A Functional Interaction between Chromogranin B and the Inositol 1,4,5-Trisphosphate Receptor/Ca2+ Channel

Abstract

Chromogranins A and B (CGA and CGB) are high capacity, low affinity calcium (Ca2+) storage proteins found in many cell types most often associated with secretory granules of secretory cells but also with the endoplasmic reticulum (ER) lumen of these cells. Both CGA and CGB associate with inositol 1,4,5-trisphosphate receptor (InsP3R) in a pH-dependent manner. At an intraluminal pH of 5.5, as found in secretory vesicles, both CGA and CGB bind to the InsP3R. When the intraluminal pH is 7.5, as found in the ER, CGA totally dissociates from InsP3R, whereas CGB only partially dissociates. To investigate the functional consequences of the interaction between the InsP3R and CGB monomers or CGA/CGB heteromers, purified mouse InsP3R type I were fused to planar lipid bilayers and activated by 2 μm InsP3. In the presence of luminal CGB monomers or CGA/CGB heteromers the InsP3R/Ca2+ channel open probability and mean open time increased significantly. The channel activity remained elevated when the pH was changed to 7.5, a reflection of CGB binding to the InsP3R even at pH 7.5. These results suggest that CGB may play an important modulatory role in the control of Ca2+ release from the ER. Furthermore, the difference in the ability of CGA and CGB to regulate the InsP3R/Ca2+ channel and the variability of CGA/CGB ratios could influence the pattern of InsP3-mediated Ca2+ release. Chromogranins A and B (CGA and CGB) are high capacity, low affinity calcium (Ca2+) storage proteins found in many cell types most often associated with secretory granules of secretory cells but also with the endoplasmic reticulum (ER) lumen of these cells. Both CGA and CGB associate with inositol 1,4,5-trisphosphate receptor (InsP3R) in a pH-dependent manner. At an intraluminal pH of 5.5, as found in secretory vesicles, both CGA and CGB bind to the InsP3R. When the intraluminal pH is 7.5, as found in the ER, CGA totally dissociates from InsP3R, whereas CGB only partially dissociates. To investigate the functional consequences of the interaction between the InsP3R and CGB monomers or CGA/CGB heteromers, purified mouse InsP3R type I were fused to planar lipid bilayers and activated by 2 μm InsP3. In the presence of luminal CGB monomers or CGA/CGB heteromers the InsP3R/Ca2+ channel open probability and mean open time increased significantly. The channel activity remained elevated when the pH was changed to 7.5, a reflection of CGB binding to the InsP3R even at pH 7.5. These results suggest that CGB may play an important modulatory role in the control of Ca2+ release from the ER. Furthermore, the difference in the ability of CGA and CGB to regulate the InsP3R/Ca2+ channel and the variability of CGA/CGB ratios could influence the pattern of InsP3-mediated Ca2+ release. Chromogranin B (CGB) 1The abbreviations used are: CGB and CGAchromogranin B and A, respectivelyERendoplasmic reticulumInsP3Rinositol 1,4,5-trisphosphate (InsP3) receptorGSTglutathione S-transferaseCHAPS3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid. belongs to the granin protein family, which also includes chromogranin A (CGA) and secretogranin II (chromogranin C). It is found in the large dense core secretory granules and in the endoplasmic reticulum (ER) lumen of most neurons, exo/endocrine cells, and neuroendocrine cells and shows a wide distribution in various areas of the brain (1.Weiler R. Marksteiner J. Bellmann R. Wohlfarter T. Schober M. Fischer-Colbrie R. Sperk G. Winkler H. Brain Res. 1990; 532: 87-94Crossref PubMed Scopus (71) Google Scholar). Levels of CGB expression in cells can be used as markers for a number of physiological and medically important pathophysiological conditions (2.Lamberts S.W. Hofland L.J. Nobels F.R. Front. Neuroendocrinol. 2001; 22: 309-339Crossref PubMed Scopus (83) Google Scholar, 3.Mahata S.K. Marksteiner J. Sperk G. Mahata M. Gruber B. Fischer-Colbrie R. Winkler H. Brain Res. Mol. Brain Res. 1992; 16: 1-12Crossref PubMed Scopus (62) Google Scholar, 4.Marksteiner J. Kaufmann W.A. Gurka P. Humpel C. J. Mol. Neurosci. 