operated Ca(2+) release-activated Ca(2+) (CRAC) channels function as an essential route for Ca(2+) entry. CRAC channels control many fundamental cellular functions including gene expression, motility, and cell proliferation, are involved in the etiology of several disease processes including a severe combined immunodeficiency syndrome, and have emerged as major targets for drug development. Although little was known of the molecular mechanisms of CRAC channel operation for several decades, the discovery of Orai1 as a prototypic CRAC channel protein and STIM1 as the endoplasmic reticulum (ER) Ca(2+) sensor has led to rapid progress in our understanding of the mechanisms and functions of CRAC channels. It is now known that activation of CRAC channels following ER Ca(2+) store depletion is governed by several events, which include the redistributions and accumulations of STIM1 and Orai1 into overlapping puncta at peripheral cellular sites, resulting in direct protein-protein interactions between the two proteins. In this chapter, I review the molecular features of the STIM and Orai proteins that regulate the gating and ion conduction mechanisms of CRAC channels.

Orai-STIM-mediated Ca2+ release from secretory granules revealed by a targeted Ca2+ and pH probe

Secretory granules (SGs) sequester significant calcium. Understanding roles for this calcium and potential mechanisms of release is hampered by the difficulty of measuring SG calcium directly in living cells. We adapted the Förster resonance energy transfer-based D1-endoplasmic reticulum (ER) probe to develop a unique probe (D1-SG) to measure calcium and pH in secretory granules. It significantly localizes to SGs and reports resting free Ca(2+) of 69 ± 15 μM and a pH of 5.8. Application of extracellular ATP to activate P2Y receptors resulted in a slow monotonic decrease in SG Ca(2+) temporally correlated with the occurrence of store-operated calcium entry (SOCE). Further investigation revealed a unique receptor-mediated mechanism of calcium release from SGs that involves SG store-operated Orai channels activated by their regulator stromal interaction molecule 1 (STIM1) on the ER. SG Ca(2+) release is completely antagonized by a SOCE antagonist, by switching to Ca(2+)-free medium, and by overexpression of a dominant-negative Orai1(E106A). Overexpression of the CRAC activation domain (CAD) of STIM1 resulted in a decrease of resting SG Ca(2+) by ∼75% and completely abolished the ATP-mediated release of Ca(2+) from SGs. Overexpression of a dominant-negative CAD construct(CAD-A376K) induced no significant changes in SG Ca(2+). Colocalization analysis suggests that, like the plasma membrane, SG membranes also possess Orai1 channels and that during SG Ca(2+) release, colocalization between SGs and STIM1 increases. We propose Orai channel opening on SG membranes as a potential mode of calcium release from SGs that may serve to raise local cytoplasmic calcium concentrations and aid in refilling intracellular calcium stores of the ER and exocytosis.

STIM proteins: dynamic calcium signal transducers

Stromal interaction molecule (STIM) proteins function in cells as dynamic coordinators of cellular calcium (Ca(2+)) signals. Spanning the endoplasmic reticulum (ER) membrane, they sense tiny changes in the levels of Ca(2+) stored within the ER lumen. As ER Ca(2+) is released to generate primary Ca(2+) signals, STIM proteins undergo an intricate activation reaction and rapidly translocate into junctions formed between the ER and the plasma membrane. There, STIM proteins tether and activate the highly Ca(2+)-selective Orai channels to mediate finely controlled Ca(2+) signals and to homeostatically balance cellular Ca(2+). Details are emerging on the remarkable organization within these STIM-induced junctional microdomains and the identification of new regulators and alternative target proteins for STIM.

Molecular regulation of CRAC channels and their role in lymphocyte function

Calcium (Ca(2+)) influx is required for the activation and function of all cells in the immune system. It is mediated mainly by store-operated Ca(2+) entry (SOCE) through Ca(2+) release-activated Ca(2+) (CRAC) channels located in the plasma membrane. CRAC channels are composed of ORAI proteins that form the channel pore and are activated by stromal interaction molecules (STIM) 1 and 2. Located in the membrane of the endoplasmic reticulum, STIM1 and STIM2 have the dual function of sensing the intraluminal Ca(2+) concentration in the ER and to activate CRAC channels. A decrease in the ER's Ca(2+) concentration induces STIM multimerization and translocation into puncta close to the plasma membrane where they bind to and activate ORAI channels. Since the identification of ORAI and STIM genes as the principal mediators of CRAC channel function, substantial advances have been achieved in understanding the molecular regulation and physiological role of CRAC channels in cells of the immune system and other organs. In this review, we discuss the mechanisms that regulate CRAC channel function and SOCE, the role of recently identified proteins and mechanisms that modulate the activation of ORAI/STIM proteins and the consequences of CRAC channel dysregulation for lymphocyte function and immunity.

