Membrane association, localization and topology of rat inositol 1,4,5-trisphosphate 3-kinase B: implications for membrane traffic and Ca2+ homoeostasis

The role of intracellular calcium stores in synaptic plasticity and memory consolidation

Memory processing requires tightly controlled signalling cascades, many of which are dependent upon intracellular calcium (Ca(2+)). Despite this, most work investigating calcium signalling in memory formation has focused on plasma membrane channels and extracellular sources of Ca(2+). The intracellular Ca(2+) release channels, ryanodine receptors (RyRs) and inositol (1,4,5)-trisphosphate receptors (IP3Rs) have a significant capacity to regulate intracellular Ca(2+) signalling. Evidence at both cellular and behavioural levels implicates both RyRs and IP3Rs in synaptic plasticity and memory formation. Pharmacobehavioural experiments using young chicks trained on a single-trial discrimination avoidance task have been particularly useful by demonstrating that RyRs and IP3Rs have distinct roles in memory formation. RyR-dependent Ca(2+) release appears to aid the consolidation of labile memory into a persistent long-term memory trace. In contrast, IP3Rs are required during long-term memory. This review discusses various functions for RyRs and IP3Rs in memory processing, including neuro- and glio-transmitter release, dendritic spine remodelling, facilitating vasodilation, and the regulation of gene transcription and dendritic excitability. Altered Ca(2+) release from intracellular stores also has significant implications for neurodegenerative conditions.

Arabidopsis inositol polyphosphate 6-/3-kinase (AtIpk2beta) is involved in axillary shoot branching via auxin signaling

The Arabidopsis (Arabidopsis thaliana) inositol polyphosphate 6-/3-kinase gene (AtIpk2beta) is known to participate in inositol phosphate metabolism. However, little is known about its physiological functions in higher plants. Here, we report that AtIpk2beta regulates Arabidopsis axillary shoot branching. By overexpressing AtIpk2beta in the wild type and mutants, we found that overexpression of AtIpk2beta leads to more axillary shoot branches. Further analysis of AtIpk2beta overexpression lines showed that axillary meristem forms earlier and the bud outgrowth rate is also accelerated, resulting in more axillary shoot branches. The AtIpk2beta promoter/beta-glucuronidase (GUS) fusion (AtIpk2betaGUS) expression pattern is similar to that of the auxin reporter DR5GUS. Moreover, AtIpk2beta can be induced in response to exogenous indole-3-acetic acid (IAA) treatments. In addition, AtIpk2beta overexpression plants exhibit IAA-related phenotypes and are more resistant to exogenous IAA treatments. Further analysis employing reverse transcription-polymerase chain reaction shows that some genes, including auxin-biosynthesis (CYP83B1), auxin-transport (PIN4), and auxin-mediated branching genes (MAX4 and SPS), are regulated by AtIpk2beta. Taken together, our data provide insights into a role for AtIpk2beta in axillary shoot branching through the auxin signaling pathway.

Subcellular localisation of human inositol 1,4,5-trisphosphate 3-kinase C: species-specific use of alternative export sites for nucleo-cytoplasmic shuttling indicates divergent roles of the catalytic and N-terminal domains

The three isoforms of human Ins(1,4,5)P3 3-kinase (IP3K) show remarkable differences in their intracellular targeting. Whereas predominant targeting to the cytoskeleton and endoplasmic reticulum has been shown for IP3K-A and IP3K-B, rat IP3K-C shuttles actively between the nucleus and cytoplasm. In the present study we examined the expression and intracellular localisation of endogenous IP3K-C in different mammalian cell lines using an isoform-specific antibody. In addition, human IP3K-C, showing remarkable differences to its rat homologue in the N-terminal targeting domain, was tagged with EGFP and used to examine active transport mechanisms into and out of the nucleus. We found both a nuclear import activity residing in its N-terminal domain and a nuclear export activity sensitive to treatment with leptomycin B. Different from the rat isoform, an exportin 1-dependent nuclear export site of the human enzyme resides outside the N-terminal targeting domain in the catalytic enzyme domain. A phylogenetic survey of vertebrate IP3K sequences indicates that in each of the three isoforms a nuclear export signal has evolved in the catalytic domain either de novo (IP3K-A) or as a substitute for an earlier evolved corresponding N-terminal signal (IP3K-B and IP3K-C). In higher vertebrates, and in particular in primates, re-export of nuclear IP3K activity may be guaranteed by the mechanism discovered.