2002; 18: 53-63Crossref PubMed Scopus (44) Google Scholar). In normal brain tissue CGB expression is enhanced after neuronal activation, providing a marker for stimulated neurons (3.Mahata S.K. Marksteiner J. Sperk G. Mahata M. Gruber B. Fischer-Colbrie R. Winkler H. Brain Res. Mol. Brain Res. 1992; 16: 1-12Crossref PubMed Scopus (62) Google Scholar). In addition to the tissue-specific distribution, a regionally specific distribution of CGB has been found intracellularly in neuronally differentiated pheochromocytoma (PC12) cells (5.Johenning F.W. Zochowski M. Conway S.J. Holmes A.B. Koulen P. Ehrlich B.E. J. Neurosci. 2002; 22: 5344-5353Crossref PubMed Google Scholar). In these cells CGB levels are elevated in the neurites rather than in the soma, which correlates with the initiation site for intracellular calcium (Ca2+) signals. The levels of CGB and chromogranin-derived peptides can be diagnostic markers for pathophysiological conditions. For example, levels of CGB are greatly reduced in the cerebro-spinal fluid of chronic schizophrenia subjects (6.Zhang B. Tan Z. Zhang C. Shi Y. Lin Z. Gu N. Feng G. He L. Neurosci. Lett. 2002; 323: 229-233Crossref PubMed Scopus (22) Google Scholar, 7.Landen M. Grenfeldt B. Davidsson P. Stridsberg M. Regland B. Gottfries C.G. Blennow K. Eur. Neuropsychopharmacol. 1999; 9: 311-315Crossref PubMed Scopus (31) Google Scholar). Moreover, the levels of CGB and chromogranin-derived peptides are diagnostically significant as neuronal markers for synaptic degeneration in Alzheimer's disease (4.Marksteiner J. Kaufmann W.A. Gurka P. Humpel C. J. Mol. Neurosci. 2002; 18: 53-63Crossref PubMed Scopus (44) Google Scholar). chromogranin B and A, respectively endoplasmic reticulum inositol 1,4,5-trisphosphate (InsP3) receptor glutathione S-transferase 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid. At the cellular level CGB is believed to have many intra- and extracellular functions. CGB functions as a heparin binding extracellular matrix protein, mediating adhesion of cells and supporting neurite outgrowth (8.Chen M. Tempst P. Yankner B.A. J. Neurochem. 1992; 58: 1691-1698Crossref PubMed Scopus (22) Google Scholar). CGB is a prohormone with numerous di- and tribasic amino acid cleavage sites that act as targets for proteolytic enzymes such as the prohormone convertase (9.Strub J.M. Sorokine O. Van Dorsselaer A. Aunis D. Metz-Boutigue M.H. J. Biol. Chem. 1997; 272: 11928-11936Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar, 10.Winkler H. Fischer-Colbrie R. Neuroscience. 1992; 49: 497-528Crossref PubMed Scopus (613) Google Scholar). Furthermore, chromogranin B is known to bind >90 mol of Ca2+/mol with a dissociation constant (Kd) of 1.5 mm (11.Yoo S. Oh Y. Kang M. Huh Y. So S. Park H. Park H. J. Biol. Chem. 2001; 276: 45806-45812Abstract Full Text Full Text PDF PubMed Scopus (53) Google Scholar), distinguishing itself as a very efficient Ca2+ storage protein. Intracellularly, CGB has also been suggested to participate in packaging and sorting other proteins into the secretory vesicles of neuroendocrine cells, thus playing key roles in secretory granule biogenesis (12.Gorr S.U. Shioi J. Cohn D.V. Am. J. Physiol. 1989; 257: E247-E254PubMed Google Scholar, 13.Yoo S.H. Albanesi J.P. J. Biol. Chem. 1990; 265: 13446-13448Abstract Full Text PDF PubMed Google Scholar, 14.Chanat E. Huttner W.B. J. Cell Biol. 1991; 115: 1505-1519Crossref PubMed Scopus (392) Google Scholar). Indeed, CGB has recently been shown to induce secretory granule biogenesis (15.Huh Y.H. Jeon S.H. Yoo S.H. J. Biol. Chem. 2003; 278: 40581-40589Abstract Full Text Full Text PDF PubMed Scopus (135) Google Scholar). CGB also localizes to the nucleus and controls transcription of many genes, including those for transcription factors (16.Yoo S.H. You S.H. Kang M.K. Huh Y.H. Lee C.S. Shim C.S. J. Biol. Chem. 2002; 277: 16011-16021Abstract Full Text Full Text PDF PubMed Scopus (44) Google Scholar). In secretory granules both CGA and CGB have been shown to interact with the InsP3R at the intravesicular pH of 5.5 (11.Yoo S. Oh Y. Kang M. Huh Y. So S. Park H. Park H. J. Biol. Chem. 2001; 276: 45806-45812Abstract Full Text Full Text PDF PubMed Scopus (53) Google Scholar, 17.Yoo S.H. Trends Neurosci. 