¹H, ¹⁵N, and ¹³C chemical shift assignments of murine calcium-binding protein 4

Calcium-binding protein 4 (CaBP4) regulates voltage-gated Ca(2+) channels in retinal rod cells and specific mutations within CaBP4 are associated with congenital stationary night blindness type 2. We report complete NMR chemical shift assignments of the Ca(2+)-saturated form of CaBP4 with Ca(2+) bound at EF1, EF3 and EF4 (BMRB no. 18877).

Reliance of ER-mitochondrial calcium signaling on mitochondrial EF-hand Ca2+ binding proteins: Miros, MICUs, LETM1 and solute carriers

Endoplasmic reticulum (ER) and mitochondria are functionally distinct with regard to membrane protein biogenesis and oxidative energy production, respectively, but cooperate in several essential cell functions, including lipid biosynthesis, cell signaling and organelle dynamics. The interorganellar cooperation requires local communication that can occur at the strategically positioned and dynamic associations between ER and mitochondria. Calcium is locally transferred from ER to mitochondria at the associations and exerts regulatory effects on numerous proteins. A common Ca(2+) sensing mechanism is the EF-hand Ca(2+) binding domain, many of which can be found in proteins of the mitochondria, including Miro1&2, MICU1,2&3, LETM1 and mitochondrial solute carriers. Recently, these proteins have triggered much interest and were described in reports with diverging conclusions. The present essay focuses on their shared features and established specific functions.

¹H, ¹³C, and ¹⁵N chemical shift assignments of neuronal calcium sensor protein, hippocalcin

Hippocalcin, a member of the neuronal calcium sensor (NCS) subclass of the calmodulin superfamily, serves as an important calcium sensor for the slow afterhyperpolarizing (sAHP) current in the hippocampus, which underlies some forms of learning and memory. Hippocalcin is also a calcium sensor for hippocampal long-term depression (LTD) and genetically linked to neurodegenerative diseases. We report NMR chemical shift assignments of Ca(2+)-free hippocalcin (BMRB no. 18627).

Structural biology of TRP channels

Membrane proteins remain challenging targets for structural biologists, despite recent technical developments regarding sample preparation and structure determination. We review recent progress towards a structural understanding of TRP channels and the techniques used to that end. We discuss available low-resolution structures from electron microscopy (EM), X-ray crystallography, and nuclear magnetic resonance (NMR) and review the resulting insights into TRP channel function for various subfamily members. The recent high-resolution structure of TRPV1 is discussed in more detail in Chapter 11. We also consider the opportunities and challenges of using the accumulating structural information on TRPs and homologous proteins for deducing full-length structures of different TRP channel subfamilies, such as building homology models. Finally, we close by summarizing the outlook of the "holy grail" of understanding in atomic detail the diverse functions of TRP channels.

Backbone (1)H, (13)C, and (15)N resonance assignments of guanylyl cyclase activating protein-1, GCAP1

Guanylyl cyclase activating protein 1 (GCAP1), a member of the neuronal calcium sensor subclass of the calmodulin superfamily, confers Ca(2+)-dependent activation of retinal guanylyl cyclase that regulates the visual light response. GCAP1 is genetically linked to retinal degenerative diseases. We report backbone NMR chemical shift assignments of Ca(2+)-saturated GCAP1 (BMRB no. 18026).

(1)H, (15)N, and (13)C chemical shift assignments of the calflagin Tb24 flagellar calcium binding protein of Trypanosoma brucei

Flagellar calcium binding proteins are expressed in a variety of trypanosomes and are potential drug targets for Chagas disease and African sleeping sickness. We report complete NMR chemical shift assignments of the flagellar calcium binding protein calflagin Tb24 of Trypanosoma brucei. (BMRB no. 18011).