Regulation of the localization and activity of inositol 1,4,5-trisphosphate 3-kinase B in intact cells by proteolysis

IP3K (inositol 1,4,5-trisphosphate 3-kinase) catalyses the Ca2+-regulated phosphorylation of the second messenger Ins(1,4,5)P3, thereby inactivating the signal to release Ca2+ and generating Ins(1,3,4,5)P4. Here we have investigated the localization and activity of IP3KB and its modulation by proteolysis. We found that the N- and C-termini (either side of residue 262) of IP3KB localized predominantly to the actin cytoskeleton and ER (endoplasmic reticulum) respectively, both in COS-7 cells and in primary astrocytes. The functional relevance of this was demonstrated by showing that full-length (actin-localized) IP3KB abolished the histamine-induced Ca2+ response in HeLa cells more effectively than truncated constructs localized to the ER or cytosol. The superior efficacy of full-length IP3KB was also attenuated by disruption of the actin cytoskeleton. By transfecting COS-7 cells with double-tagged IP3KB, we show that the translocation from actin to ER may be a physiologically regulated process caused by Ca2+-modulated constitutive proteolysis in intact cells.

Structural insights into enzyme regulation for inositol 1,4,5-trisphosphate 3-kinase B

D-Myoinositol 1,4,5-trisphophate 3-kinases (IP(3)-3Ks) play important roles in metazoan cellular signaling. It has been demonstrated that mice without a functional version of IP(3)-3K isoform B are deficient in peripheral T-cells, indicating that IP(3)-3KB is essential to the developing immune system. The recent apo IP(3)-3KA structure exhibited a helix at the catalytic domain N-terminus exhibited a helix at the N-terminus of the catalytic domain, with a tryptophan indole moiety mimicking the binding mode of the substrate ATP purine ring, suggesting a mechanism of autoinhibition. Here we present the structure of the complete catalytic domain of IP(3)-3KB, including the CaM binding domain in complex with Mg(2+) and ATP. The crystal structure reveals a homodimeric arrangement of IP(3)-3KB catalytic domains, mediated via an intermolecular antiparallel beta-sheet formed from part of the CaM binding region. Residues from the putative autoinhibitory helix are rearranged into a loop configuration, with extensive interactions with the bound ATP. Mutagenesis of residues from this region reveals that substitution of the putative autoinhibitory tryptophan generates a hyperactive enzyme which retains Ca(2+)/CaM sensitivity. The IP(3)-3KB structure suggests a mechanism of enzyme activation, and raises the possibility that an interaction between IP(3)-3KB molecules may occur as part of the catalytic or regulatory cycle.

Identification of mitogen-activated protein kinase docking sites in enzymes that metabolize phosphatidylinositols and inositol phosphates

BACKGROUND

Reversible interactions between the components of cellular signaling pathways allow for the formation and dissociation of multimolecular complexes with spatial and temporal resolution and, thus, are an important means of integrating multiple signals into a coordinated cellular response. Several mechanisms that underlie these interactions have been identified, including the recognition of specific docking sites, termed a D-domain and FXFP motif, on proteins that bind mitogen-activated protein kinases (MAPKs). We recently found that phosphatidylinositol-specific phospholipase C-gamma1 (PLC-gamma1) directly binds to extracellular signal-regulated kinase 2 (ERK2), a MAPK, via a D-domain-dependent mechanism. In addition, we identified D-domain sequences in several other PLC isozymes. In the present studies we sought to determine whether MAPK docking sequences could be recognized in other enzymes that metabolize phosphatidylinositols (PIs), as well as in enzymes that metabolize inositol phosphates (IPs).

RESULTS

We found that several, but not all, of these enzymes contain identifiable D-domain sequences. Further, we found a high degree of conservation of these sequences and their location in human and mouse proteins; notable exceptions were PI 3-kinase C2-gamma, PI 4-kinase type IIbeta, and inositol polyphosphate 1-phosphatase.

CONCLUSION

The results indicate that there may be extensive crosstalk between MAPK signaling and signaling pathways that are regulated by cellular levels of PIs or IPs.

