Interaction between an intraluminal loop peptide of the inositol 1,4,5-trisphosphate receptor and the near N-terminal peptide of chromogranin A

The acidocalcisome inositol-1,4,5-trisphosphate receptor of Trypanosoma brucei is stimulated by luminal polyphosphate hydrolysis products

Acidocalcisomes are acidic calcium stores rich in polyphosphate (polyP) and are present in trypanosomes and also in a diverse range of other organisms. Ca²⁺ is released from these organelles through a channel, inositol 1,4,5-trisphosphate receptor (TbIP₃R), which is essential for growth and infectivity of the parasite Trypanosoma brucei However, the mechanism by which TbIP₃R controls Ca²⁺ release is unclear. In this work, we expressed TbIP₃R in a chicken B lymphocyte cell line in which the genes for all three vertebrate IP₃Rs were stably ablated (DT40-3KO). We show that IP₃-mediated Ca²⁺ release depends on Ca²⁺ but not on ATP concentration and is inhibited by heparin, caffeine, and 2-aminomethoxydiphenyl borate (2-APB). Excised patch clamp recordings from nuclear membranes of DT40 cells expressing only TbIP₃R disclosed that luminal inorganic orthophosphate (P_i) or pyrophosphate (PP_i), and neutral or alkaline pH can stimulate IP₃-generated currents. In contrast, polyP or acidic pH did not induce these currents, and nuclear membranes obtained from cells expressing rat IP₃R were unresponsive to polyP or its hydrolysis products. Our results are consistent with the notion that polyP hydrolysis products within acidocalcisomes or alkalinization of their luminal pH activate TbIP₃R and Ca²⁺ release. We conclude that TbIP₃R is well-adapted to its role as the major Ca²⁺ release channel of acidocalcisomes in T. brucei.

Inositol 1,4,5-trisphosphate receptors and their protein partners as signalling hubs

Inositol 1,4,5‐trisphosphate receptors (IP₃Rs) are expressed in nearly all animal cells, where they mediate the release of Ca²⁺ from intracellular stores. The complex spatial and temporal organization of the ensuing intracellular Ca²⁺ signals allows selective regulation of diverse physiological responses. Interactions of IP₃Rs with other proteins contribute to the specificity and speed of Ca²⁺ signalling pathways, and to their capacity to integrate information from other signalling pathways. In this review, we provide a comprehensive survey of the proteins proposed to interact with IP₃Rs and the functional effects that these interactions produce. Interacting proteins can determine the activity of IP₃Rs, facilitate their regulation by multiple signalling pathways and direct the Ca²⁺ that they release to specific targets. We suggest that IP₃Rs function as signalling hubs through which diverse inputs are processed and then emerge as cytosolic Ca²⁺ signals.

Secretory granules in inositol 1,4,5-trisphosphate-dependent Ca2+ signaling in the cytoplasm of neuroendocrine cells

Of all the intracellular organelles, secretory granules contain by far the highest calcium concentration; secretory granules of typical neuroendocrine chromaffin cells contain approximately 40 mM Ca(2+) and occupy approximately 20% cell volume, accounting for >60% of total cellular calcium. They also contain the majority of cellular inositol 1,4,5-trisphosphate receptors (IP(3)Rs) in addition to the presence of >2 mM of chromogranins A and B that function as high-capacity, low-affinity Ca(2+) storage proteins. Chromogranins A and B also interact with the IP(3)Rs and activate the IP(3)R/Ca(2+) channels. In experiments with both neuroendocrine PC12 and nonneuroendocrine NIH3T3 cells, in which the number of secretory granules present was changed by either suppression or induction of secretory granule formation, secretory granules were demonstrated to account for >70% of the IP(3)-induced Ca(2+) releases in the cytoplasm. Moreover, the IP(3) sensitivity of secretory granule IP(3)R/Ca(2+) channels is at least approximately 6- to 7-fold more sensitive than those of the endoplasmic reticulum, thus enabling secretory granules to release Ca(2+) ahead of the endoplasmic reticulum. Further, there is a direct correlation between the number of secretory granules and the IP(3) sensitivity of cytoplasmic IP(3)R/Ca(2+) channels and the increased ratio of IP(3)-induced cytoplasmic Ca(2+) release, highlighting the importance of secretory granules in the IP(3)-dependent Ca(2+) signaling. Given that secretory granules are present in all secretory cells, these results presage critical roles of secretory granules in the control of cytoplasmic Ca(2+) concentrations in other secretory cells.-Yoo, S. H. Secretory granules in inositol 1,4,5-trisphosphate-dependent Ca(2+) signaling in the cytoplasm of neuroendocrine cells.