2000; 23: 424-428Abstract Full Text Full Text PDF PubMed Scopus (64) Google Scholar). Purified InsP3R interacts directly with CGA and CGB at pH 5.5. CGA dissociates from the InsP3R at pH 7.5, whereas CGB remains partially associated (18.Yoo S.H. So S.H. Kweon H.S. Lee J.S. Kang M.K. Jeon C.J. J. Biol. Chem. 2000; 275: 12553-12559Abstract Full Text Full Text PDF PubMed Scopus (62) Google Scholar). Both chromogranin proteins form a complex with the InsP3R in vivo (11.Yoo S. Oh Y. Kang M. Huh Y. So S. Park H. Park H. J. Biol. Chem. 2001; 276: 45806-45812Abstract Full Text Full Text PDF PubMed Scopus (53) Google Scholar). The functional aspect of this coupling has been investigated for CGA alone using single channel experiments and Ca2+ flux studies (19.Thrower E. Park H. So S. Yoo S. Ehrlich B. J. Biol. Chem. 2002; 277: 15801-15806Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar). In the presence of CGA the open probability and mean open time of the InsP3R channel increases 10-fold. Despite the role of secretory granules of secretory cells and ER as major InsP3-sensitive intracellular Ca2+ stores (13.Yoo S.H. Albanesi J.P. J. Biol. Chem. 1990; 265: 13446-13448Abstract Full Text PDF PubMed Google Scholar, 20.Gerasimenko O.V. Gerasimenko J.V. Belan P.V. Petersen O.H. Cell. 1996; 84: 473-480Abstract Full Text Full Text PDF PubMed Scopus (217) Google Scholar, 21.Nguyen T. Chin W.C. Verdugo P. Nature. 1998; 395: 908-912Crossref PubMed Scopus (160) Google Scholar) and the abundance of CGB in these (10.Winkler H. Fischer-Colbrie R. Neuroscience. 1992; 49: 497-528Crossref PubMed Scopus (613) Google Scholar) and a variety of other cell types (1.Weiler R. Marksteiner J. Bellmann R. Wohlfarter T. Schober M. Fischer-Colbrie R. Sperk G. Winkler H. Brain Res. 1990; 532: 87-94Crossref PubMed Scopus (71) Google Scholar), the functional interaction of InsP3R and CGB is less well characterized. Given also the tendency of CGA/CGB mixture to spontaneously form a CGA/CGB heterodimer at a near physiological pH 7.5 and a CGA2CGB2 heterotetramer at the intragranular pH 5.5 (22.Yoo S.H. Lewis M.S. J. Biol. Chem. 1996; 271: 17041-17046Abstract Full Text Full Text PDF PubMed Scopus (42) Google Scholar), it is important to understand the effects of CGB and CGA/CGB heteromers. In this study, we examined the effect of CGB and CGA/CGB heteromers the channel of the InsP3R type CGB increased the open probability and mean open time of the channel by in to this functional effect was less to in pH when with the effect of CGA These results the functional interaction of InsP3R with CGB monomers and CGA/CGB heteromers and suggest that of these a important role in the pattern of InsP3-mediated Ca2+ release. InsP3R specific to amino of type was with a and was The was the the S.H. J. Biol. Chem. Full Text PDF PubMed Google Scholar), and the of the was been (11.Yoo S. Oh Y. Kang M. Huh Y. So S. Park H. Park H. J. Biol. Chem. 2001; 276: 45806-45812Abstract Full Text Full Text PDF PubMed Scopus (53) Google Scholar). InsP3R with CGA Yoo S.H. Lett. 1997; PubMed Scopus Google Scholar) and CGB S.H. Kang Lett. 1997; PubMed Scopus Google Scholar) were by and into and proteins were in cells and purified The binding were in a pH 5.5 mm pH 5.5, 2 mm and or in a pH 7.5 mm pH 7.5, 2 mm and and the interaction was by an of purified or protein with the purified InsP3R in of with mixture for at the mixture was with of to of the InsP3R was by using of the pH 7.5 but with The InsP3R was a and by using the InsP3R of the flux studies the InsP3R type I was from as S.H. Jeon C.J. J. Biol. Chem. 2000; 275: Full Text Full Text PDF PubMed Scopus Google Scholar). were with of I mm pH mm mm mm μm μm and at for at The were at for