Ins(1,4,5)P3 metabolism and the family of IP3-3Kinases

The release of Ca2+ from intracellular stores is triggered by the second messenger inositol (1,4,5)-trisphosphate (Ins(1,4,5)P3). The regulation of this process is critically important for cellular homeostasis. Ins(1,4,5)P3 is rapidly metabolised, either to inositol (1,4)-bisphosphate (Ins(1,4)P2) by inositol polyphosphate 5-phosphatases or to inositol (1,3,4,5)-tetrakisphosphate (Ins(1,3,4,5)P4) by one of a family of inositol (1,4,5)P3 3-kinases (IP3-3Ks). Three isoforms of IP3-3K have now been identified in mammals; they have a conserved C-terminal catalytic domain, but divergent N-termini. This review discusses the metabolism of Ins(1,4,5)P3, compares the IP3-3K isoforms and addresses potential mechanisms by which their activity might be regulated.

Assembly factors of F1Fo-ATP synthase across genomes

Work with respiration-deficient strains of Saccharomyces cerevisiae has provided evidence that assembly of the mitochondrial ATP synthase is dependent on proteins that serve substrate-specific, chaperone-type functions: Atp10p, Atp11p, Atp12p, Atp22p, and Fmc1p. Atp11p and Atp12p mediate the formation of the F1 moiety via interaction with subunits F1-beta and F1-alpha, respectively. The role of Fmc1p is less clear. Atp10p and Atp22p are essential for the formation of the F(O) part, during which Atp10p assists in the incorporation of the F(O)-a subunit. Here we present a comprehensive analysis of ATP synthase assembly factors from all available genomes. The mechanism of the F1 assembly is preserved in all eukaryotic lineages that are capable of ATP synthesis via oxidative phosphorylation and requires Atp11p and Atp12p. Conversely, composition of the F(O) part as well as its assembly is more versatile. We found two distinct subtypes of the F(O)-a subunit, one of which seems to be dependent on the action of Atp10p while the other does not. Restricted occurrence of Fmc1p and Atp22p suggests the existence of lineage-specific assembly factors. Our phylogenetic data served as a source for comparative sequence analysis, which identified evolutionarily conserved residues, putative functional domains and their basic structural features for Atp10p, Atp11p, and Atp12p orthologs. These results provide the basis for detailed molecular analysis of the ATP synthase-specific chaperones.

Antiproliferative plant and synthetic polyphenolics are specific inhibitors of vertebrate inositol-1,4,5-trisphosphate 3-kinases and inositol polyphosphate multikinase

Inositol-1,4,5-trisphosphate 3-kinases (IP3K) A, B, and C as well as inositol polyphosphate multikinase (IPMK) catalyze the first step in the formation of the higher phosphorylated inositols InsP5 and InsP6 by metabolizing Ins(1,4,5)P3 to Ins(1,3,4,5)P4. In order to clarify the special role of these InsP3 phosphorylating enzymes and of subsequent anabolic inositol phosphate reactions, a search was conducted for potent enzyme inhibitors starting with a fully active IP3K-A catalytic domain. Seven polyphenolic compounds could be identified as potent inhibitors with IC50 < 200 nM (IC50 given): ellagic acid (36 nM), gossypol (58 nM), (-)-epicatechin-3-gallate (94 nM), (-)-epigallocatechin-3-gallate (EGCG, 120 nM), aurintricarboxylic acid (ATA, 150 nM), hypericin (170 nM), and quercetin (180 nM). All inhibitors displayed a mixed-type inhibition with respect to ATP and a non-competitive inhibition with respect to Ins(1,4,5)P3. Examination of these inhibitors toward IP3K-A, -B, and -C and IPMK from mammals revealed that ATA potently inhibits all kinases while the other inhibitors do not markedly affect IPMK but differentially inhibit IP3K isoforms. We identified chlorogenic acid as a specific IPMK inhibitor whereas the flavonoids myricetin, 3',4',7,8-tetrahydroxyflavone and EGCG inhibit preferentially IP3K-A and IP3K-C. Mutagenesis studies revealed that both the calmodulin binding and the ATP [corrected] binding domain in IP3K are involved in inhibitor binding. Their absence in IPMK and the presence of a unique insertion in IPMK were found to be important for selectivity differences from IP3K. The fact that all identified IP3K and IPMK inhibitors have been reported as antiproliferative agents and that IP3Ks or IPMK often are the best binding targets deserves further investigation concerning their antitumor potential.