Target binding to S100B reduces dynamic properties and increases Ca(2+)-binding affinity for wild type and EF-hand mutant proteins

Mutations in the second EF-hand (D61N, D63N, D65N, and E72A) of S100B were used to study its Ca(2+) binding and dynamic properties in the absence and presence of a bound target, TRTK-12. With (D63N)S100B as an exception ((D63N)K(D)=50±9 μM), Ca(2+) binding to EF2-hand mutants were reduced by more than 8-fold in the absence of TRTK-12 ((D61N)K(D)=412±67 μM, (D65N)K(D)=968±171 μM, and (E72A)K(D)=471±133 μM), when compared to wild-type protein ((WT)K(D)=56±9 μM). For the TRTK-12 complexes, the Ca(2+)-binding affinity to wild type ((WT+TRTK)K(D)=12±10 μM) and the EF2 mutants was increased by 5- to 14-fold versus in the absence of target ((D61N+TRTK)K(D)=29±1.2 μM, (D63N+TRTK)K(D)=10±2.2 μM, (D65N+TRTK)K(D)=73±4.4 μM, and (E72A+TRTK)K(D)=18±3.7 μM). In addition, R(ex), as measured using relaxation dispersion for side-chain (15)N resonances of Asn63 ((D63N)S100B), was reduced upon TRTK-12 binding when measured by NMR. Likewise, backbone motions on multiple timescales (picoseconds to milliseconds) throughout wild type, (D61N)S100B, (D63N)S100B, and (D65N)S100B were lowered upon binding TRTK-12. However, the X-ray structures of Ca(2+)-bound (2.0Å) and TRTK-bound (1.2Å) (D63N)S100B showed no change in Ca(2+) coordination; thus, these and analogous structural data for the wild-type protein could not be used to explain how target binding increased Ca(2+)-binding affinity in solution. Therefore, a model for how S100B-TRTK-12 complex formation increases Ca(2+) binding is discussed, which considers changes in protein dynamics upon binding the target TRTK-12.

Molecular structure and target recognition of neuronal calcium sensor proteins

BACKGROUND

Neuronal calcium sensor (NCS) proteins, a sub-branch of the calmodulin superfamily, are expressed in the brain and retina where they transduce calcium signals and are genetically linked to degenerative diseases. The amino acid sequences of NCS proteins are highly conserved but their physiological functions are quite distinct. Retinal recoverin and guanylate cyclase activating proteins (GCAPs) both serve as calcium sensors in retinal rod cells, neuronal frequenin (NCS1) modulate synaptic activity and neuronal secretion, K+ channel interacting proteins (KChIPs) regulate ion channels to control neuronal excitability, and DREAM (KChIP3) is a transcriptional repressor that regulates neuronal gene expression.

SCOPE OF REVIEW

Here we review the molecular structures of myristoylated forms of NCS1, recoverin, and GCAP1 that all look very different, suggesting that the sequestered myristoyl group helps to refold these highly homologous proteins into very different structures. The molecular structure of NCS target complexes have been solved for recoverin bound to rhodopsin kinase, NCS-1 bound to phosphatidylinositol 4-kinase, and KChIP1 bound to A-type K+ channels.

MAJOR CONCLUSIONS

We propose the idea that N-terminal myristoylation is critical for shaping each NCS family member into a unique structure, which upon Ca2+-induced extrusion of the myristoyl group exposes a unique set of previously masked residues, thereby exposing a distinctive ensemble of hydrophobic residues to associate specifically with a particular physiological target. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signaling.

Crystal structure of cardiac troponin C regulatory domain in complex with cadmium and deoxycholic acid reveals novel conformation