to the which was in II mm pH mm mm mm μm μm for and at for at The was with an of mm pH 7.5, mm mm μm μm and to an InsP3R with mm pH 7.5, mm mm The was with of this and the InsP3R was by of pH mm mm μm μm The was by pH and with an of mm pH mm and to a with mm pH 7.5, mm and was and at For experiments the InsP3R type I was in and purified from mouse using heparin affinity and as H. S. P. J. Biol. Chem. 2000; 275: Full Text Full Text PDF PubMed Scopus Google Scholar). The purified InsP3R was into by the addition of of purified protein to of of in and for were as S.H. Jeon C.J. J. Biol. Chem. 2000; 275: Full Text Full Text PDF PubMed Scopus Google Scholar). experiments were using types of at both pH 5.5 and 7.5. CGB in CGA and CGB in at the of or chromogranin to be used for control Ca2+ from the was by in were at by using a with a was at the of of with of and of For the of Ca2+ the were after addition of the to of the The of were to Ca2+ using a R. T. 1989; PubMed Scopus Google Scholar). after the addition of was used to intravesicular the of in InsP3R by a of to was to the the of and the at the of was with the at at lipid bilayers were by a of in a in a a The was with The are to the and to the lumen of the The of mm pH mm mm 2 μm The of mm to pH 5.5 purified InsP3R was used in these experiments the pH could be changed using mm and mm channel were using a and were with an to to 2 to a and using InsP3R were to the and by the addition of 2 μm to the InsP3R activation, single channel activity was CGB or a mixture of CGA/CGB was to the and InsP3R single channel activity was The pH the was changed by the addition of to pH 7.5, and InsP3R single channel activity was These experiments were in the presence of of the to the and in the presence and of of CGB or the CGA/CGB mixture in the at pH 5.5. InsP3R single channel activity was CGB from the InsP3R at pH 7.5, the was in the presence of luminal CGB or the CGA/CGB mixture at this are as the mean of at pH-dependent of InsP3R with and was a interaction between the purified InsP3R and CGA or of CGA and CGB were in E. and The interaction between the InsP3R and and proteins was examined at pH 5.5 and 7.5 shown in of the and a mixture of CGA and CGB with the InsP3R at pH 5.5. When at pH 7.5 CGA to interact with the InsP3R but CGB with the InsP3R, at a reduced level a affinity of CGB for the InsP3R. of CGB and CGA/CGB InsP3-mediated release studies were to investigate the effects of CGB the for InsP3R type for CGA (19.Thrower E. Park H. So S. Yoo S. Ehrlich B. J. Biol. Chem. 2002; 277: 15801-15806Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar), Ca2+ release from was at pH both in the presence and of CGB Ca2+ was using μm The of Ca2+ was to be The Ca2+ release in the of CGB a for of When CGB was the at pH 5.5, the pH at which CGA and CGB with the InsP3R (18.Yoo S.H. So S.H. Kweon H.S. Lee J.S. Kang M.K. Jeon C.J. J. Biol. Chem. 2000; 275: 12553-12559Abstract Full Text Full Text PDF PubMed Scopus (62) Google Scholar), Ca2+ release was enhanced a for of when the pH was at 7.5, the at were to those at pH 5.5 in the presence of CGB for of is from that with CGA pH a and the at pH 7.5 was to that in the of CGA (19.Thrower E. Park H. So S. Yoo S. Ehrlich B. J. Biol. Chem. 2002; 277: 15801-15806Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar). Ca2+ release from the was also in the presence of an mixture of CGA/CGB The for was the in the presence of the as that in the presence of CGB alone for was μm at pH 5.5 and μm at pH in InsP3R investigate of the InsP3R by interaction with CGB or a we of the of the InsP3R by the of it was to the of the InsP3R is changed as a of a in the InsP3R shown in the InsP3R was by in the presence of both CGB or the CGA/CGB The at was and the results were to the PubMed Scopus Google Scholar), is the in the of is the in the presence of is the and is the of The from the was for the InsP3R in the of When CGB was this to and at the pH 7.5 and 5.5, respectively this it is that at of the InsP3R are less to the when CGB is