Intracellular Ca2+ Sensing: Its Role in Calcium Homeostasis and Signaling

Ca2+ is a ubiquitous intracellular messenger that controls diverse cellular functions but can become toxic and cause cell death. Selective control of specific targets depends on spatiotemporal patterning of the calcium signal and decoding it by multiple, tunable, and often strategically positioned Ca2+-sensing elements. Ca2+ is detected by specialized motifs on proteins that have been biochemically characterized decades ago. However, the field of Ca2+ sensing has been reenergized by recent progress in fluorescent technology, genetics, and cryo-EM. These approaches exposed local Ca2+-sensing mechanisms inside organelles and at the organellar interfaces, revealed how Ca2+ binding might work to open some channels, and identified human mutations and disorders linked to a variety of Ca2+-sensing proteins. Here we attempt to place these new developments in the context of intracellular calcium homeostasis and signaling. Ca2+ is a ubiquitous intracellular messenger that controls diverse cellular functions but can become toxic and cause cell death. Selective control of specific targets depends on spatiotemporal patterning of the calcium signal and decoding it by multiple, tunable, and often strategically positioned Ca2+-sensing elements. Ca2+ is detected by specialized motifs on proteins that have been biochemically characterized decades ago. However, the field of Ca2+ sensing has been reenergized by recent progress in fluorescent technology, genetics, and cryo-EM. These approaches exposed local Ca2+-sensing mechanisms inside organelles and at the organellar interfaces, revealed how Ca2+ binding might work to open some channels, and identified human mutations and disorders linked to a variety of Ca2+-sensing proteins. Here we attempt to place these new developments in the context of intracellular calcium homeostasis and signaling. Intracellular free Ca2+ concentration varies widely depending on its location. The cytoplasmic [Ca2+] ([Ca2+]c) under resting conditions is ∼10−7 M, 104 times lower than [Ca2+] in the extracellular milieu (∼10−3 M). Inside the cell, Ca2+ levels in the nuclear matrix ([Ca2+]n) and in the mitochondrial matrix ([Ca2+]mt) are similar to that in the cytoplasm. However, other intracellular organelles, known as Ca2+ stores, can accumulate Ca2+ and maintain a higher [Ca2+] than the cytoplasm (1–5 × 10−4 M). The main internal Ca2+ store is the endoplasmic reticulum (ER), and, in muscle cells, the sarcoplasmic reticulum (SR). The low [Ca2+]c is maintained through the action of the plasma membrane Ca2+ transport ATPase (PMCA) and the Na+/Ca2+ exchanger (NCX) in a resting cell. Upon elevated [Ca2+]c, this activity is complemented by the sarcoendoplasmic reticulum Ca2+-ATPase (SERCA), which fills the ER/SR Ca2+ store and, to a lesser extent, by the mitochondrial Ca2+ uniporter (mtCU) (Figure 1A, red arrows). All of these proteins sense and are activated by Ca2+, and, therefore, any elevations in [Ca2+]c stimulate removal of cytoplasmic Ca2+, resulting in homeostatic control of [Ca2+]c (Figure 1A, green arrows). Nevertheless, various cell stimuli, such as membrane depolarization, extracellular signaling molecules, or intracellular messengers, promote an increase of [Ca2+]c from 100 nM to 1 μM or more. This increase results from either the influx of extracellular Ca2+ via plasma membrane (PM) Ca2+ channels or the release of Ca2+ from internal stores, mostly via the 1,4,5-triphosphate receptor (IP3R) and ryanodine receptor (RyR) from the ER/SR (Figure 1A, blue arrows). The [Ca2+]c increase is usually steep, followed by a decay giving rise to [Ca2+]c spikes or repetitive [Ca2+]c oscillations