A plasma membrane-targeted cytosolic domain of STIM1 selectively activates ARC channels, an arachidonate-regulated store-independent Orai channel

The Orai family of calcium channels includes the store-operated CRAC channels and store-independent, arachidonic acid (AA)-regulated ARC channels. Both depend on STIM1 for their activation but, whereas CRAC channel activation involves sensing the depletion of intracellular calcium stores via a luminal N terminal EF-hand of STIM1 in the endoplasmic reticulum (ER) membrane, ARC channels are exclusively activated by the pool of STIM1 that constitutively resides in the plasma membrane (PM). Here, the EF-hand is extracellular and unlikely to ever lose its bound calcium, suggesting that STIM1-dependent activation of ARC channels is very different from that of CRAC channels. We now show that attachment of the cytosolic portion of STIM1 to the inner face of the PM via an N terminal Lck-domain sequence is sufficient to enable normal AA-dependent activation of ARC channels, while failing to allow activation of store-operated CRAC channels. Introduction of a point mutation within the Lck-domain resulted in the loss of both PM localization and ARC channel activation. Reversing the orientation of the PM-anchored STIM1 C terminus via a C-terminal CAAX-box fails to support either CRAC or ARC channel activation. Finally, the Lck-anchored STIM1 C-terminal domain also enabled the exclusive activation of the ARC channels following physiological agonist addition. These data demonstrate that simple tethering of the cytosolic C-terminal domain of STIM1 to the inner face of the PM is sufficient to allow the full, normal and exclusive activation of ARC channels, and that the N-terminal regions of STIM1 (including the EF-hand domain) play no significant role in this activation.

Ca(2+) release-activated Ca(2+) (CRAC) current, structure, and function

Store-operated Ca(2+) entry describes the phenomenon that connects a depletion of internal Ca(2+) stores to an activation of plasma membrane-located Ca(2+) selective ion channels. Tremendous progress towards the underlying molecular mechanism came with the discovery of the two respective limiting components, STIM and Orai. STIM1 represents the ER-located Ca(2+) sensor and transmits the signal of store depletion to the plasma membrane. Here it couples to and activates Orai, the highly Ca(2+)-selective pore-forming subunit of Ca(2+) release-activated Ca(2+) channels. In this review, we focus on the molecular steps that these two proteins undergo from store-depletion to their coupling, the activation, and regulation of Ca(2+) currents.

Structural and mechanistic insights into the activation of Stromal interaction molecule 1 (STIM1)

Calcium influx through the Ca(2+) release-activated Ca(2+) (CRAC) channel is an essential process in many types of cells. Upon store depletion, the calcium sensor in the endoplasmic reticulum, STIM1, activates Orai1, a CRAC channel in the plasma membrane. We have determined the structures of SOAR from Homo sapiens (hSOAR), which is part of STIM1 and is capable of constitutively activating Orai1, and the entire coiled coil region of STIM1 from Caenorhabditis elegans (ceSTIM1-CCR) in an inactive state. Our studies reveal that the formation of a SOAR dimer is necessary to activate the Orai1 channel. Mutations that disrupt SOAR dimerization or remove the cluster of positive residues abolish STIM1 activation of Orai1. We identified a possible inhibitory helix within the structure of ceSTIM1-CCR that tightly interacts with SOAR. Functional studies suggest that the inhibitory helix may keep the C-terminus of STIM1 in an inactive state. Our data allowed us to propose a model for STIM1 activation.