The amino-terminal regulatory domain of cardiac troponin C (cNTnC) plays an important role as the calcium sensor for the troponin complex. Calcium binding to cNTnC results in conformational changes that trigger a cascade of events that lead to cardiac muscle contraction. The cardiac N-terminal domain of TnC consists of two EF-hand calcium binding motifs, one of which is dysfunctional in binding calcium. Nevertheless, the defunct EF-hand still maintains a role in cNTnC function. For its structural analysis by X-ray crystallography, human cNTnC with the wild-type primary sequence was crystallized under a novel crystallization condition. The crystal structure was solved by the single-wavelength anomalous dispersion method and refined to 2.2 Å resolution. The structure displays several novel features. Firstly, both EF-hand motifs coordinate cadmium ions derived from the crystallization milieu. Secondly, the ion coordination in the defunct EF-hand motif accompanies unusual changes in the protein conformation. Thirdly, deoxycholic acid, also derived from the crystallization milieu, is bound in the central hydrophobic cavity. This is reminiscent of the interactions observed for cardiac calcium sensitizer drugs that bind to the same core region and maintain the "open" conformational state of calcium-bound cNTnC. The cadmium ion coordination in the defunct EF-hand indicates that this vestigial calcium binding site retains the structural and functional elements that allow it to coordinate a cadmium ion. However, it is a result of, or concomitant with, large and unusual structural changes in cNTnC.

SNF1-related protein kinases 2 are negatively regulated by a plant-specific calcium sensor

SNF1-related protein kinases 2 (SnRK2s) are plant-specific enzymes involved in environmental stress signaling and abscisic acid-regulated plant development. Here, we report that SnRK2s interact with and are regulated by a plant-specific calcium-binding protein. We screened a Nicotiana plumbaginifolia Matchmaker cDNA library for proteins interacting with Nicotiana tabacum osmotic stress-activated protein kinase (NtOSAK), a member of the SnRK2 family. A putative EF-hand calcium-binding protein was identified as a molecular partner of NtOSAK. To determine whether the identified protein interacts only with NtOSAK or with other SnRK2s as well, we studied the interaction of an Arabidopsis thaliana orthologue of the calcium-binding protein with selected Arabidopsis SnRK2s using a two-hybrid system. All kinases studied interacted with the protein. The interactions were confirmed by bimolecular fluorescence complementation assay, indicating that the binding occurs in planta, exclusively in the cytoplasm. Calcium binding properties of the protein were analyzed by fluorescence spectroscopy using Tb(3+) as a spectroscopic probe. The calcium binding constant, determined by the protein fluorescence titration, was 2.5 ± 0.9 × 10(5) M(-1). The CD spectrum indicated that the secondary structure of the protein changes significantly in the presence of calcium, suggesting its possible function as a calcium sensor in plant cells. In vitro studies revealed that the activity of SnRK2 kinases analyzed is inhibited in a calcium-dependent manner by the identified calcium sensor, which we named SCS (SnRK2-interacting calcium sensor). Our results suggest that SCS is involved in response to abscisic acid during seed germination most probably by negative regulation of SnRK2s activity.

1H, 15N, and 13C chemical shift assignments of calcium-binding protein 1 with Ca2+ bound at EF1, EF3 and EF4

Calcium-binding protein 1 (CaBP1) regulates inositol 1,4,5-trisphosphate receptors (InsP(3)Rs) and a variety of voltage-gated Ca(2+) channels in the brain. We report complete NMR chemical shift assignments of the Ca(2+)-saturated form of CaBP1 with Ca(2+) bound at EF1, EF3 and EF4 (residues 1-167, BMRB no. 16862).

Structural basis for prokaryotic calcium-mediated regulation by a Streptomyces coelicolor calcium binding protein

The important and diverse regulatory roles of Ca(2+) in eukaryotes are conveyed by the EF-hand containing calmodulin superfamily. However, the calcium-regulatory proteins in prokaryotes are still poorly understood. In this study, we report the three-dimensional structure of the calcium-binding protein from Streptomyces coelicolor, named CabD, which shares low sequence homology with other known helix-loop-helix EF-hand proteins. The CabD structure should provide insights into the biological role of the prokaryotic calcium-binding proteins. The unusual structural features of CabD compared with prokaryotic EF-hand proteins and eukaryotic sarcoplasmic calcium-binding proteins, including the bending conformation of the first C-terminal α-helix, unpaired ligand-binding EF-hands and the lack of the extreme C-terminal loop region, suggest it may have a distinct and significant function in calcium-mediated bacterial physiological processes, and provide a structural basis for potential calcium-mediated regulatory roles in prokaryotes.