that are supported by multiple positive and negative feedback effects of Ca2+ favoring synchronized activation and rapid deactivation of the Ca2+ channels and by the homeostatic regulation of the Ca2+ removal mechanisms (Figure 1A, green arrows). The Ca2+-regulated proteins present different thresholds for activity depending on their function. For example, PMCA and SERCA pumps have high affinities for Ca2+ and low pumping rate (≈30 and ≈10 Hz, respectively) (Juhaszova et al., 2000Juhaszova M. Church P. Blaustein M.P. Stanley E.F. Location of calcium transporters at presynaptic terminals.Eur. J. Neurosci. 2000; 12: 839-846Crossref PubMed Scopus (75) Google Scholar, Lytton et al., 1992Lytton J. Westlin M. Burk S.E. Shull G.E. MacLennan D.H. Functional comparisons between isoforms of the sarcoplasmic or endoplasmic reticulum family of calcium pumps.J. Biol. Chem. 1992; 267: 14483-14489PubMed Google Scholar), which makes them suitable to respond to modest elevations in [Ca2+]c and to reestablish the resting Ca2+ level. NCX and MCU show a lower affinity for Ca2+ and greater transport rates (150–300 Hz for NCX; Boyman et al., 2009Boyman L. Mikhasenko H. Hiller R. Khananshvili D. Kinetic and equilibrium properties of regulatory calcium sensors of NCX1 protein.J. Biol. Chem. 2009; 284: 6185-6193Crossref PubMed Scopus (53) Google Scholar) and, thus, can limit larger [Ca2+]c transients. Each cell type presents a unique combination of Ca2+ channels and pumps to create a cell-type- and agonist-specific calcium signal that suits their physiological requirements (Berridge et al., 2000Berridge M.J. Lipp P. Bootman M.D. The versatility and universality of calcium signalling.Nat. Rev. Mol. Cell Biol. 2000; 1: 11-21Crossref PubMed Scopus (4440) Google Scholar). The low resting [Ca2+]c and the calcium signal have to be tightly regulated because almost every aspect of cell function is controlled by Ca2+, including secretion, gene expression, muscle contraction, and metabolism, and any unregulated [Ca2+] elevations would cause cell injury or cell death (Figure 1B; Clapham, 2007Clapham D.E. Calcium signaling.Cell. 2007; 131: 1047-1058Abstract Full Text Full Text PDF PubMed Scopus (2957) Google Scholar, Hajnóczky et al., 2006Hajnóczky G. Csordás G. Das S. Garcia-Perez C. Saotome M. Sinha Roy S. Yi M. Mitochondrial calcium signalling and cell death: approaches for assessing the role of mitochondrial Ca2+ uptake in apoptosis.Cell Calcium. 2006; 40: 553-560Crossref PubMed Scopus (489) Google Scholar, Neher and Sakaba, 2008Neher E. Sakaba T. Multiple roles of calcium ions in the regulation of neurotransmitter release.Neuron. 2008; 59: 861-872Abstract Full Text Full Text PDF PubMed Scopus (623) Google Scholar). Furthermore, regulation of organelle-specific cell functions might depend on propagation of the [Ca2+]c signal into specific organelles, like the nucleus, for gene-regulatory events (Zhang et al., 2009Zhang S.J. Zou M. Lu L. Lau D. Ditzel D.A. Delucinge-Vivier C. Aso Y. Descombes P. Bading H. Nuclear calcium signaling controls expression of a large gene pool: identification of a gene program for acquired neuroprotection induced by synaptic activity.PLoS Genet. 