Effects of rapamycin-induced oligomerization of parvalbumin, Stim1 and Orai1 in puncta formation

Elevations of cytosolic Ca2+ from the endoplasmic reticulum (ER) regulate a diverse range of cellular processes. When these luminal stores become depleted, the transmembrane ER protein Stim1 oligomerizes and translocates within the ER membrane to puncta junctions to couple with Orai1 channels, activating store-operated calcium entry (SOCE). Stim1 oligomerization and puncta formation have generally been associated with its luminal domains, however, studies have implicated that the cytoplasmic domains may contribute to this oligomerization. Studies have also suggested that intermediate or regulating elements may be required to fine-tune puncta formation and activation of SOCE. Here we made fusion proteins of Stim1 and Orai1 with FRB and FKBP12 domains that associate in the presence of rapamycin. Rapamycin-induced coupling of Stim1 to Stim1, Orai1 to Orai1 and Stim1 to Orai1 was found to be insufficient for puncta formation. Rapamycin was then used to recruit the cytosolic Ca2+ buffer protein parvalbumin (Pav) to Stim1 in order to buffer the local cytosolic Ca2+ near the ER membrane. Interestingly, Pav buffering near the ER caused puncta formation that was indistinguishable from those caused by thapsigargin. Our results suggest that Stim1 oligomerization and puncta formation may be additionally regulated either by local Ca2+ levels near the ER membrane or by as yet unidentified Ca2+-dependent proteins interacting with the cytoplasmic domains of Stim1.

Morphological and functional aspects of STIM1-dependent assembly and disassembly of store-operated calcium entry complexes

The SOCE (store-operated Ca2+ entry) pathway is a central component of cell signalling that links the Ca2+-filling state of the ER (endoplasmic reticulum) to the activation of Ca2+-permeable channels at the PM (plasma membrane). SOCE channels maintain a high free Ca2+ concentration within the ER lumen required for the proper processing and folding of proteins, and fuel the long-term cellular Ca2+ signals that drive gene expression in immune cells. SOCE is initiated by the oligomerization on the membrane of the ER of STIMs (stromal interaction molecules) whose luminal EF-hand domain switches from globular to an extended conformation as soon as the free Ca2+ concentration within the ER lumen ([Ca2+]ER) decreases below basal levels of ~500 μM. The conformational changes induced by the unbinding of Ca2+ from the STIM1 luminal domain promote the formation of higher-order STIM1 oligomers that move towards the PM and exposes activating domains in STIM1 cytosolic tail that bind to Ca2+ channels of the Orai family at the PM and induce their activation. Both SOCE and STIM1 oligomerization are reversible events, but whether restoring normal [Ca2+]ER levels is sufficient to initiate the deoligomerization of STIM1 and to control the termination of SOCE is not known. The translocation of STIM1 towards the PM involves the formation of specialized compartments derived from the ER that we have characterized at the ultrastructural level and termed the pre-cortical ER, the cortical ER and the thin cortical ER. Pre-cortical ER structures are thin ER tubules enriched in STIM1 extending along microtubules and located deep inside cells. The cortical ER is located in the cell periphery in very close proximity (8–11 nm) to the plasma membrane. The thin cortical ER consists of thinner sections of the cortical ER enriched in STIM1 and devoid of chaperones that appear to be specialized ER compartments dedicated to Ca2+ signalling.

Structure, regulation and biophysics of I(CRAC), STIM/Orai1

Ca(2+) release activated Ca(2+) (CRAC) channels mediate robust Ca(2+) influx when the endoplasmic reticulum Ca(2+) stores are depleted. This essential process for T-cell activation as well as degranulation of mast cells involves the Ca(2+) sensor STIM1, located in the endoplasmic reticulum and the Ca(2+) selective Orai1 channel in the plasma membrane. Our review describes the CRAC signaling pathway, the activation of which is initiated by a drop in the endoplasmic Ca(2+) level sensed by STIM1. This in term induces multimerisation and puncta-formation of STIM1 proteins is followed by their coupling to and activation of Orai channels. Consequently Ca(2+) entry is triggered through the Orai pore into the cytosol with subsequent closure of the channel by Ca(2+)-dependent inactivation. We will portray a mechanistic view of the events coupling STIM1 to Orai activation based on their structure and biophysics.

