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]. To explore this diversity further, we have isolated and sequenced partial clones of two Ins(1,4,5)P $ R mRNAs from the mouse embryonic C $ H10T" # cell line. These clones showed between 94.2 and 94.9 % sequence identity with the corresponding rat Ins(1,4,5)P $ R-II and Ins(1,4,5)P $ R-III isoforms. Based on these newly obtained sequences we have determined the relative expression of the different Ins(1,4,5)P $ R mRNAs in cultured cells and in animal tissues of mouse origin by a ratio reverse transcriptase polymerase chain reaction (RT-

We investigated the interaction of the 12 kDa FK506-binding protein (FKBP12) with two ryanodine-receptor isoforms (RyR1 and RyR3) and with two myo-inositol 1,4,5-trisphosphate (IP $ ) receptor isoforms (IP $ R1 and IP $ R3). Using glutathione Stransferase (GST)-FKBP12 affinity chromatography, we could efficiently extract RyR1 (42p7 % of the solubilized RyR1) from terminal cisternae of skeletal muscle as well as RyR3 (32p4 % of the solubilized RyR3) from RyR3-overexpressing HEK-293 cells. These interactions were completely abolished by FK506 (20 µM) but were largely unaffected by RyR-channel modulators. In contrast, neither IP $ R1 nor IP $ R3 from various sources, including rabbit cerebellum, A7r5 smooth-muscle cells and IP $ R-overexpressing Sf9 insect cells from Spodoptera frugiperda, were retained on the GST-FKBP12 matrix. Moreover, immunoprecipitation experiments indicated a high-affinity interaction of FKBP12 with RyR1 but not with IP $ R1. In order to determine the FKBP12-binding site, we fragmented both RyR1 and IP $ R1

Anti-apoptotic Bcl-2 proteins are upregulated in different cancers, including diffuse large B-cell lymphoma (DLBCL) and chronic lymphocytic leukemia (CLL), enabling survival by inhibiting pro-apoptotic Bcl-2-family members and inositol 1,4,5-trisphosphate (IP 3 ) receptor (IP 3 R)-mediated Ca 2+ -signaling. A peptide tool (Bcl-2/IP 3 R Disruptor-2; BIRD-2) was developed to abrogate the interaction of Bcl-2 with IP 3 Rs by targeting Bcl-2′s BH4 domain. BIRD-2 triggers cell death in primary CLL cells and in DLBCL cell lines. Particularly, DLBCL cells with high levels of IP 3 R2 were sensitive to BIRD-2. Here, we report that BIRD-2-induced cell death in DLBCL cells does not only depend on high IP 3 R2-expression levels, but also on constitutive IP 3 signaling, downstream of the tonically active B-cell receptor. The basal Ca 2+ level in SU-DHL-4 DLBCL cells was significantly elevated due to the constitutive IP 3 production. This constitutive IP 3 signaling fulfilled a prosurvival role, since inhibition of phospholipase C (PLC) using U73122 (2.5 µM) caused cell death in SU-DHL-4 cells. Milder inhibition of IP 3 signaling using a lower U73122 concentration (1 µM) or expression of an IP 3 sponge suppressed both BIRD-2-induced Ca 2+ elevation and apoptosis in SU-DHL-4 cells. Basal PLC/IP 3 signaling also fulfilled a pro-survival role in other DLBCL cell lines, including Karpas 422, RI-1 and SU-DHL-6 cells, whereas PLC inhibition protected these cells against BIRD-2-evoked apoptosis. Finally, U73122 treatment also suppressed BIRD-2-induced cell death in primary CLL, both in unsupported systems and in co-cultures with CD40L-expressing fibroblasts. Thus, constitutive IP 3 signaling in lymphoma and leukemia cells is not only important for cancer cell survival, but also represents a vulnerability, rendering cancer cells dependent on Bcl-2 to limit IP 3 R activity. BIRD-2 seems to switch constitutive IP 3 signaling from pro-survival into pro-death, presenting a plausible therapeutic strategy.

Bcl-2 protein has emerged as a critical regulator of intracellular Ca 2+ dynamics by directly targeting and inhibiting the IP3 receptor (IP3R), a major intracellular Ca 2+ -release channel. Here, we demonstrate that such inhibition occurs under conditions of basal, but not high IP3R activity, since overexpressed and purified Bcl-2 (or its BH4 domain) can inhibit IP3R function provoked by low concentration of agonist or IP3, while fails to attenuate against high concentration of agonist or IP3. Surprisingly, Bcl-2 remained capable of inhibiting IP3R1 channels lacking the residues encompassing the previously identified Bcl-2-binding site (a.a. 1380-1408) located in the ARM2 domain, part of the modulatory region. Using a plethora of computational, biochemical and biophysical methods, we demonstrate that Bcl-2 and more particularly its BH4 domain bind to the ligand-binding domain (LBD) of IP3R1. In line with this finding, the interaction between the LBD and Bcl-2 (or its BH4 domain) was sensitive to IP3 and adenophostin A, ligands of the IP3R. Consistent with this, the BH4 domain of Bcl-2 counteracted the binding of IP3 to the LBD. Collectively, our work reveals a novel mechanism by which Bcl-2 influences IP3R activity at the level of the LBD. This allows for exquisite modulation of Bcl-2's inhibitory properties on IP3Rs that is tunable to the level of IP3 signaling in cells.

The pro-and antiapoptotic proteins belonging to the B-cell lymphoma-2 (Bcl-2) family exert a critical control over cell-death processes by enabling or counteracting mitochondrial outer membrane permeabilization. Beyond this mitochondrial function, several Bcl-2 family members have emerged as critical modulators of intracellular Ca 2+ homeostasis and dynamics, showing proapoptotic and antiapoptotic functions. Bcl-2 family proteins specifically target several intracellular Ca 2+ -transport systems, including organellar Ca 2+ channels: inositol 1,4,5-trisphosphate receptors (IP 3 Rs) and ryanodine receptors (RyRs), Ca 2+ -release channels mediating Ca 2+ flux from the endoplasmic reticulum, as well as voltage-dependent anion channels (VDACs), which mediate Ca 2+ flux across the mitochondrial outer membrane into the mitochondria. Although the formation of protein complexes between Bcl-2 proteins and these channels has been extensively studied, a major advance during recent years has been elucidating the complex interaction of Bcl-2 proteins with IP 3 Rs. Distinct interaction sites for different Bcl-2 family members were identified in the primary structure of IP 3 Rs. The unique molecular profiles of these Bcl-2 proteins may account for their distinct functional outcomes when bound to IP 3 Rs. Furthermore, Bcl-2 inhibitors used in cancer therapy may affect IP 3 R function as part of their proapoptotic effect and/or as an adverse effect in healthy cells. B-CELL LYMPHOMA-2 (Bcl-2) FAMILY OF PROTEINST he Bcl-2 family of proteins consists of proand antiapoptotic members, which are characterized by the presence of at least one of the four highly conserved α-helical motifs, termed Bcl-2 homology (BH) domains (Adams and Cory 1998). The antiapoptotic family members, such as Bcl-2, Bcl-Xl, and Mcl-1, contain all four BH domains where the BH1, BH2, and BH3 domains form a hydrophobic cleft (Fig. 1A). The hydrophobic cleft is separated from the amino-terminal BH4 domain by an unstructured

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