Integration of Diverse Research Methods to Analyze and Engineer Ca-Binding Proteins: From Prediction to Production

In recent years, increasingly sophisticated computational and bioinformatics tools have evolved for the analyses of protein structure, function, ligand interactions, modeling and energetics. This includes the development of algorithms to recursively evaluate side-chain rotamer permutations, identify regions in a 3D structure that meet some set of search parameters, calculate and minimize energy values, and provide high-resolution visual tools for theoretical modeling. Here we discuss the interdependency between different areas of bioinformatics, the evolution of different algorithm design approaches, and finally the transition from theoretical models to real-world design and application as they relate to Ca(2+)-binding proteins. Within this context, it has become evident that significant pre-experimental design and calculations can be modeled through computational methods, thus eliminating potentially unproductive research and increasing our confidence in the correlation between real and theoretical models. Moving from prediction to production, it is anticipated that bioinformatics tools will play an increasingly significant role in research and development, improving our ability to both understand the physiological roles of Ca(2+) and other metals and to extend that knowledge to the design of function-specific synthetic proteins capable of fulfilling different roles in medical diagnostics and therapeutics.

(1)H, (15)N, and (13)C chemical shift assignments of neuronal calcium sensor-1 homolog from fission yeast

The neuronal calcium sensor (NCS) proteins regulate signal transduction processes and are highly conserved from yeast to humans. We report complete NMR chemical shift assignments of the NCS homolog from fission yeast (Schizosaccharomyces pombe), referred to in this study as Ncs1p. (BMRB no. 16446).

Effects of Ca2+, Mg2+, and myristoylation on guanylyl cyclase activating protein 1 structure and stability

Guanylyl cyclase activating protein 1 (GCAP1), a member of the neuronal calcium sensor (NCS) subclass of the calmodulin superfamily, confers Ca(2+)-dependent activation of retinal guanylyl cylcase (RetGC) during phototransduction in vision. Here we analyze the energetics of Ca(2+) and Mg(2+) binding to the individual EF-hands, characterize metal-induced conformational changes, and evaluate structural effects of myristoylation as studied by isothermal titration calorimetry (ITC), differential scanning calorimetry (DSC), and nuclear magnetic resonance (NMR). GCAP1 binds cooperatively to Ca(2+) at EF3 and EF4 (DeltaH(EF3) = -3.5 kcal/mol, and DeltaH(EF4) = -0.9 kcal/mol) with nanomolar affinity (K(EF3) = 80 nM, and K(EF4) = 200 nM), and a third Ca(2+) binds entropically at EF2 (DeltaH(EF2) = 3.1 kcal/mol, and K(EF2) = 0.9 microM). GCAP1 binds functionally to Mg(2+) at EF2 (DeltaH(EF2) = 4.3 kcal/mol, and K(EF2) = 0.7 mM) required for RetGC activation. Ca(2+) and/or Mg(2+) binding to GCAP1 dramatically alters DSC and NMR spectra, indicating metal-induced protein conformational changes in EF2, EF3, and EF4. Myristoylation of GCAP1 does not significantly alter its metal binding energetics or NMR spectra, suggesting that myristoylation does not influence the structure of the metal-binding EF-hands. Myristoylation also has almost no effect on protein folding stability measured by DSC. NMR resonances of myristate attached to GCAP1 are exchange-broadened, upfield-shifted, and insensitive to Ca(2+), consistent with the myristoyl group being sequestered inside the protein as seen in the crystal structure. We conclude that the protein environment near the myristate is not influenced by Mg(2+) or Ca(2+) binding but instead is constitutively dynamic and may play a role in promoting interactions of GCAP1 with the cyclase.