The Ca(2+) sensor stromal interaction molecule 1 (STIM1) is necessary and sufficient for the store-operated Ca(2+) entry function of transient receptor potential canonical (TRPC) 1 and 4 channels in endothelial cells

We addressed the requirement for stromal interaction molecule 1 (STIM1), the endoplasmic reticulum (ER) Ca(2+)-sensor, and Orai1, a Ca(2+) selective channel, in regulating Ca(2+) entry through the store-operated channels mouse transient receptor potential canonical (TRPC) 4 or human TRPC1. Studies were made using murine and human lung endothelial cells (ECs) challenged with thrombin known to induce Ca(2+) entry via TRPC1/4. Deletion or knockdown of TRPC4 abolished Ca(2+) entry secondary to depletion of ER Ca(2+) stores, preventing the disruption of the endothelial barrier. Knockdown of STIM1 (but not of Orai1or Orai3) or expression of the dominant-negative STIM1(K684E-K685E) mutant in ECs also suppressed Ca(2+) entry secondary to store depletion. Ectopic expression of WT-STIM1 or WT-Orai1 in TRPC4(-/-)-ECs failed to rescue Ca(2+) entry; however, WT-TRPC4 expression in TRPC4(-/-)-ECs restored Ca(2+) entry indicating the requirement for TRPC4 in mediating store-operated Ca(2+) entry. Moreover, expression of the dominant-negative Orai1(R91W) mutant or Orai3(E81W) mutant in WT-ECs failed to prevent thrombin-induced Ca(2+) entry. In contrast, expression of the dominant-negative TRPC4(EE647-648KK) mutant in WT-ECs markedly reduced thrombin-induced Ca(2+) entry. In ECs expressing YFP-STIM1, ER-store Ca(2+) depletion induced formation of fluorescent membrane puncta in WT but not in TRPC4(-/-) cells, indicating that mobilization of STIM1 and engagement of its Ca(2+) sensing function required TRPC4 expression. Coimmunoprecipitation studies showed coupling of TRPC1 and TRPC4 with STIM1 on depletion of ER Ca(2+) stores. Thus, TRPC1 and TRPC4 can interact with STIM1 to form functional store-operated Ca(2+)-entry channels, which are essential for mediating Ca(2+) entry-dependent disruption of the endothelial barrier.

Store-operated calcium channels: new perspectives on mechanism and function

Store-operated calcium channels (SOCs) are a nearly ubiquitous Ca(2+) entry pathway stimulated by numerous cell surface receptors via the reduction of Ca(2+) concentration in the ER. The discovery of STIM proteins as ER Ca(2+) sensors and Orai proteins as structural components of the Ca(2+) release-activated Ca(2+) (CRAC) channel, a prototypic SOC, opened the floodgates for exploring the molecular mechanism of this pathway and its functions. This review focuses on recent advances made possible by the use of STIM and Orai as molecular tools. I will describe our current understanding of the store-operated Ca(2+) entry mechanism and its emerging roles in physiology and disease, areas of uncertainty in which further progress is needed, and recent findings that are opening new directions for research in this rapidly growing field.

Modulation of STIM1 and capacitative Ca2+ entry by the endoplasmic reticulum luminal oxidoreductase ERp57

STIM1 is an endoplasmic reticulum (ER) membrane Ca(2+) sensor responsible for activation of store-operated Ca(2+) influx. We discovered that STIM1 oligomerization and store-operated Ca(2+) entry (SOC) are modulated by the ER oxidoreductase ERp57. ERp57 interacts with the ER luminal domain of STIM1, with this interaction involving two conserved cysteine residues, C(49) and C(56). SOC is accelerated in the absence of ERp57 and inhibited in C(49) and C(56) mutants of STIM1. We show that ERp57, by ER luminal interaction with STIM1, has a modulatory role in capacitative Ca(2+) entry. This is the first demonstration of a protein involved in ER intraluminal regulation of STIM1.