2009; 5: e1000604Crossref PubMed Scopus (229) Google Scholar) and the mitochondrial matrix for oxidative metabolism (Griffiths and Rutter, 2009Griffiths E.J. Rutter G.A. Mitochondrial calcium as a key regulator of mitochondrial ATP production in mammalian cells.Biochim. Biophys. Acta. 2009; 1787: 1324-1333Crossref PubMed Scopus (255) Google Scholar). The information encoded in the calcium signal is deciphered by various intracellular Ca2+-binding motifs. These motifs are present in effector proteins, including Ca2+ channel proteins (i.e., IP3R and RyR) and proteins mediating Ca2+-controlled cell functions (i.e., isocitrate dehydrogenase [ICDH]) (Figure 2A). Ca2+-binding motifs are also present in specialized Ca2+-sensing proteins that couple changes in [Ca2+] to a wide variety of cellular functions depending on their localization, pattern of modulation, and the Ca2+ source. These proteins either simply associate with the effector proteins (e.g., calmodulin [CaM] or troponin C) or display enzyme activity (e.g., calcineurin or calpain) to relay the effect of Ca2+ binding to the effector proteins (Figures 2B and 2C, respectively). CaM can also confer Ca2+ sensitivity to enzymes like the Ca2+/CaM-dependent protein kinase (CaMK) that phosphorylates many effectors of Ca2+ to alter their activity (Figure 2D). Depending on the loop geometry of their Ca2+-binding site(s), Ca2+-binding proteins can be classified into three families: the EF-hand proteins, the annexins, and the C2 domain proteins. The EF-hand denotes a Ca2+-binding motif that contains a Ca2+-coordinated loop that is flanked by two α helices orientated almost perpendicular to one another. The bound Ca2+ ion is coordinated by seven ligands (primarily carboxylate) in a pentagonal bipyramid arrangement (Strynadka and James, 1989Strynadka N.C. James M.N. Crystal structures of the helix-loop-helix calcium-binding proteins.Annu. Rev. Biochem. 1989; 58: 951-998Crossref PubMed Google Scholar). EF-hand domains are the most common Ca2+-binding motifs found in proteins. This family of proteins presents a wide range of functions that are as diverse as Ca2+ buffering in the cytoplasm, signal transduction between compartments, and gene expression in the nucleus (Figure 1B). The diversity of biological functions carried out by these proteins in a wide range of [Ca2+] is possible because Ca2+ binds to EF-hand domains with different affinities, extending from 10−6 M to 10−3 M (Gifford et al., 2007Gifford J.L. Walsh M.P. Vogel H.J. Structures and metal-ion-binding properties of the Ca2+-binding helix-loop-helix EF-hand motifs.Biochem. J. 2007; 405: 199-221Crossref PubMed Scopus (652) Google Scholar). Some Ca2+-binding proteins with relatively high affinity behave as Ca2+ buffer proteins that modulate the shape and/or duration of Ca2+ signals and help maintain Ca2+ homeostasis. In contrast, Ca2+ sensors having affinity constants ranging between 10−5 M and 10−7 M can detect and respond to a physiologically relevant change in intracellular [Ca2+]. These differences in function correlate with differences in the conformational changes induced by Ca2+ binding. Ca2+ binding to EF-hands of Ca2+ sensor proteins induces a conformational change characterized by a significant opening of their structure that permits their interaction with downstream targets (Zhang et al., 1995Zhang M. Tanaka T. Ikura M. Calcium-induced conformational transition revealed by the solution structure of apo calmodulin.Nat. Struct. Biol. 1995; 2: 758-767Crossref PubMed Scopus (651) Google Scholar). On the contrary, Ca2+ buffer proteins stay in a "closed" conformation upon Ca2+ binding that is similar to their Ca2+-free state (Skelton et al., 1994Skelton N.J. Kördel J. Akke M. Forsén S. Chazin W.J. Signal transduction versus buffering activity in Ca(2+)-binding proteins.Nat. Struct. Biol. 1994; 1: 239-245Crossref PubMed Scopus (148) Google Scholar). A ubiquitously expressed and well characterized protein specialized for Ca2+ sensing is CaM. CaM has two globular domains, each containing a pair of EF-hand motifs, connected by a central helix. Activation by Ca2+ binding causes each of the EF-hand domains of CaM to undergo a significant opening of their structure. As a result, the hydrophobic