1H, 15N, and 13C chemical shift assignments of calcium-binding protein 1 (CaBP1)

Calcium-binding protein 1 (CaBP1) regulates inositol 1,4,5-trisphosphate receptors (InsP(3)Rs) and a variety of voltage-gated Ca2+ channels in the brain. We report complete NMR chemical shift assignments of Ca2+-free CaBP1 (residues 1-167, BMRB no. 15197).

Crystal structure of apo-calmodulin bound to the first two IQ motifs of myosin V reveals essential recognition features

A 2.5-A resolution structure of calcium-free calmodulin (CaM) bound to the first two IQ motifs of the murine myosin V heavy chain reveals an unusual CaM conformation. The C-terminal lobe of each CaM adopts a semi-open conformation that grips the first part of the IQ motif (IQxxxR), whereas the N-terminal lobe adopts a closed conformation that interacts more weakly with the second part of the motif (GxxxR). Variable residues in the IQ motif play a critical role in determining the precise structure of the bound CaM, such that even the consensus residues of different motifs show unique interactions with CaM. This complex serves as a model for the lever arm region of many classes of unconventional myosins, as well as other IQ motif-containing proteins such as neuromodulin and IQGAPs.

Calcium-dependent and -independent interactions of the S100 protein family

The S100 proteins comprise at least 25 members, forming the largest group of EF-hand signalling proteins in humans. Although the proteins are expressed in many tissues, each S100 protein has generally been shown to have a preference for expression in one particular tissue or cell type. Three-dimensional structures of several S100 family members have shown that the proteins assume a dimeric structure consisting of two EF-hand motifs per monomer. Calcium binding to these S100 proteins, with the exception of S100A10, results in an approx. 40 degrees alteration in the position of helix III, exposing a broad hydrophobic surface that enables the S100 proteins to interact with a variety of target proteins. More than 90 potential target proteins have been documented for the S100 proteins, including the cytoskeletal proteins tubulin, glial fibrillary acidic protein and F-actin, which have been identified mostly from in vitro experiments. In the last 5 years, efforts have concentrated on quantifying the protein interactions of the S100 proteins, identifying in vivo protein partners and understanding the molecular specificity for target protein interactions. Furthermore, the S100 proteins are the only EF-hand proteins that are known to form both homo- and hetero-dimers, and efforts are underway to determine the stabilities of these complexes and structural rationales for their formation and potential differences in their biological roles. This review highlights both the calcium-dependent and -independent interactions of the S100 proteins, with a focus on the structures of the complexes, differences and similarities in the strengths of the interactions, and preferences for homo- compared with hetero-dimeric S100 protein assembly.

Purification, crystallization and preliminary X-ray diffraction of human S100A15

Human S100A15 is a novel member of the S100 family of EF-hand calcium-binding proteins and was recently identified in psoriasis, where it is significantly upregulated in lesional skin. The protein is implicated as an effector in calcium-mediated signal transduction pathways. Although its biological function is unclear, the association of the 11.2 kDa S100A15 with psoriasis suggests that it contributes to the pathogenesis of the disease and could provide a molecular target for therapy. To provide insight into the function of S100A15, the protein was crystallized to visualize its structure and to further the understanding of how the many similar calcium-binding mediator proteins in the cell distinguish their cognate target molecules. The S100A15 protein has been cloned, expressed and purified to homogeneity and produced two crystal forms. Crystals of form I are triclinic, with unit-cell parameters a = 33.5, b = 44.3, c = 44.8 angstroms, alpha = 71.2, beta = 68.1, gamma = 67.8 degrees and an estimated two molecules in the asymmetric unit, and diffract to 1.7 angstroms resolution. Crystals of form II are monoclinic, with unit-cell parameters a = 82.1, b = 33.6, c = 52.2 angstroms, beta = 128.2 degrees and an estimated one molecule in the asymmetric unit, and diffract to 2.0 angstroms resolution. This structural analysis of the human S100A15 will further aid in the phylogenic comparison between the other members of the S100 protein family, especially the highly homologous paralog S100A7.