Identification and subcellular distribution of endogenous Ins(1,4,5)P(3) 3-kinase B in mouse tissues

Inositol 1,4,5-trisphosphate 3-kinase (IP(3)-3K) catalyses the phosphorylation of inositol 1,4,5-trisphosphate to inositol 1,3,4,5-tetrakisphosphate. cDNAs encoding three mammalian isoforms have been reported and referred to as IP(3)-3KA, IP(3)-3KB, and IP(3)-3KC. IP(3)-3KB is particularly sensitive to proteolysis at the N-terminus, a mechanism known to generate active fragments of lower molecular mass. Endogenous IP(3)-3KB has therefore not been formally identified in tissues. We have probed a series of murine tissues with an antibody directed against the C-terminus of IP(3)-3KB and used IP(3)-3KB deficient mouse tissues as negative controls. IP(3)-3KB was shown to be particularly well expressed in brain, lung, and thymus with molecular masses of 110-120kDa. The identification of the native IP(3)-3KB by Western blotting for the first time will facilitate further studies of regulation of its activity by specific proteases and/or phosphorylation.

Identification of the actin-binding domain of Ins(1,4,5)P3 3-kinase isoform B (IP3K-B)

Dewaste et al. [Dewaste, Moreau, De Smedt, Bex, De Smedt, Wuytaack, Missiaen and Erneux (2003) Biochem. J. 374, 41-49] showed that over-expressed EGFP (enhanced green fluorescent protein) fused to Ins(1,4,5)P3 3-kinase B (IP3K-B) co-localizes with the cytoskeleton, as well as with the endoplasmic reticulum and the plasma membrane. The domains responsible for these subcellular localizations are not yet identified. For the endogenous enzyme, we confirmed both actin and endoplasmic reticulum localization by employing a high affinity antibody against IP3K-B. F-actin targeting is exclusively dependent on the non-catalytic N-terminal region of IP3K-B. By expressing fragments of this N-terminal domain as EGFP-fusion proteins and inspecting transfected cells by confocal microscopy, we characterized a distinct 63-amino-acid domain comprising amino acids 108-170 of the enzyme which is responsible for F-actin targeting. A truncation of this fragment from both sides revealed that the full size of this segment is essential for this function. Deletion of this segment in a full-length over-expressed IP3K-B-EGFP-fusion protein completely abolished F-actin interaction. Direct interaction of this actin-binding segment with only F-actin, but not with G-actin, was observed in vitro using a bacterially expressed, affinity-purified GST (glutathione S-transferase)-Rattus norvegicus IP3K (aa 108-170) fusion protein. Helix-breaking mutations within this isolated segment abolished the F-actin binding properties both in vitro and when over-expressed in cells, indicating that an intact secondary structure is essential for actin targeting. The segment shows sequence similarities to the actin-binding region in IP3K-A, but no similarity to other actin-binding domains.

Calpain cleavage of the B isoform of Ins(1,4,5)P3 3-kinase separates the catalytic domain from the membrane anchoring domain

Inositol (1,4,5)-trisphosphate [Ins(1,4,5)P3] is one of the key intracellular second messengers in cells and mobilizes Ca2+ stores in the ER (endoplasmic reticulum). Ins(1,4,5)P3 has a short half-life within the cell, and is rapidly metabolized through one of two pathways, one of which involves further phosphorylation of the inositol ring: Ins(1,4,5)P3 3-kinase (IP3-3K) phosphorylates Ins(1,4,5)P3, resulting in the formation of inositol (1,3,4,5)-tetrakisphosphate [Ins(1,3,4,5)P4]. There are three known isoforms of IP3-3K, designated IP3-3KA, IP3-3KB and IP3-3KC. These have differing N-termini, but highly conserved C-termini harbouring the catalytic domain. The three IP3-3K isoforms have different subcellular locations and the B-kinase is uniquely present in both cytosolic and membrane-bound pools. As it is the N-terminus of the B-kinase that differs most from the A- and C-kinases, we have hypothesized that this portion of the protein may be responsible for membrane localization. Although there are no known membrane-targeting protein motifs within the sequence of IP3-3KB, it is found to be tightly associated with the ER membrane. Here, we show that specific regions of the N-terminus of IP3-3KB are necessary and sufficient for efficient membrane localization of the protein. We also report that, in the presence of Ca2+, the kinase domain of IP3-3KB is cleaved from the membrane-anchoring region by calpain.

