Inositol 1,4,5-trisphosphate receptors are downregulated in mouse oocytes in response to sperm or adenophostin A but not to increases in intracellular Ca(2+) or egg activation

Fertilization in mammals stimulates a series of Ca(2+) oscillations that continue for 3-4 h. Cell-cycle-dependent changes in the ability to release Ca(2+) are one mechanism that leads to the inhibition of Ca(2+) transients after fertilization. The downregulation of InsP(3)Rs at fertilization may be an additional mechanism for inhibiting Ca(2+) transients. In the present study we examine the mechanism of this InsP(3)R downregulation. We find that neither egg activation nor Ca(2+) transients are necessary or sufficient for the stimulation of InsP(3)R downregulation. First, parthenogenetic activation fails to stimulate downregulation. Second, downregulation persists when fertilization-induced Ca(2+) transients and egg activation are inhibited using BAPTA. Third, downregulation can be induced in immature oocytes that do not undergo egg activation. Other than fertilization, the only stimulus that downregulated InsP(3)Rs was microinjection of the potent InsP(3)R agonist adenophostin A. InsP(3)R downregulation was inhibited by the cysteine protease inhibitor ALLN but MG132 and lactacystin were not effective. Finally, we have injected maturing oocytes with adenophostin A and produced MII eggs depleted of InsP(3)Rs. We show that sperm-induced Ca(2+) signaling is inhibited in such InsP(3)R-depleted eggs. These data show that InsP(3)R binding is sufficient for downregulation and that Ca(2+) signaling at fertilization is mediated via the InsP(3)R.     

Molecular cloning and characterization of the inositol 1,4,5-trisphosphate receptor in Drosophila melanogaster.

We isolated a cDNA encoding an inositol 1,4,5-trisphosphate receptor (InsP3R) of Drosophila melanogaster. The predicted Drosophila InsP3R (2,833 amino acids) has extensive sequence similarity to the mouse InsP3R. The polypeptide encoded by the cDNA was functionally expressed and showed characteristic InsP3-binding activity. The Drosophila InsP3R gene is located at the region 83A5-9 on the third chromosome and expresses throughout development but predominantly in the adult. Localization of the InsP3R mRNA in adult tissues suggests strong expression in the retina and antenna, indicating the involvement of the InsP3R in visual and olfactory transduction. In addition, the InsP3R mRNA is abundant in the legs and thorax, which are enriched with a muscular system. Such localization is apparently consistent with the quantitatively predominant sites for [3H]InsP3 binding in Drosophila and the fleshfly (Boettcherisca peregrina). The present study points to the likely functional importance of the InsP3/Ca2+ signaling system in Drosophila.     

Expression and distribution of InsP(3) receptor subtypes in proliferating vascular smooth muscle cells

The expression and distribution of types 1, 2, and 3 inositol 1,4, 5-trisphosphate receptor (InsP(3)R) in proliferating, primary cultures of rat aortic smooth muscle were compared to fully developed and differentiated rat aortic smooth muscle. Subtype-specific InsP(3)R antibodies revealed that the expression of type 1 InsP(3)R was similar in cultured aortic cells and aorta homogenate but expression of type 2 and 3 InsP(3)R subtypes was increased 3-fold in cultured aortic cells. The distribution of the type 1 InsP(3)R was located throughout the cytoplasm; type 2 InsP(3)R was found closely associated with the nucleus and at the plasma membrane; type 3 InsP(3)R was distributed predominantly around the nucleus. Alterations in InsP(3)R subtype expression and localization may have important functions in regulating intracellular calcium release around the nucleus when vascular smooth muscle cells switch to a more proliferating phenotype.     

Cardiac type 2 inositol 1,4,5-trisphosphate receptor: interaction and modulation by calcium/calmodulin-dependent protein kinase II