The EF-hand domain: a globally cooperative structural unit

EF-hand Ca(2+)-binding proteins participate in both modulation of Ca(2+) signals and direct transduction of the ionic signal into downstream biochemical events. The range of biochemical functions of these proteins is correlated with differences in the way in which they respond to the binding of Ca(2+). The EF-hand domains of calbindin D(9k) and calmodulin are homologous, yet they respond to the binding of calcium ions in a drastically different manner. A series of comparative analyses of their structures enabled the development of hypotheses about which residues in these proteins control the calcium-induced changes in conformation. To test our understanding of the relationship between protein sequence and structure, we specifically designed the F36G mutation of the EF-hand protein calbindin D(9k) to alter the packing of helices I and II in the apoprotein. The three-dimensional structure of apo F36G was determined in solution by nuclear magnetic resonance spectroscopy and showed that the design was successful. Surprisingly, significant structural perturbations also were found to extend far from the site of mutation. The observation of such long-range effects provides clear evidence that four-helix EF-hand domains should be treated as a single globally cooperative unit. A hypothetical mechanism for how the long-range effects are transmitted is described. Our results support the concept of energetic and structural coupling of the key residues that are crucial for a protein's fold and function.

An extended hydrophobic core induces EF-hand swapping

The structure of calbindin D(9k) with two substitutions was determined by X-ray crystallography at 1.8-A resolution. Unlike wild-type calbindin D(9k), which is a monomeric protein with two EF-hands, the structure of the mutated calbindin D(9k) reveals an intertwined dimer. In the dimer, two EF-hands of the monomers have exchanged places, and thus a 3D domain-swapped dimer has been formed. EF-hand I of molecule A is packed toward EF-hand II of molecule B and vice versa. The formation of a hydrophobic cluster, in a region linking the EF-hands, promotes the conversion of monomers to 3D domain-swapped dimers. We propose a mechanism by which domain swapping takes place via the apo form of calbindin D(9k). Once formed, the calbindin D(9k) dimers are remarkably stable, as with even larger misfolded aggregates like amyloids. Thus calbindin D(9k) dimers cannot be converted to monomers by dilution. However, heating can be used for conversion, indicating high energy barriers separating monomers from dimers.

Folding units in calcium vector protein of amphioxus: Structural and functional properties of its amino- and carboxy-terminal halves

Muscle of amphioxus contains large amounts of a four EF-hand Ca2+-binding protein, CaVP, and its target, CaVPT. To study the domain structure of CaVP and assess the structurally important determinants for its interaction with CaVPT, we expressed CaVP and its amino (N-CaVP) and carboxy-terminal halves (C-CaVP). The interactive properties of recombinant and wild-type CaVP are very similar, despite three post-translational modifications in the wild-type protein. N-CaVP does not bind Ca2+, shows a well-formed hydrophobic core, and melts at 44 degrees C. C-CaVP binds two Ca2+ with intrinsic dissociation constants of 0.22 and 140 microM (i.e., very similar to the entire CaVP). The metal-free domain in CaVP and C-CaVP shows no distinct melting transition, whereas its 1Ca2+ and 2Ca2+) forms melt in the 111 degrees -123 degrees C range, suggesting that C-CaVP and the carboxy- domain of CaVP are natively unfolded in the metal-free state and progressively gain structure upon binding of 1Ca2+ and 2Ca2+. Thermal denaturation studies provide evidence for interdomain interaction: the apo, 1Ca2+ and 2Ca2+ states of the carboxy-domain destabilize to different degrees the amino-domain. Only C-CaVP forms a Ca2+-dependent 1:1 complex with CaVPT. Our results suggest that the carboxy-terminal domain of CaVP interacts with CaVPT and that the amino-terminal lobe modulates this interaction.