binding sites within the central helix of CaM are exposed to interact with downstream targets (Zhang et al., 1995Zhang M. Tanaka T. Ikura M. Calcium-induced conformational transition revealed by the solution structure of apo calmodulin.Nat. Struct. Biol. 1995; 2: 758-767Crossref PubMed Scopus (651) Google Scholar). Ca2+-activated CaM (Ca2+/CaM) interact in a Ca2+-dependent manner with either their target enzymes, leading to their own activation (e.g., CaMK and calcineurin), or the activation of their target proteins, resulting in the regulation of their function in a Ca2+-dependent manner (e.g., Orai; Figure 3B). The CaM-dependent activation of enzymes may occur by direct or sequential mechanisms (e.g. CaMK and calcineurin, respectively). In the first case, CaM interaction and activation of target enzymes only occur under elevated [Ca2+]c, whereas, in the sequential mechanism, partial Ca2+ activation of CaM, under resting Ca2+ conditions, is enough to interact with target enzymes and form an inactive low-affinity complex. For its activation, this complex requires further binding of Ca2+ to CaM's EF hands. This specific mechanism would provide a sensitive switch for control of enzyme activity within a narrow range of free [Ca2+] (Kincaid and Vaughan, 1986Kincaid R.L. Vaughan M. Direct comparison of Ca2+ requirements for calmodulin interaction with and activation of protein phosphatase.Proc. Natl. Acad. Sci. USA. 1986; 83: 1193-1197Crossref PubMed Scopus (31) Google Scholar). In addition to Ca2+/CaM interaction with downstream targets, Ca2+-free CaM (apo-CaM) can also interact with target proteins in a reversible or irreversible manner and regulate their activities. Therefore, CaM interaction with its target proteins is not only facilitated by its Ca2+-induced conformational change, but the interaction can also be mediated through Ca2+-independent binding sites called IQ motifs. These motifs, with the sequence IQXXXRGXXXR, provide binding sites for CaM and other proteins of the EF-hand family (Cheney and Mooseker, 1992Cheney R.E. Mooseker M.S. Unconventional myosins.Curr. Opin. Cell Biol. 1992; 4: 27-35Crossref PubMed Scopus (336) Google Scholar). Among the many downstream targets of CaM, CaMK enzymes are one of the best characterized (Swulius and Waxham, 2008Swulius M.T. Waxham M.N. Ca(2+)/calmodulin-dependent protein kinases.Cell. Mol. Life Sci. 2008; 65: 2637-2657Crossref PubMed Scopus (235) Google Scholar). As a kinase enzyme, CaMK catalyzes the transfer of phosphate from the gamma position of ATP to the hydroxyl group of Ser, Thr, or Tyr within protein substrates. Therefore, this CaM-dependent enzyme transduces the intracellular calcium signals into changes in the phosphorylation state and activity of target proteins. CaMK also performs autophosphorylation to increase its affinity for CaM, thus resulting in their association at low [Ca2+]c. The CaMK capacity to trap CaM enables these enzymes to detect the frequency of the calcium signals (Meyer et al., 1992Meyer T. Hanson P.I. Stryer L. Schulman H. Calmodulin trapping by calcium-calmodulin-dependent protein kinase.Science. 1992; 256: 1199-1202Crossref PubMed Scopus (513) Google Scholar). Depending on the downstream targets of CaMK, the members of this family can be classified into two classes: multifunctional kinases and substrate-specific kinases. Multifunctional kinases have multiple downstream targets (e.g., CaMK kinase [CaMKK], CaMKI, CaMKII, and CaMKIV), and their activation can lead to signaling that affects many downstream pathways controlling a variety of cellular functions. In contrast, substrate-specific kinases have only one known downstream target (e.g., CaMKIII, phosphorylase kinase, and the