The type 2 inositol 1,4,5-trisphosphate receptor (InsP(3)R2) was identified previously as the predominant isoform in cardiac ventricular myocytes. Here we reported the subcellular localization of InsP(3)R2 to the cardiomyocyte nuclear envelope (NE). The other major known endo/sarcoplasmic reticulum calcium-release channel (ryanodine receptor) was not localized to the NE, indicating functional segregation of these channels and possibly a unique role for InsP(3)R2 in regulating nuclear calcium dynamics. Immunoprecipitation experiments revealed that the NE InsP(3)R2 associates with Ca(2+)/calmodulin-dependent protein kinase IIdelta (CaMKIIdelta), the major isoform expressed in cardiac myocytes. Recombinant InsP(3)R2 and CaMKIIdelta(B) also co-immunoprecipitated after co-expression in COS-1 cells. Additionally, the amino-terminal 1078 amino acids of the InsP(3)R2 were sufficient for interaction with CaMKIIdelta(B) and associated upon mixing following separate expression. CaMKII can also phosphorylate InsP(3)R2, as demonstrated by (32)P labeling. Incorporation of CaMKII-treated InsP(3)R2 into planar lipid bilayers revealed that InsP(3)-mediated channel open probability is significantly reduced ( approximately 11 times) by phosphorylation via CaMKII. We concluded that the InsP(3)R2 and CaMKIIdelta likely represent two central components of a multiprotein signaling complex, and this raises the possibility that calcium release via InsP(3)R2 in the myocyte NE may activate local CaMKII signaling, which may feedback on InsP(3)R2 function.     

Functional characterization of the type 1 inositol 1,4,5-trisphosphate receptor coupling domain SII(+/-) splice variants and the Opisthotonos mutant form

The type 1 inositol (1,4,5)-trisphosphate receptor (InsP3R1) plays a critical role in Ca2+ signaling in cells. Neuronal and nonneuronal isoforms of the InsP3R1 differ by alternative splicing in the coupling domain of the InsP3R1 (SII site). Deletion of 107 amino acids from the coupling domain of the InsP3R1 results in epileptic-like behaviors in opisthotonos (opt) spontaneous mouse mutant. Using Spodoptera frugiperda cells expression system, we compared single-channel behavior of recombinant InsP3R1-SII(+), InsP3R1-SII(-), and InsP3R1-opt channels in planar lipid bilayers. The main results of our study are: 1) the InsP3R1-SII(-) has a higher conductance (94 pS) and the InsP3R1-opt has a lower conductance (64 pS) than the InsP3R1-SII(+) (81 pS); 2) the bell-shaped Ca2+-dependence peaks at 200-300 nM Ca2+ for all three InsP3R1 isoforms; 3) the bell-shaped Ca2+-dependence is wider for the InsP3R1-SII(+) and narrower for the InsP3R1-SII(-) and InsP3R1-opt; 4) the apparent affinity for ATP is sixfold lower for the InsP3R1-SII(-) (1.4 mM) and 20-fold lower for the InsP3R1-opt (5.3 mM) than for the InsP3R1-SII(+) (0.24 mM); 5) the InsP3R1-SII(-) is approximately twofold more active than the InsP3R1-SII(+) in the absence of ATP. Obtained results provide novel information about the molecular determinants of the InsP3R1 function.     

Electroconvulsive Shock Reduces Inositol 1,4,5-Trisphosphate 3-Kinase mRNA Expression in Rat Dentate Gyrus

Abstract: We investigated the expression of inositol 1,4,5-trisphosphate (InsP₃) 3-kinase mRNA after a single electroconvulsive shock (ECS) with in situ hybridization histochemistry in rat brain. At 6 h after ECS, the expression was markedly decreased in the dentate gyrus, and the decrease was maintained until 9 h with a slight recovery. The InsP₃ 3-kinase mRNA content returned to basal levels after 12 h. We could not detect any apparent changes in the expression of InsP₃ 3-kinase mRNA in the CA1–CA3 areas of hippocampus, the striatum, and the cerebral cortex at any time point examined. In the temporal pattern, the reduction of the expression in the dentate gyrus was preceded by the induction of c- fos after ECS. These observations suggest that the InsP₃ 3-kinase might be one of the genes whose expression can be altered by ECS.     