Remodelling of the endoplasmic reticulum during store-operated calcium entry

SOCE (store-operated calcium entry) is a ubiquitous cellular mechanism linking the calcium depletion of the ER (endoplasmic reticulum) to the activation of PM (plasma membrane) Ca2+-permeable channels. The activation of SOCE channels favours the entry of extracellular Ca2+ into the cytosol, thereby promoting the refilling of the depleted ER Ca2+ stores as well as the generation of long-lasting calcium signals. The molecules that govern SOCE activation comprise ER Ca2+ sensors [STIM1 (stromal interaction molecule 1) and STIM2], PM Ca2+-permeable channels {Orai and TRPC [TRP (transient receptor potential) canonical]} and regulatory Ca2+-sensitive cytosolic proteins {CRACR2 [CRAC (Ca2+ release-activated Ca2+ current) regulator 2]}. Upon Ca2+ depletion of the ER, STIM molecules move towards the PM to bind and activate Orai or TRPC channels, initiating calcium entry and store refilling. This molecular rearrangement is accompanied by the formation of specialized compartments derived from the ER, the pre-cER (cortical ER) and cER. The pre-cER appears on the electron microscope as thin ER tubules enriched in STIM1 that extend along microtubules and that are devoid of contacts with the PM. The cER is located in immediate proximity to the PM and comprises thinner sections enriched in STIM1 and devoid of chaperones that might be dedicated to calcium signalling. Here, we review the molecular interactions and the morphological changes in ER structure that occur during the SOCE process.

STIM proteins and the endoplasmic reticulum-plasma membrane junctions

Eukaryotic organelles can interact with each other through stable junctions where the two membranes are kept in close apposition. The junction that connects the endoplasmic reticulum to the plasma membrane (ER-PM junction) is unique in providing a direct communication link between the ER and the PM. In a recently discovered signaling process, STIM (stromal-interacting molecule) proteins sense a drop in ER Ca(2+) levels and directly activate Orai PM Ca(2+) channels across the junction space. In an inverse process, a voltage-gated PM Ca(2+) channel can directly open ER ryanodine-receptor Ca(2+) channels in striated-muscle cells. Although ER-PM junctions were first described 50 years ago, their broad importance in Ca(2+) signaling, as well as in the regulation of cholesterol and phosphatidylinositol lipid transfer, has only recently been realized. Here, we discuss research from different fields to provide a broad perspective on the structures and unique roles of ER-PM junctions in controlling signaling and metabolic processes.

Stoichiometric requirements for trapping and gating of Ca2+ release-activated Ca2+ (CRAC) channels by stromal interaction molecule 1 (STIM1)

Store-operated Ca(2+) entry depends critically on physical interactions of the endoplasmic reticulum (ER) Ca(2+) sensor stromal interaction molecule 1 (STIM1) and the Ca(2+) release-activated Ca(2+) (CRAC) channel protein Orai1. Recent studies support a diffusion-trap mechanism in which ER Ca(2+) depletion causes STIM1 to accumulate at ER-plasma membrane (PM) junctions, where it binds to Orai1, trapping and activating mobile CRAC channels in the overlying PM. To determine the stoichiometric requirements for CRAC channel trapping and activation, we expressed mCherry-STIM1 and Orai1-GFP at varying ratios in HEK cells and quantified CRAC current (I(CRAC)) activation and the STIM1:Orai1 ratio at ER-PM junctions after store depletion. By competing for a limited amount of STIM1, high levels of Orai1 reduced the junctional STIM1:Orai1 ratio to a lower limit of 0.3-0.6, indicating that binding of one to two STIM1s is sufficient to immobilize the tetrameric CRAC channel at ER-PM junctions. In cells expressing a constant amount of STIM1, CRAC current was a highly nonlinear bell-shaped function of Orai1 expression and the minimum stoichiometry for channel trapping failed to evoke significant activation. Peak current occurred at a ratio of ∼2 STIM1:Orai1, suggesting that maximal CRAC channel activity requires binding of eight STIM1s to each channel. Further increases in Orai1 caused channel activity and fast Ca(2+)-dependent inactivation to decline in parallel. The data are well described by a model in which STIM1 binds to Orai1 with negative cooperativity and channels open with positive cooperativity as a result of stabilization of the open state by STIM1.