myosin light-chain kinases), and, therefore, they usually have a specific function within the cell or tissue where they are expressed. Calcineurin and calpain can directly bind and sense Ca2+ that affects their protein phosphatase and protease function, respectively. Calcineurin is regulated by Ca2+ both directly and via CaM. Calcineurin has been implicated in a wide variety of biological responses, including lymphocyte activation and neuronal and muscle development (Schulz and Yutzey, 2004Schulz R.A. Yutzey K.E. Calcineurin signaling and NFAT activation in cardiovascular and skeletal muscle development.Dev. Biol. 2004; 266: 1-16Crossref PubMed Scopus (236) Google Scholar). On the other hand, calpain is uniquely regulated by Ca2+ binding to its EF-hand domains. Members of the calpain family have been linked to various biological processes, including integrin-mediated cell migration, cytoskeletal remodeling, cell differentiation, and apoptosis (Suzuki and Sorimachi, 1998Suzuki K. Sorimachi H. A novel aspect of calpain activation.FEBS Lett. 1998; 433: 1-4Abstract Full Text Full Text PDF PubMed Scopus (140) Google Scholar). Emerging literature highlights a sub-branch of the CaM family, the neuronal calcium sensor (NCS) proteins (Burgoyne, 2007Burgoyne R.D. Neuronal calcium sensor proteins: generating diversity in neuronal Ca2+ signalling.Nat. Rev. Neurosci. 2007; 8: 182-193Crossref PubMed Scopus (418) Google Scholar). Some NCS proteins are uniquely expressed in neurons, whereas other members (such as NCS-1) are also expressed in other tissues (Kapp-Barnea et al., 2003Kapp-Barnea Y. Melnikov S. Shefler I. Jeromin A. Sagi-Eisenberg R. Neuronal calcium sensor-1 and phosphatidylinositol 4-kinase beta regulate IgE receptor-triggered exocytosis in cultured mast cells.J. Immunol. 2003; 171: 5320-5327Crossref PubMed Scopus (36) Google Scholar). NCS proteins are implicated in the regulation of several neuronal functions. Tissue-specific expression of Ca2+-sensing proteins like NCS can provide for selective control of specific pathways in different paradigms. Annexins and C2 domain proteins present a unique architecture of their Ca2+-binding sites that allows them to peripherally dock onto negatively charged membrane surfaces in their Ca2+-bound conformation. As a result, these families are considered to provide a link between Ca2+ signaling and membrane functions (Figure 1B). The Ca2+-binding sites of annexins do not present an EF-hand-type helix-loop-helix structure, and only five of the seven coordination sites are provided by protein oxygen. The other two coordination sites are provided by water molecules, which can be replaced by phosphoryl groups when the annexin binds lipid (i.e., Ca2+- and phospholipid-binding motif) (Swairjo et al., 1995Swairjo M.A. Concha N.O. Kaetzel M.A. Dedman J.R. Seaton B.A. Ca(2+)-bridging mechanism and phospholipid head group recognition in the membrane-binding protein annexin V.Nat. Struct. Biol. 1995; 2: 968-974Crossref PubMed Scopus (281) Google Scholar). Knockout and knockdown approaches have revealed that multiples steps in the endocytosis and exocytosis process depend on annexin (Ali et al., 1989Ali S.M. Geisow M.J. Burgoyne R.D. A role for calpactin in calcium-dependent exocytosis in adrenal chromaffin cells.Nature. 1989; 340: 313-315Crossref PubMed Scopus (235) Google Scholar, Mayran et al., 2003Mayran N. Parton R.G. Gruenberg J. 