Functional and biochemical analysis of the type 1 inositol (1,4,5)-trisphosphate receptor calcium sensor

Modulation of the type 1 inositol (1,4,5)-trisphosphate receptors (InsP(3)R1) by cytosolic calcium (Ca(2+)) plays an essential role in their signaling function, but structural determinants and mechanisms responsible for the InsP(3)R1 regulation by Ca(2+) are poorly understood. Using DT40 cell expression system and Ca(2+) imaging assay, in our previous study we identified a critical role of E2100 residue in the InsP(3)R1 modulation by Ca(2+). By using intrinsic tryptophan fluorescence measurements in the present study we determined that the putative InsP(3)R1 Ca(2+)-sensor region (E1932-R2270) binds Ca(2+) with 0.16 micro M affinity. We further established that E2100D and E2100Q mutations decrease Ca(2+)-binding affinity of the putative InsP(3)R1 Ca(2+)-sensor region to 1 micro M. In planar lipid bilayer experiments with recombinant InsP(3)R1 expressed in Spodoptera frugiperda cells we discovered that E2100D and E2100Q mutations shifted the peak of the InsP(3)R1 bell-shaped Ca(2+) dependence from 0.2 micro M to 1.5 micro M Ca(2+). In agreement with the biochemical data, we found that the apparent affinities of Ca(2+) activating and inhibitory sites of the InsP(3)R1 were 0.2 micro M for the wild-type channels and 1-2 micro M Ca(2+) for the E2100D and E2100Q mutants. The results obtained in our study support the hypothesis that E2100 residue forms a part of the InsP(3)R1 Ca(2+) sensor.     

The inositol 1,4,5-trisphosphate receptors

The inositol (1,4,5)-trisphosphate receptors (InsP3R) are the intracellular calcium (Ca2+) release channels that play a key role in Ca2+ signaling in cells. Three InsP3R isoforms-InsP3R type 1 (InsP3R1), InsP3R type 2 (InsP3R2), and InsP3R type 3 (InsP3R3) are expressed in mammals. A single InsP3R isoform is expressed in Drosophila melanogaster (DmInsP3R) and Caenorhabditis elegans (CeInsP3R). The progress made during last decade towards understanding the function and the properties of the InsP3R is briefly reviewed in this chapter. The main emphasis is on studies that revealed structural determinants responsible for the ligand recognition by the InsP3R, ion permeability of the InsP3R, modulation of the InsP3R by cytosolic Ca2+, ATP and PKA phosphorylation and on the recently identified InsP3R-binding partners. The main focus is on the InsP3R1, but the recent information about properties of other InsP3R isoforms is also discussed.     

Association of type 1 inositol 1,4,5-trisphosphate receptor with AKAP9 (Yotiao) and protein kinase A

Inositol 1,4,5-trisphosphate receptors (InsP(3)R) play a key role in intracellular calcium (Ca(2+)) signaling. Three InsP(3)R isoforms are expressed in mammals. Type 1 InsP(3)R (InsP(3)R1) is a predominant neuronal isoform. Neuronal InsP(3)R1 is one of the major substrates of protein kinase A (PKA) phosphorylation. In our previous study (Tang, T. S., Tu, H., Wang, Z., and Bezprozvanny, I. (2003) J. Neurosci. 23, 403-415) we discovered a direct association between InsP(3)R1 and protein phosphatase 1 alpha (PP1 alpha). In functional experiments we demonstrated that phosphorylation by PKA activates InsP(3)R1 and that dephosphorylation by PP1 alpha inhibits InsP(3)R1. To extend these findings, here we investigated the possibility of InsP(3)R1-PKA association. In a series of biochemical experiments we demonstrate the following findings. 1) InsP(3)R1 and PKA associate in the brain. 2) InsP(3)R1-PKA association is mediated by the AKAP9 (Yotiao) multi-functional PKA anchoring protein. 3) InsP(3)R1-AKAP9 association is mediated via the leucine/isoleucine zipper (LIZ) motif in the InsP(3)R1 coupling domain and the fourth LIZ motif in AKAP9. 4) The InsP(3)R association with AKAP9 is specific for type 1 InsP(3)R. 5) Both the SII(+) and the SII(-) coupling domain splice variants of InsP(3)R1 bind to AKAP9. 6) Binding to AKAP9 promotes association of neuronal InsP(3)R1 with the NR1 NMDA receptor; and 7) neuronal InsP(3)R1 associate with PP1 directly via carboxy-terminus and indirectly via AKAP9. The obtained results advance our understanding of cross-talk between cAMP and InsP(3)/Ca(2+) signaling pathways in the brain.     