the of the InsP3R. Furthermore, the suggest that the are less when the pH was at 5.5 than at 7.5. also were of the InsP3R in the presence of a CGA/CGB mixture in the The of in the of CGA/CGB was to and at the pH 7.5 and 5.5, in the presence of the CGA/CGB the of the InsP3R in the presence of of CGB and CGA/CGB InsP3R The effects of CGB and CGA/CGB InsP3R were investigated at the single channel level using InsP3R into planar lipid In the of luminal CGB and in the presence of Ca2+ and InsP3R single channel activity was channel of were and a single of mean open was with a that between and The open probability was effect of the the mean open time of the pH pH pH pH pH CGB pH pH pH pH in a effect of the the open probability of the pH pH pH pH pH pH pH pH pH in a the addition of of CGB to the luminal a in channel activity was the of the single channel remained significant in mean open and were of mean open time were but the were increased control the open time remained in the as control a open time was that was at than the open time control conditions and are the open in the presence of Furthermore, the increased from in control conditions to in the presence of the pH of the luminal to pH a dissociation of CGB from the InsP3R, the was reduced to this is a in channel the was elevated when with control levels A in luminal pH to 7.5 the to even but the of control that CGB remained to the InsP3R. The mean open were reduced to and and of an to the channel activity In the of experiments a mixture of CGA/CGB was In the of CGA/CGB the of the single channel was 2 the was and the mean open time was to that in the control experiments the addition of of CGA/CGB to the luminal a large in channel activity was The was and of mean open time were and the for the open were increased control the of mean open was less than that with CGB alone and that with CGA A in the luminal pH to 7.5 to but the of control The elevated a coupling of CGA/CGB with the InsP3R, the CGB of the The mean open were and to CGB alone at pH 7.5 The addition of heparin to the channel activity with studies the effect of CGA the activity of the InsP3R (19.Thrower E. Park H. So S. Yoo S. Ehrlich B. J. Biol. Chem. 2002; 277: 15801-15806Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar), control experiments were The addition of CGB or CGA/CGB in the of InsP3R channel the addition of CGB or CGA/CGB to the luminal in the of InsP3R effect the the addition of CGB or CGA/CGB to the in the presence of InsP3R and channel of CGB or CGA/CGB the for channel activity was a of both in the presence of CGB at pH 5.5 and 7.5 and in at an of the was in the presence of CGB pH when with control with an in at 2 μm At pH 7.5 the channel has a at the at 2 μm the is less than that at pH 5.5. experiments to the were in the presence of the CGA/CGB in the presence of a of at the open probability was in the presence of CGA/CGB pH when with control levels of the at pH 7.5 that the at the were to those for CGA/CGB at pH 5.5, but at 2 μm the was less at pH 7.5 than that at pH 5.5. The of channel activity after the addition of CGB or the mixture of CGA/CGB was at pH 5.5 and 7.5 of the pH used the effect of CGB alone or CGA/CGB was that the of the ability to bind to the InsP3R at pH 7.5, In the we examined the functional interaction between the InsP3R type I and CGB monomers or CGA/CGB heteromers and found that both CGB and CGA/CGB heteromers enhanced of the InsP3R/Ca2+ channel in the presence of InsP3. A interaction between the InsP3R and CGA or CGB was S.H. Trends Neurosci. 2000; 23: 424-428Abstract Full Text Full Text PDF PubMed Scopus (64) Google Scholar), and the in cells (5.Johenning F.W. Zochowski M. Conway S.J. Holmes A.B. Koulen P. Ehrlich B.E. J. Neurosci. 