The three isoenzymes of human inositol-1,4,5-trisphosphate 3-kinase show specific intracellular localization but comparable Ca2+ responses on transfection in COS-7 cells

Inositol 1,4,5-trisphosphate [Ins(1,4,5) P3] 3-kinase catalyses the phosphorylation of InsP3 to inositol 1,3,4,5-tetrakisphosphate. cDNAs encoding three human isoenzymes of InsP3 3-kinase (A, B and C) have been reported previously [Choi, Kim, Lee, Moon, Sim, Kim, Chung and Rhee (1990) Science 248, 64-66; Dewaste, Pouillon, Moreau, Shears, Takazawa and Erneux (2000) Biochem. J. 352, 343-351; Dewaste, Roymans, Moreau and Erneux (2002) Biochem. Biophys. Res. Commun. 291, 400-405; Takazawa, Perret, Dumont and Erneux (1991) Biochem. Biophys. Res. Commun. 174, 529-535]. The localization of InsP3 3-kinase isoenzymes fused at their N-terminus to the green fluorescent protein has been studied by confocal microscopy. The A isoform appeared to associate with the cytoskeleton, whereas the C isoform was totally cytoplasmic. The B isoform had a more complex localization: it appeared in the plasma membrane, cytoskeleton and in the endoplasmic reticulum. The three human isoenzymes of InsP3 3-kinase can thus be distinguished by their N-terminal sequence, sensitivity to Ca2+/calmodulin and localization on transfection in COS-7 cells. We have compared the cytosolic Ca2+ responses induced by ATP in COS-7 cells transfected with the three isoenzymes. Cells expressing high levels of any of the three isoforms no longer respond to ATP, whereas cells expressing low levels of each enzyme showed a reduced response consisting of one to three Ca2+ spikes in response to 100 microM ATP. These effects were seen only in wild-type InsP3 3-kinase-transfected cells. 3-Kinase-dead mutant cells behaved as vector-transfected cells. The results highlight the potential role of the three isoforms of InsP3 3-kinase as direct InsP3 metabolizing enzymes and direct regulators of Ca2+ responses to extracellular signals.

Rat inositol 1,4,5-trisphosphate 3-kinase C is enzymatically specialized for basal cellular inositol trisphosphate phosphorylation and shuttles actively between nucleus and cytoplasm

The calcium-liberating second messenger inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) is converted to inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4) by Ins(1,4,5)P3 3-kinases (IP3Ks) that add a fourth phosphate group to the 3-position of the inositol ring. Two isoforms of IP3Ks (named A and B) from different vertebrate species have been well studied. Recently the cloning and examination of a human full-length cDNA encoding a novel isoform, termed human IP3K-C (HsIP3K-C), has been reported. In the present study we report the cloning of a full-length cDNA encoding a rat homologue of HsIP3K-C with a unique mRNA expression pattern, which differs remarkably from the tissue distribution of HsIP3K-C. Of the rat tissues examined, rat IP3K-C (RnIP3K-C) is mainly present in heart, brain, and testis and shows the strongest expression in an epidermal tissue, namely tongue epithelium. RnIP3K-C has a calculated molecular mass of approximately 74.5 kDa and shows an overall identity of approximately 75% with HsIP3K-C. A bacterially expressed, enzymatically active and Ca2+-calmodulin-regulated fragment of this isoform displays remarkable enzymatic properties like a very low Km for Ins(1,4,5)P3 ( approximately 0.2 microm), substrate inhibition by high concentrations of Ins(1,4,5)P3, allosteric product activation by Ins(1,3,4,5)P4 in absence of Ca2+-calmodulin (Ka(app) 0.52 microm), and the ability to efficiently phosphorylate a second InsP3 substrate, inositol 2,4,5-trisphosphate, to inositol 2,4,5,6-tetrakisphosphate in the presence of Ins(1,3,4,5)P4. Furthermore, the RnIP3K-C fused with a fluorescent protein tag is actively transported into and out of the nucleus when transiently expressed in mammalian cells. A leucine-rich nuclear export signal and an uncharacterized nuclear import activity are localized in the N-terminal domain of the protein and determine its nucleocytoplasmic shuttling. These findings point to a particular role of RnIP3K-C in nuclear inositol trisphosphate phosphorylation and cellular growth.