Functional characterization of mammalian inositol 1,4,5-trisphosphate receptor isoforms

Inositol 1,4,5-trisphosphate receptors (InsP3R) play a key role in intracellular calcium (Ca2+) signaling. Three mammalian InsP3R isoforms--InsP3R type 1 (InsP3R1), InsP3R type 2 (InsP3R2), and InsP3R type 3 (InsP3R3) are expressed in mammals, but the functional differences between the three mammalian InsP3R isoforms are poorly understood. Here we compared single-channel behavior of the recombinant rat InsP3R1, InsP3R2, and InsP3R3 expressed in Sf9 cells, reconstituted into planar lipid bilayers and recorded with 50 mM Ba2+ as a current carrier. We found that: 1), for all three mammalian InsP3R isoforms the size of the unitary current is 1.9 pA and single-channel conductance is 74-80 pS; 2), in optimal recording conditions the maximal single-channel open probability for all three mammalian InsP3R isoforms is in the range 30-40%; 3), in optimal recording conditions the mean open dwell time for all three mammalian InsP3R isoforms is 7-8 ms, the mean closed dwell time is approximately 10 ms; 4), InsP3R2 has the highest apparent affinity for InsP(3) (0.10 microM), followed by InsP3R1 (0.27 microM), and then by InsP3R3 (0.40 microM); 5), InsP3R1 has a high-affinity (0.13 mM) ATP modulatory site, InsP3R2 gating is ATP independent, and InsP3R3 has a low-affinity (2 mM) ATP modulatory site; 6), ATP modulates InsP3R1 gating in a noncooperative manner (n(Hill) = 1.3); 7), ATP modulates InsP3R3 gating in a highly cooperative manner (n(Hill) = 4.1). Obtained results provide novel information about functional properties of mammalian InsP3R isoforms.     

Single-channel properties in endoplasmic reticulum membrane of recombinant type 3 inositol trisphosphate receptor

The inositol 1,4,5-trisphosphate receptor (InsP(3)R) is an intracellular Ca(2+)-release channel localized in endoplasmic reticulum (ER) with a central role in complex Ca(2+) signaling in most cell types. A family of InsP(3)Rs encoded by several genes has been identified with different primary sequences, subcellular locations, variable ratios of expression, and heteromultimer formation. This diversity suggests that cells require distinct InsP(3)Rs, but the functional correlates of this diversity are largely unknown. Lacking are single-channel recordings of the recombinant type 3 receptor (InsP(3)R-3), a widely expressed isoform also implicated in plasma membrane Ca(2+) influx and apoptosis. Here, we describe functional expression and single-channel recording of recombinant rat InsP(3)R-3 in its native membrane environment. The approach we describe suggests a novel strategy for expression and recording of recombinant ER-localized ion channels in the ER membrane. Ion permeation and channel gating properties of the rat InsP(3)R-3 are strikingly similar to those of Xenopus type 1 InsP(3)R in the same membrane. Using two different two-electrode voltage clamp protocols to examine calcium store-operated calcium influx, no difference in the magnitude of calcium influx was observed in oocytes injected with rat InsP(3)R-3 cRNA compared with control oocytes. Our results suggest that if cellular expression of multiple InsP(3)R isoforms is a mechanism to modify the temporal and spatial features of [Ca(2+)](i) signals, then it must be achieved by isoform-specific regulation or localization of various types of InsP(3)Rs that have relatively similar Ca(2+) permeation properties.     

Single channel function of recombinant type-1 inositol 1,4,5-trisphosphate receptor ligand binding domain splice variants.

In this study we describe the expression and function of the two rat type-1 inositol 1,4,5-trisphosphate receptor (InsP3R) ligand binding domain splice variants (SI+/-/SII+). Receptor protein from COS-1 cells transfected with the type-1 InsP3R expression plasmids (pInsP3R-T1, pInsP3R-T1ALT) or control DNA were incorporated into planar lipid bilayers and the single channel properties of the recombinant receptors were defined. The unitary conductance of the two splice variants were approximately 290 pS with Cs+ as charge carrier and approximately 65 pS with Ca2+ as charge carrier. Both InsP3R expression products consistently behaved like those of the native type-1 receptor isoform isolated from cerebellum in terms of their InsP3, Ca2+, and heparin sensitivity. An InsP3 receptor ligand binding domain truncation lacking the 310 amino-terminal amino acids (pInsP3R-DeltaT1ALT) formed tetrameric complexes but failed to bind InsP3 with high affinity, and did not form functional Ca2+ channels when reconstituted in lipid bilayers. These data suggest that 1) the ligand binding alternative splice site is functionally inert in terms of InsP3 binding and single channel function, and 2) the single channel properties of the expressed recombinant type-1 channel are essentially identical to those of the native channel. This work establishes a foundation from which molecular/biophysical approaches can be used to define the structure-function properties of the InsP3 receptor channel family.     