2002; 22: 5344-5353Crossref PubMed Google Scholar). Furthermore, a mixture of CGA/CGB with the purified InsP3R at both pH 5.5 and 7.5, the interaction at pH 7.5 was reduced with that at pH 5.5 The interaction between the InsP3R and the at pH is with the physiological roles of secretory granules the intragranular pH of secretory granules and the Ca2+ increases as the granules from the to the The interaction of the InsP3R with CGB monomers and with CGA/CGB at pH 5.5 an open probability for the Ca2+ channel of and which is the activity of the InsP3R to in a functional interaction at pH 5.5 is to the of these proteins in secretory granules (18.Yoo S.H. So S.H. Kweon H.S. Lee J.S. Kang M.K. Jeon C.J. J. Biol. Chem. 2000; 275: 12553-12559Abstract Full Text Full Text PDF PubMed Scopus (62) Google Scholar). of intracellular Ca2+ is for the of and of secretory vesicles, and it is that the interaction in this the vesicles the intraluminal pH and the Ca2+ is elevated H. E. Neuroscience. PubMed Scopus Google Scholar). The vesicles near the and is to for secretory granules H. E. Neuroscience. PubMed Scopus Google Scholar). The increased between CGB and the InsP3R as the vesicles the probability of the protein complex shows an increased of the InsP3R to by InsP3. enhanced channel activity release Ca2+ into the between the and the the the Ca2+ for The functional interaction between the InsP3R and CGB remains even when the pH is at a near physiological that this protein has roles in numerous in a The addition of CGB or CGA/CGB heteromers to the InsP3R at pH 7.5 an open probability of and the level of at this pH was elevated from those when using InsP3R T. H. Z. J. Neurosci. 2003; 23: PubMed Google Scholar, Koulen P. K. Ehrlich B.E. M.H. S. A. 2003; PubMed Scopus Google Scholar). The between the InsP3R and CGB at pH 7.5 can have important roles in the ER, CGB is found and the InsP3R is associated with the ER the distribution of the InsP3R is to be CGB has been shown to be in a regionally specific as in neuronally differentiated cells (5.Johenning F.W. Zochowski M. Conway S.J. Holmes A.B. Koulen P. Ehrlich B.E. J. Neurosci. 2002; 22: 5344-5353Crossref PubMed Google Scholar). In these cells CGB is to the which is the intracellular Ca2+ (5.Johenning F.W. Zochowski M. Conway S.J. Holmes A.B. Koulen P. Ehrlich B.E. J. Neurosci. 2002; 22: 5344-5353Crossref PubMed Google Scholar). It is that this be found in other cell types as are and the of CGB with the InsP3R may an important and of intracellular Ca2+ the effect of a mixture of CGA/CGB is the as CGB When in CGA and CGB form a heterodimer at pH 7.5 and a CGA2CGB2 heterotetramer at pH 5.5 (22.Yoo S.H. Lewis M.S. J. Biol. Chem. 1996; 271: 17041-17046Abstract Full Text Full Text PDF PubMed Scopus (42) Google Scholar). In the experiments the of the channel was after the addition of CGB alone or a mixture of CGA/CGB at a of or which from the InsP3R at pH 7.5 (18.Yoo S.H. So S.H. Kweon H.S. Lee J.S. Kang M.K. Jeon C.J. J. Biol. Chem. 2000; 275: 12553-12559Abstract Full Text Full Text PDF PubMed Scopus (62) Google Scholar) and was effect the InsP3R/Ca2+ channel (19.Thrower E. Park H. So S. Yoo S. Ehrlich B. J. Biol. Chem. 2002; 277: 15801-15806Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar), the InsP3R/Ca2+ effect of CGB at pH 7.5 an important physiological the ratios of CGA to CGB cells (10.Winkler H. Fischer-Colbrie R. Neuroscience. 1992; 49: 497-528Crossref PubMed Scopus (613) Google Scholar) and the intraluminal pH levels of the in which CGA and CGB are found also the InsP3R/Ca2+ roles of CGA and CGB also that the ratios of CGA to CGB found in cells could be very important in the time to in Ca2+ storage the pH of the enhanced Ca2+ release be in this difference in the ability of CGA and CGB to regulate the InsP3R could influence the pattern of InsP3-mediated Ca2+ release as CGA/CGB ratios in of the In the ability of CGB to the InsP3R/Ca2+ channel at pH 7.5 is to have a InsP3-mediated Ca2+ release from the ER. In we have a functional interaction between the intracellular Ca2+ release the InsP3R, with the Ca2+ storage protein, with and with CGA/CGB heteromers. In of these the the in the purified InsP3R activity these can be the have the to as of Ca2+ release in a variety of cell types both from secretory granules and from the ER. and for and the B.