Identification of residues participating in the interaction between an intraluminal loop of inositol 1,4,5-trisphosphate receptor and a conserved N-terminal region of chromogranin B

The inositol 1,4,5-trisphosphate receptor (IP3R) is a membrane channel that conducts calcium ions from the intracellular calcium stores. Despite a wealth of information on the cytoplasmic regulation of the IP3R, little is known about its regulation on the luminal side of the calcium stores. Here, we report studies on the IP3R intraluminal loop L3-2 and a conserved N-terminal region of chromogranin B. The IP3R loop is an important part of the channel's pore-forming region, and the chromogranin peptide has been shown to competitively inhibit calcium signaling by IP3R. Using the NMR titration approach, we showed that a part of the L3-2 is involved in a specific interaction with the chromogranin B peptide. Further NMR resonance assignments revealed that the 14th-20th residues of L3-2 are the keys to the binding to the chromogranin B peptide. Through detailed analysis of the data, we suggest a mechanism of IP3R regulation by chromogranin B involving conformational exchanges of the L3-2 region. Our report presents the findings of the first study on the interaction between the luminal loop of the IP3 receptor and its regulator at residue-resolution. The approaches described here should help to guide further studies on the interactions between the IP3R and other luminal side regulators.

The inositol 1,4,5-trisphosphate receptor (IP3R) and its regulators: sometimes good and sometimes bad teamwork

In both nonexcitable and excitable cells, the inositol 1,4,5-trisphosphate receptor (IP(3)R) is the primary cytosolic target responsible for the initiation of intracellular calcium (Ca(2+)) signaling. To fulfill this function, the IP(3)R depends on interaction with accessory subunits and regulatory proteins. These include proteins that reside in the lumen of the endoplasmic reticulum (ER), such as chromogranin A and B and ERp44, and cytosolic proteins, such as neuronal Ca(2+) sensor 1, huntingtin, cytochrome c, IP(3)R-binding protein released with inositol 1,4,5-trisphosphate, Homer, and 4.1N. Specific interactions between these modulatory proteins and the IP(3)R have been described, making it clear that the controlled modulation of the IP(3)R by its binding partners is necessary for physiological cell regulation. The functional coupling of these modulators with the IP(3)R can control apoptosis, intracellular pH, the initiation and regulation of neuronal Ca(2+) signaling, exocytosis, and gene expression. The pathophysiological relevance of IP(3)R modulation is apparent when the functional interaction of these proteins is enhanced or abolished by mutation or overexpression. The subsequent deregulation of the IP(3)R leads to pathological changes in Ca(2+) signaling, signal initiation, the amplitude and frequency of Ca(2+) signals, and the duration of the Ca(2+) elevation. Consequences of this deregulation include abnormal growth and apoptosis. Complex regulation of Ca(2+) signaling is required for the cell to live and function, and this difficult task can only be managed when the IP(3)R teams up and acts properly with its numerous binding partners.

Structural insights into the regulatory mechanism of IP3 receptor

Inositol 1,4,5-trisphosphate receptors (IP(3)R) are intracellular Ca(2+) release channels whose opening requires binding of two intracellular messengers IP(3) and Ca(2+). The regulation of IP(3)R function has also been shown to involve a variety of cellular proteins. Recent biochemical and structural analyses have deepened our understanding of how the IP(3)-operated Ca(2+) channel functions. Specifically, the atomic resolution structure of the IP(3)-binding region has provided a sound structural basis for the receptor interaction with the natural ligand. Electron microscopic studies have also shed light on the overall shape of the tetrameric receptor. This review aims to provide comprehensive overview of the current information available on the structure and function relationship of IP(3)R.