The type III inositol 1,4,5-trisphosphate receptor is associated with aggressiveness of colorectal carcinoma

The inositol 1,4,5-trisphosphate receptor (InsP3R) mediates Ca(2+) signaling in epithelia and regulates cellular functions such as secretion, apoptosis and cell proliferation. Loss of one or more InsP3R isoform has been implicated in disease processes such as cholestasis. Here we examined whether gain of expression of InsP3R isoforms also may be associated with development of disease. Expression of all three InsP3R isoforms was evaluated in tissue from colorectal carcinomas surgically resected from 116 patients. Type I and II InsP3Rs were seen in both normal colorectal mucosa and colorectal cancer, while type III InsP3R was observed only in colorectal cancer. Type III InsP3R expression in the advancing margins of tumors correlated with depth of invasion, lymph node metastasis, liver metastasis, and TNM stage. Heavier expression of type III InsP3R also was associated with decreased 5-year survival. shRNA knockdown of type III InsP3R in CACO-2 colon cancer cells enhanced apoptosis, while over-expression of the receptor decreased apoptosis. Thus, type III InsP3R becomes expressed in colon cancer, and its expression level is directly related to aggressiveness of the tumor, which may reflect inhibition of apoptosis by the receptor. These findings suggest a previously unrecognized role for Ca(2+) signaling via this InsP3R isoform in colon cancer.     

Modulation of mammalian inositol 1,4,5-trisphosphate receptor isoforms by calcium: a role of calcium sensor region

In the accompanying article, we compared main functional properties of the three mammalian inositol 1,4,5-trisphosphate receptors (InsP3R) isoforms. In this article we focused on modulation of mammalian InsP3R isoforms by cytosolic Ca2+. We found that: 1), when recorded in the presence of 2 microM InsP3 and 0.5 mM ATP all three mammalian InsP3R isoforms display bell-shaped Ca2+ dependence in physiological range of Ca2+ concentrations (pCa 8-5); 2), in the same experimental conditions InsP3R3 is most sensitive to modulation by Ca2+ (peak at 107 nM Ca2+), followed by InsP3R2 (peak at 154 nM Ca2+), and then by InsP3R1 (peak at 257 nM Ca2+); 3), increase in ATP concentration to 5 mM had no significant effect of Ca2+ dependence of InsP3R1 and InsP3R2; 4), increase in ATP concentration to 5 mM converted Ca2+ dependence of InsP3R3 from "narrow" shape to "square" shape; 5), ATP-induced change in the shape of InsP3R3 Ca2+ dependence was mainly due to an >200-fold reduction in the apparent affinity of the Ca2+-inhibitory site; 6), the apparent Ca2+ affinity of the Ca2+ sensor region (Cas) determined in biochemical experiments is equal to 0.23 microM Ca2+ for RT1-Cas, 0.16 microM Ca2+ for RT2-Cas, and 0.10 microM Ca2+ for RT3-Cas; and 7), Ca2+ sensitivity of InsP3R1 and InsP3R3 isoforms recorded in the presence of 2 microM InsP3 and 0.5 mM ATP or 2 microM InsP3 and 5 mM ATP can be exchanged by swapping their Cas regions. Obtained results provide novel information about functional properties of mammalian InsP3R isoforms and support the importance of the Ca2+ sensor region (Cas) in determining the sensitivity of InsP3R isoforms to modulation by Ca2+.     

ATP binding to a unique site in the type-1 S2- inositol 1,4,5-trisphosphate receptor defines susceptibility to phosphorylation by protein kinase A