Solvation energetics and conformational change in EF-hand proteins

Calmodulin and other members of the EF-hand protein family are known to undergo major changes in conformation upon binding Ca(2+). However, some EF-hand proteins, such as calbindin D9k, bind Ca(2+) without a significant change in conformation. Here, we show the importance of a precise balance of solvation energetics to conformational change, using mutational analysis of partially buried polar groups in the N-terminal domain of calmodulin (N-cam). Several variants were characterized using fluorescence, circular dichroism, and NMR spectroscopy. Strikingly, the replacement of polar side chains glutamine and lysine at positions 41 and 75 with nonpolar side chains leads to dramatic enhancement of the stability of the Ca(2+)-free state, a corresponding decrease in Ca(2+)-binding affinity, and an apparent loss of ability to change conformation to the open form. The results suggest a paradigm for conformational change in which energetic strain is accumulated in one state in order to modulate the energetics of change to the alternative state.

erythraea: a molten globule when deprived of Ca(2+)

Calerythrin, a four-EF-hand calcium-binding protein from Saccharopolyspora erythraea, exists in an equilibrium between ordered and less ordered states with slow exchange kinetics when deprived of Ca(2+) and at low temperatures, as observed by NMR. As the temperature is raised, signal dispersion in NMR spectra reduces, and intensity of near-UV CD bands decreases. Yet far-UV CD spectra indicate only a small decrease in the amount of secondary structure, and SAXS data show that no significant change occurs in the overall size and shape of the protein. Thus, at elevated temperatures, the equilibrium is shifted toward a state with characteristics of a molten globule. The fully structured state is reached by Ca(2+)-titration. Calcium first binds cooperatively to the C-terminal sites 3 and 4 and then to the N-terminal site 1, which is paired with an atypical, nonbinding site 2. EF-hand 2 still folds together with the C-terminal half of the protein, as deduced from the order of appearance of backbone amide cross peaks in the NMR spectra of partially Ca(2+)-saturated states.

Overexpression of CALNUC (nucleobindin) increases agonist and thapsigargin releasable Ca2+ storage in the Golgi

We previously demonstrated that CALNUC, a Ca2+-binding protein with two EF-hands, is the major Ca2+-binding protein in the Golgi by 45Ca2+ overlay (Lin, P., H. Le-Niculescu, R. Hofmeister, J.M. McCaffery, M. Jin, H. Henneman, T. McQuistan, L. De Vries, and M. Farquhar. 1998. J. Cell Biol. 141:1515-1527). In this study we investigated CALNUC's properties and the Golgi Ca2+ storage pool in vivo. CALNUC was found to be a highly abundant Golgi protein (3.8 microg CALNUC/mg Golgi protein, 2.5 x 10(5) CALNUC molecules/NRK cell) and to have a single high affinity, low capacity Ca2+-binding site (Kd = 6.6 microM, binding capacity = 1.1 micromol Ca2+/micromol CALNUC). 45Ca2+ storage was increased by 2.5- and 3-fold, respectively, in HeLa cells transiently overexpressing CALNUC-GFP and in EcR-CHO cells stably overexpressing CALNUC. Deletion of the first EF-hand alpha helix from CALNUC completely abolished its Ca2+-binding capability. CALNUC was correctly targeted to the Golgi in transfected cells as it colocalized and cosedimented with the Golgi marker, alpha-mannosidase II (Man II). Approximately 70% of the 45Ca2+ taken up by HeLa and CHO cells overexpressing CALNUC was released by treatment with thapsigargin, a sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) (Ca2+ pump) blocker. Stimulation of transfected cells with the agonist ATP or IP3 alone (permeabilized cells) also resulted in a significant increase in Ca2+ release from Golgi stores. By immunofluorescence, the IP3 receptor type 1 (IP3R-1) was distributed over the endoplasmic reticulum and codistributed with CALNUC in the Golgi. These results provide direct evidence that CALNUC binds Ca2+ in vivo and together with SERCA and IP3R is involved in establishment of the agonist-mobilizable Golgi Ca2+ store.