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For instance, C2 domains of classical protein kinase C isoforms and synaptotagmins bind to the anionic head group of phosphatidylserine (Corbalán-García et al., 1999Corbalán-García S. Rodríguez-Alfaro J.A. Gómez-Fernández J.C. Determination of the calcium-binding sites of the C2 domain of protein kinase Calpha that are critical for its translocation to the plasma membrane.Biochem. J. 1999; 337: 513-521Crossref PubMed Scopus (60) Google Scholar, Fukuda et al., 1996Fukuda M. Kojima T. Mikoshiba K. Phospholipid composition dependence of Ca2+-dependent phospholipid binding to the C2A domain of synaptotagmin IV.J. Biol. Chem. 1996; 271: 8430-8434Crossref PubMed Scopus (94) Google Scholar), whereas the C2 domain of cPLA2 binds to the neutral phosphatidylcholine (Nalefski et al., 1998Nalefski E.A. McDonagh T. Somers W. Seehra J. Falke J.J. Clark J.D. Independent folding and ligand specificity of the C2 calcium-dependent lipid binding domain of cytosolic phospholipase A2.J. Biol. 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Structural and functional conservation of key domains in InsP3 and ryanodine receptors.Nature. 2012; 483: 108-112Crossref PubMed Scopus (141) Google Scholar), and the Ca2+-activated K+ (BK) channels (Hite et al., 2017Hite R.K. Tao X. MacKinnon R. Structural basis for gating the high-conductance Ca(2+)-activated K(+) channel.Nature. 2017; 541: 52-57Crossref PubMed Scopus (105) Google Scholar, Russo et al., 2009Russo G.J. Louie K. Wellington A. Macleod G.T. Hu F. Panchumarthi S. Zinsmaier K.E. Drosophila Miro is required for both anterograde and retrograde axonal mitochondrial transport.J. Neurosci. 2009; 29: 5443-5455Crossref PubMed Scopus (162) Google Scholar). Studies using single-particle cryo-EM identified a pair of EF-hand domains at the central domain of RyR1 (4060–4134) (des Georges et al., 2016des Georges A. Clarke O.B. Zalk R. Yuan Q. Condon K.J. Grassucci R.A. Hendrickson W.A. Marks A.R. Frank J. Structural basis for gating and activation of RyR1.Cell. 2016; 167: 145-157.e17Abstract Full Text Full Text PDF PubMed Scopus (231) Google Scholar, Wei et al., 2016Wei R. Wang X. Zhang Y. Mukherjee S. Zhang L. Chen Q. Huang X. Jing S. Liu C. Li S. et al.Structural insights into Ca(2+)-activated long-range allosteric channel gating of RyR1.Cell Res. 2016; 26: 977-994Crossref PubMed Scopus (66) Google Scholar) and modulator binding sites for Ca2+, ATP, and caffeine at the interdomain interfaces of the C-terminal domain (4957–5037) (des Georges et al., 2016des Georges A. Clarke O.B. Zalk R. Yuan Q. Condon K.J. Grassucci R.A. Hendrickson W.A. Marks A.R. Frank J. Structural basis for gating and activation of RyR1.Cell. 2016; 167: 145-157.e17Abstract Full Text Full Text PDF PubMed Scopus (231) Google Scholar). Although the IP3R structure in its apo-state has been recently elucidated at near-atomic (4.7-Å) resolution (Fan et al., 2015Fan G. Baker M.L. Wang Z. Baker M.R. Sinyagovskiy P.A. Chiu W. Ludtke S.J. Serysheva I.I. Gating machinery of InsP3R channels revealed by electron cryomicroscopy.Nature. 2015; 527: 336-341Crossref PubMed Scopus (153) Google Scholar), more studies are needed to define the molecular architecture of the domains that control channel gating. To date, the only information available is given by mapping the sequence conservation across the RyR and IP3R family. This analysis revealed that the Ca2+-binding domain described at the C-terminal of RyR1 is conserved, whereas the pair of EF-hands located at the central domain of RyR1 are absent in IP3R, thus suggesting that these EF-hands are not involved in Ca2+ activation (des Georges et al., 2016des Georges A. Clarke O.B. Zalk R. Yuan Q. Condon K.J. Grassucci R.A. Hendrickson W.A. Marks A.R. Frank J. Structural basis for gating and activation of RyR1.Cell. 2016; 167: 145-157.e17Abstract Full Text Full Text PDF PubMed Scopus (231) Google Scholar). This hypothesis is supported by the fact that deletion or sequence scrambling of EF-hand domains