Differential roles for STIM1 and STIM2 in store-operated calcium entry in rat neurons

The interaction between Ca²⁺ sensors STIM1 and STIM2 and Ca²⁺ channel-forming protein ORAI1 is a crucial element of store-operated calcium entry (SOCE) in non-excitable cells. However, the molecular mechanism of SOCE in neurons remains unclear. We addressed this issue by establishing the presence and function of STIM proteins. Real-time polymerase chain reaction from cortical neurons showed that these cells contain significant amounts of Stim1 and Stim2 mRNA. Thapsigargin (TG) treatment increased the amount of both endogenous STIM proteins in neuronal membrane fractions. The number of YFP-STIM1/ORAI1 and YFP-STIM2/ORAI1 complexes was also enhanced by such treatment. The differences observed in the number of STIM1 and STIM2 complexes under SOCE conditions and the differential sensitivity to SOCE inhibitors suggest their distinct roles. Endoplasmic reticulum (ER) store depletion by TG enhanced intracellular Ca²⁺ levels in loaded with Fura-2 neurons transfected with YFP-STIM1 and ORAI1, but not with YFP-STIM2 and ORAI1, which correlated well with the number of complexes formed. Moreover, the SOCE inhibitors ML-9 and 2-APB reduced Ca²⁺ influx in neurons expressing YFP-STIM1/ORAI1 but produced no effect in cells transfected with YFP-STIM2/ORAI1. Moreover, in neurons transfected with YFP-STIM2/ORAI1, the increase in constitutive calcium entry was greater than with YFP-STIM1/ORAI1. Our data indicate that both STIM proteins are involved in calcium homeostasis in neurons. STIM1 mainly activates SOCE, whereas STIM2 regulates resting Ca²⁺ levels in the ER and Ca²⁺ leakage with the additional involvement of STIM1.

Evolutionary origins of STIM1 and STIM2 within ancient Ca2+ signaling systems

Human stromal interaction molecule (STIM) proteins are parts of elaborate eukaryotic Ca(2+) signaling systems that include numerous plasma membrane (PM), endoplasmic reticulum (ER), and mitochondrial Ca(2+) transporters, channels and regulators. STIM2 and STIM1 function as Ca(2+) sensors with different sensitivities for ER Ca(2+). They translocate to ER-PM junctions and open PM Orai Ca(2+) influx channels when receptor-mediated Ca(2+) release lowers ER Ca(2+) levels. The resulting increase in cytosolic Ca(2+) leads to the activation of numerous Ca(2+) effector proteins that in turn regulate differentiation, cell contraction, secretion and other cell functions. In this review, we use an evolutionary perspective to survey molecular activation mechanisms in the Ca(2+) signaling system, with a particular focus on regulatory motifs and functions of the two STIM proteins. We discuss the presence and absence of STIM genes in different species, the order of appearance of STIM versus Orai, and the evolutionary addition of new signaling domains to STIM proteins.

STIM1 couples to ORAI1 via an intramolecular transition into an extended conformation

Upon depletion of ER calcium stores, STIM1 and ORAI1 associate and induce calcium release-activated calcium (CRAC) currents. This study reveals that STIM1 undergoes an intramolecular transition into an extended conformation that is involved in ORAI1 binding and activation.

Stromal interaction molecule (STIM1) and ORAI1 are key components of the Ca²⁺ release-activated Ca²⁺ (CRAC) current having an important role in T-cell activation and mast cell degranulation. CRAC channel activation occurs via physical interaction of ORAI1 with STIM1 when endoplasmic reticulum Ca²⁺ stores are depleted. Here we show, utilizing a novel STIM1-derived Förster resonance energy transfer sensor, that the ORAI1 activating small fragment (OASF) undergoes a C-terminal, intramolecular transition into an extended conformation when activating ORAI1. The C-terminal rearrangement of STIM1 does not require a functional CRAC channel, suggesting interaction with ORAI1 as sufficient for this conformational switch. Extended conformations were also engineered by mutations within the first and third coiled-coil domains in the cytosolic portion of STIM1 revealing the involvement of hydrophobic residues in the intramolecular transition. Corresponding full-length STIM1 mutants exhibited enhanced interaction with ORAI1 inducing constitutive CRAC currents, even in the absence of store depletion. We suggest that these mutant STIM1 proteins imitate a physiological activated state, which mimics the intramolecular transition that occurs in native STIM1 upon store depletion.