Progastrin Is Directed to the Regulated Secretory Pathway by Synergistically Acting Basic and Acidic Motifs

Bioactivation of prohormones occurs in the granules of the regulated secretory pathway of endocrine cells, which release hormones in response to external stimulation. How secretory granules are formed and how the cargo is selected is still unclear, but it has been shown for several prohormones and processing enzymes that domains within the prohormone structure can act as "sorting signals" for this pathway. The domains mediate interactions with other proteins or with the membrane or facilitate aggregation of the (pro)peptides. We have now searched for domains in progastrin that are active in sorting the prohormone into secretory granules. Truncation studies showed that the N-terminal 30 residues of progastrin are dispensable, whereas the last 49 residues are sufficient for correct biosynthesis of bioactive gastrin. Thus, further N-terminal truncation abolished gastrin expression. C-terminal truncation of 8 residues resulted in an increase in basal secretion as did point mutations in the dibasic processing sites of progastrin. These mutants, however, still responded to secretagogues, suggesting a residual sorting capacity to the regulated pathway. Amino acid substitutions in an acidic, polyglutamate motif within gastrin-17, the main bioactive, cellular gastrin form, did not alter secretion per se, but when these residues were substituted in C-terminally truncated mutants, double mutants increased in basal secretion and did not respond to secretagogue stimulation. This implies that the mutants are constitutively secreted. Our data suggest that the dibasic processing sites constitute the most important sorting domain of progastrin, and these sites act in synergy with the acidic domain.

Inositol 1,4,5-trisphosphate receptor/Ca(2+) channel modulatory role of chromogranins A and B

The secretory granules function as the major IP(3)-sensitive intracellular Ca(2+) store of secretory cells. Recently it was found that the secretory granules contain three isoforms of inositol 1,4,5-trisphosphate receptor (IP(3)R)/Ca(2+) channels and high-capacity, low-affinity Ca(2+) storage proteins chromogranins A (CgA) and B (CgB). The IP(3)R/Ca(2+) channel was shown to directly interact with CgA and CgB at the intragranular pH 5.5, and this coupling led to modulation of the IP(3)R/Ca(2+) channel activity by the coupled chromogranins. These results provide the molecular structural basis of the IP(3)-mediated Ca(2+) release mechanism of secretory granules.

Comparative sorting of neuroendocrine secretory proteins: a search for common ground in a mosaic of sorting models and mechanisms

Endocrine, neuroendocrine and exocrine cells store regulated secretory proteins in secretory granules, while constitutive and constitutive-like secretory proteins are secreted directly without storage. Sorting of secretory proteins takes place in the trans-Golgi network (sorting for entry) or immature secretory granules (sorting by retention). The relative contribution of these sorting steps and the sorting signals and mechanisms involved in each step has been the subject of intense studies and debate in recent years. New evidence now suggests that: (1) two proteins with structurally similar sorting signals can use different sorting mechanisms; (2) one protein with multiple sorting signals can be sorted differently in different cell types; and (3) one cell type can recognize different sorting signals and use different sorting mechanisms. The latter finding suggests that sorting must be a regulated event. While the current image of sorting is complex, recent findings are pointing to common features that form a mosaic of related sorting mechanisms.

Interaction of chromogranin B and the near N-terminal region of chromogranin B with an intraluminal loop peptide of the inositol 1,4, 5-trisphosphate receptor

Given the interaction of the inositol 1,4,5-trisphosphate receptor (IP(3)R) with chromogranins A (CGA) and B (CGB), two major Ca(2+) storage proteins of secretory granules that have been shown to be IP(3)-sensitive intracellular Ca(2+) store of neuroendocrine cells, we have investigated the potential interaction of the intraluminal loop regions of the IP(3)R with both intact CGB and the conserved near N-terminal region of CGB. The interaction studies carried out with CGB and glutathione S-transferase fusion proteins of intraluminal loop regions of bovine type 1 IP(3)R showed that CGB interacts with intraluminal loop 3-2 (the second loop formed between transmembrane regions 5 and 6) of the IP(3)R at both pH 5.5 and 7.5. Analytical ultracentrifugation studies also indicated that CGB interacts with the same intraluminal loop region of the IP(3)R and the interaction was much stronger than that between CGA and the loop. Moreover, the conserved near N-terminal region of CGB also interacted with the intraluminal loop region of the IP(3)R. The CGB interaction with the IP(3)R intraluminal loop peptide at pH 7.5 showed a DeltaG(0) value of -8.1 kcal/mol at 37 degrees C for a 1:1 stoichiometry, indicating a K(d) of approximately 1.9 micrometer. These results give insight into the molecular organization of the IP(3)-sensitive Ca(2+) store.