in RyR2 and RyR1 did not affect activation of the channel by Ca2+ (Fessenden et al., 2004Fessenden J.D. Feng W. Pessah I.N. Allen P.D. Mutational analysis of putative calcium binding motifs within the skeletal ryanodine receptor isoform, RyR1.J. Biol. Chem. 2004; 279: 53028-53035Crossref PubMed Scopus (46) Google Scholar, Guo et al., 2016Guo W. Sun B. Xiao Z. Liu Y. Wang Y. Zhang L. Wang R. Chen S.R. The EF-hand Ca2+ Binding Domain Is Not Required for Cytosolic Ca2+ Activation of the Cardiac Ryanodine Receptor.J. Biol. Chem. 2016; 291: 2150-2160Crossref PubMed Scopus (35) Google Scholar). In addition, studies of BK channels in the Ca2+-bound and Ca2+-free states have revealed the molecular basis of channel gating by voltage and Ca2+. At the level of Ca2+ sensing, this channel presents a "gating ring" at the cytoplasm that is formed by four Ca2+ sensors. Each sensor includes two regulators of K+ conductance (RCK) that regulate the conductance of K+ through the binding of two Ca2+ ions and an Mg2+ ion. Moreover, the central pore gate domain (located in the transmembrane domain) appeared to be connected to both the voltage sensors, also located in the transmembrane domain, and the Ca2+ sensors, located in the cytoplasm. Therefore, these data suggest a new shared pathway for channel activation (Hite et al., 2017Hite R.K. Tao X. MacKinnon R. Structural basis for gating the high-conductance Ca(2+)-activated K(+) channel.Nature. 2017; 541: 52-57Crossref PubMed Scopus (105) Google Scholar, Tao et al., 2017Tao X. Hite R.K. MacKinnon R. Cryo-EM structure of the open high-conductance Ca(2+)-activated K(+) channel.Nature. 2017; 541: 46-51Crossref PubMed Scopus (139) Google Scholar). Ca2+ regulates many different cellular functions. To achieve this versatility, the calcium signal displays a range of spatial and temporal patterns detected differently by various Ca2+ sensors. Although the bulk [Ca2+]c peaks at around 1 μM, close to the open Ca2+ channels, [Ca2+]c can reach 10–100 μM. These "nanodomains" provide a meaningful signal for low-affinity Ca2+-sensing motifs unresponsive to fluctuations in global [Ca2+]c. A major direction of recent progress on local Ca2+ sensing has been focused on detection of Ca2+ within organelles and at organellar interfaces (Figure 3A). An example is the process known as store-operated Ca2+ entry (SOCE), whereby Ca2+ influx across the plasma membrane is activated in response to a decrease in the ER Ca2+ content (Figure 3B). The main role of SOCE is to refill the intracellular Ca2+ stores to maintain the primary source of intracellular Ca2+ mobilization and a favorable environment for protein folding in the ER lumen. Essential components of the molecular machinery responsible for SOCE have been recently discovered. Among them, STIM1 (and its STIM2 isoform) is the ER transmembrane protein responsible for sensing the changes in the ER luminal [Ca2+] ([Ca2+]ER) through a pair of Ca2+-binding EF-hand domain that are exposed to the ER lumen (Liou et al., 2005Liou J. Kim M.L. Heo W.D. Jones J.T. Myers J.W. Ferrell Jr., J.E. Meyer T. STIM is a Ca2+ sensor essential for Ca2+-store-depletion-triggered Ca2+ influx.Curr. Biol. 2005; 15: 1235-1241Abstract Full Text Full Text PDF PubMed Scopus (1751) Google Scholar, Roos et al., 2005Roos J. DiGregorio P.J. Yeromin A.V. Ohlsen K. Lioudyno M. Zhang S. Safrina O. Kozak J.A. Wagner S.L. Cahalan M.D. et al.STIM1, an essential and conserved component of store-operated Ca2+ channel function.J. Cell Biol. 2005; 169: 435-445Crossref PubMed Scopus (1519) Google Scholar, Zhang et al., 2005Zhang S.L. Yu Y. Roos J. Kozak J.A. Deerinck T.J. Ellisman M.H. Stauderman K.A. 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