The subtype- and splice variant-specific modulation of inositol 1,4,5-trisphosphate receptors (InsP3R) by interaction with cellular factors plays a fundamental role in defining the characteristics of Ca2+ release in individual cell types. In this study, we investigate the binding properties and functional consequences of the expression of a putative nucleotide binding fold (referred to as the ATPC site) unique to the S2- splice variant of the type-1 InsP3R (InsP3R-1), the predominant splice variant in peripheral tissue. A glutathione S-transferase fusion protein encompassing amino acids 1574-1765 of the S2- InsP3R-1 and including the glycine-rich motif Gly-Tyr-Gly-Glu-Lys-Gly bound ATP specifically as measured by fluorescent trinitrophenyl-ATP binding. This binding was completely abrogated by a point mutation (G1690A) in the nucleotide binding fold. The functional sensitivity of S2- InsP3R-1 constructs was evaluated in DT40-3KO-M3 cells, a null background for InsP3R, engineered to express muscarinic M3 receptors. The S2- InsP3R-1 containing the G1690A mutation was markedly less sensitive to agonist stimulation than wild type S2- InsP3R-1 or receptors containing a similar (Gly --> Ala) mutation in the established nucleotide binding sites in InsP3R-1 (the ATPA and ATPB sites). The ATP sensitivity of InsP3-induced Ca2+ release, however, was not altered by the G1690A mutation when measured in permeabilized DT40-3KO cells, suggesting a unique role for the ATPC site. Ca2+ release was dramatically potentiated following activation of cAMP-dependent protein kinase in DT40-3KO cells transiently expressing wild type S2- InsP3R or Gly --> Ala mutations in the ATPA and ATPB sites, but phosphorylation of the receptor and the potentiation of Ca2+ release were absent in cells expressing the G1690A mutation in S2- InsP3R. These data indicate that ATP binding specifically to the ATPC site in S2- InsP3R-1 controls the susceptibility of the receptor to protein kinase A-mediated phosphorylation, contributes to the functional sensitivity of the S2- InsP3R-1 and ultimately the sensitivity of cells to agonist stimulation.     

The type III inositol 1,4,5-trisphosphate receptor is associated with aggressiveness of colorectal carcinoma

The inositol 1,4,5-trisphosphate receptor (InsP3R) mediates Ca²⁺ signaling in epithelia and regulates cellular functions such as secretion, apoptosis and cell proliferation. Loss of one or more InsP3R isoform has been implicated in disease processes such as cholestasis. Here we examined whether gain of expression of InsP3R isoforms also may be associated with development of disease. Expression of all three InsP3R isoforms was evaluated in tissue from colorectal carcinomas surgically resected from 116 patients. Type I and II InsP3Rs were seen in both normal colorectal mucosa and colorectal cancer, while type III InsP3R was observed only in colorectal cancer. Type III InsP3R expression in the advancing margins of tumors correlated with depth of invasion, lymph node metastasis, liver metastasis, and TNM stage. Heavier expression of type III InsP3R also was associated with decreased 5-year survival. shRNA knockdown of type III InsP3R in CACO-2 colon cancer cells enhanced apoptosis, while over-expression of the receptor decreased apoptosis. Thus, type III InsP3R becomes expressed in colon cancer, and its expression level is directly related to aggressiveness of the tumor, which may reflect inhibition of apoptosis by the receptor. These findings suggest a previously unrecognized role for Ca²⁺ signaling via this InsP3R isoform in colon cancer.     

ATP modulation of Ca2+ release by type-2 and type-3 inositol (1, 4, 5)-triphosphate receptors. Differing ATP sensitivities and molecular determinants of action

ATP enhances Ca(2+) release from inositol (1,4,5)-trisphosphate receptors (InsP(3)R). However, the three isoforms of InsP(3)R are reported to respond to ATP with differing sensitivities. Ca(2+) release through InsP(3)R1 is positively regulated at lower ATP concentrations than InsP(3)R3, and InsP(3)R2 has been reported to be insensitive to ATP modulation. We have reexamined these differences by studying the effects of ATP on InsP(3)R2 and InsP(3)R3 expressed in isolation on a null background in DT40 InsP(3)R knockout cells. We report that the Ca(2+)-releasing activity as well as the single channel open probability of InsP(3)R2 was enhanced by ATP, but only at submaximal InsP(3) levels. Further, InsP(3)R2 was more sensitive to ATP modulation than InsP(3)R3 under similar experimental conditions. Mutations in the ATPB sites of InsP(3)R2 and InsP(3)R3 were generated, and the functional consequences of these mutations were tested. Surprisingly, mutation of the ATPB site in InsP(3)R3 had no effect on ATP modulation, suggesting an additional locus for the effects of ATP on this isoform. In contrast, ablation of the ATPB site of InsP(3)R2 eliminated the enhancing effects of ATP. Furthermore, this mutation had profound effects on the patterns of intracellular calcium signals, providing evidence for the physiological significance of ATP binding to InsP(3)R2.