Coupling of the IP3 receptor/Ca2+ channel with Ca2+ storage proteins chromogranins A and B in secretory granules

The secretory granules of neuroendocrine cells, which function as an inositol (1,4,5)-trisphosphate-sensitive intracellular Ca2+ store, contain both the inositol (1,4,5)-trisphosphate receptor/Ca2+ channel and the high-capacity low-affinity Ca2+ storage proteins, chromogranins A and B. Chromogranins A and B, which exist in approximately 2 mm range in the secretory granules, can bind 50-100 mol of Ca2+/mol with dissociation constants of 2-4 mm. These proteins interact directly with the inositol (1,4,5)-trisphosphate receptor/ Ca2+ channel at the intragranular pH 5.5, not only changing the conformation of the inositol (1,4,5)-trisphosphate receptor/Ca2+ channel but also modulating the channel activity. Given the homo- and heterotetrameric existence of both the inositol (1,4,5)-trisphosphate receptor/Ca2+ channel and chromogranins A and B, these tetrameric proteins appear to interact, thus controlling the intracellular Ca2+ concentration.

Antibacterial and antifungal activities of vasostatin-1, the N-terminal fragment of chromogranin A

Vasostatin-1, the natural N-terminal 1-76 chromogranin A (CGA)-derived fragment in bovine sequence, has been purified from chromaffin secretory granules and identified by sequencing and matrix-assisted laser desorption time-of-flight mass spectrometry. This peptide, which displays antibacterial activity against Gram-positive bacteria at micromolar concentrations, is also able to kill a large variety of filamentous fungi and yeast cells in the 1-10 microM range. We have found that the C-terminal moiety of vasostatin-1 is essential for the antifungal activity, and shorter active peptides have been synthesized. In addition, from the comparison with the activity displayed by related peptides (human recombinant and rat synthetic fragments), we could determine that antibacterial and antifungal activities have different structural requirements. To assess for such activities in vivo, CGA and CGA-derived fragments were identified in secretory material released from human polymorphonuclear neutrophils upon stimulation. Vasostatin-1, which is stored in a large variety of cells (endocrine, neuroendocrine, and neurons) and which is liberated from stimulated chromaffin and immune cells upon stress, may represent a new component active in innate immunity.

Inositol 1,4,5-trisphosphate receptors are strongly expressed in the nervous system, pharynx, intestine, gonad and excretory cell of Caenorhabditis elegans and are encoded by a single gene (itr-1)

Inositol 1,4,5-trisphosphate (InsP3) activates receptors (InsP3Rs) that mediate intracellular Ca(2+ )release, thereby modulating intracellular calcium signals and regulating important aspects of cellular physiology and gene expression. To further our understanding of InsP3Rs we have characterised InsP3Rs and the InsP3R gene, itr-1, from the model organism Caenorhabditis elegans. cDNAs encoding InsP3Rs were cloned enabling us to: (a) identify three putative transcription start sites that result in alternative mRNA 5' ends: (b) detect alternative splicing at three sites and: (c) determine the full genomic organisation of the itr-1 gene. The InsP3R protein (ITR-1) is approximately 42 % identical with known InsP3Rs and possesses conserved structural features. When the putative InsP3 binding domain was expressed in Escherichia coli, specific binding of InsP3 was detected. Using antibodies against ITR-1 we detected a protein of 220 kDa in C. elegans membranes. These antibodies and itr-1::GFP (green fluorescent protein) reporter constructs were used to determine the expression pattern of itr-1 in C. elegans. Strong expression was observed in the intestine, pharynx, nerve ring, excretory cell and gonad. These results demonstrate the high degree of structural and functional conservation of InsP3Rs from nematodes to mammals and the utility of C. elegans as a system for studies on InsP3R mediated signalling.