Arabidopsis inositol polyphosphate 6-/3-kinase is a nuclear protein that complements a yeast mutant lacking a functional ArgR-Mcm1 transcription complex

Inositol 1,4,5-trisphosphate 3-kinase, and more generally inositol polyphosphate kinases (Ipk), play important roles in signal transduction in animal cells; however, their functions in plant cells remain to be elucidated. Here, we report the molecular cloning of a cDNA (AtIpk2beta) from a higher plant, Arabidopsis. Arabidopsis AtIpk2beta is a 33-kD protein that exhibits weak homology ( approximately 25% identical amino acids) with Ipk proteins from animals and yeast and lacks a calmodulin binding site, as revealed by sequence analysis and calmodulin binding assays. However, recombinant AtIpk2beta phosphorylates inositol 1,4,5-trisphosphate to inositol 1,4,5,6-tetrakisphosphate and also converts it to inositol 1,3,4,5,6-pentakisphosphate [Ins(1,3,4,5,6)P(5)]. AtIpk2beta also phosphorylates inositol 1,3,4,5-tetrakisphosphate to Ins(1,3,4,5,6)P(5). Thus, the enzyme is a D3/D6 dual-specificity inositol phosphate kinase. AtIpk2beta complements a yeast ARG82/IPK2 mutant lacking a functional ArgR-Mcm1 transcription complex. This complex is involved in regulating Arg metabolism-related gene expression and requires inositol polyphosphate kinase activity to function. AtIpk2beta was found to be located predominantly in the nucleus of plant cells, as demonstrated by immunolocalization and fusion to green fluorescent protein. RNA gel blot analysis and promoter-beta-glucuronidase reporter gene studies demonstrated AtIpk2beta gene expression in various organs tested. These data suggest a role for AtIpk2beta as a transcriptional control mediator in plants.

Control of oxidative stress resistance by IP3 kinase in Drosophila melanogaster

Oxidative damage is thought to be a major causal factor of aging, and is implicated in several human pathologies such as Alzheimer's and Parkinson's diseases. Nevertheless the genetical determinants of in vivo oxidative stress response are still poorly understood. To identify cellular components whose deregulation leads to oxidative stress resistance, we performed a genetic screen in Drosophila melanogaster. We thus identified in this screen Drosophila Inositol 1,4,5-triphosphate kinase I (D-IP3K1), a Drosophila gene homologous to mammalian IP3Ks. In vertebrates, IP3Ks phosphorylate the second messenger Inositol 1,4,5-triphosphate (IP3) to produce Inositol 1,3,4,5 tetrakiphosphate (IP4). IP3 binding to its receptor (IP3R) triggers Ca(2+) release from the endoplasmic reticulum (ER) to the cytosol, whereas IP4 physiological role remains elusive. We show here that ubiquitous overexpression of D-IP3K1 confers resistance of flies to H(2)O(2)- but not to paraquat-induced oxidative stress. Additional genetic analysis with other members of IP3 and IP4 signaling pathways led us to propose that the D-IP3K1 protective effect is mainly mediated through the reduction of IP3 level (which probably results in reduced Ca(2+) release from internal stores), rather than through the rise of IP4 level.

Simulations of inositol phosphate metabolism and its interaction with InsP(3)-mediated calcium release

Inositol phosphates function as second messengers for a variety of extracellular signals. Ins(1,4,5)P(3) generated by phospholipase C-mediated hydrolysis of phosphatidylinositol bisphosphate, triggers numerous cellular processes by regulating calcium release from internal stores. The Ins(1,4,5)P(3) signal is coupled to a complex metabolic cascade involving a series of phosphatases and kinases. These enzymes generate a range of inositol phosphate derivatives, many of which have signaling roles of their own. We have integrated published biochemical data to build a mass action model for InsP(3) metabolism. The model includes most inositol phosphates that are currently known to interact with each other. We have used this model to study the effects of a G-protein coupled receptor stimulus that activates phospholipase C on the inositol phosphates. We have also monitored how the metabolic cascade interacts with Ins(1,4,5)P(3)-mediated calcium release. We find temporal dynamics of most inositol phosphates to be strongly influenced by the elaborate networking. We also show that Ins(1,3,4,5)P(4) plays a key role in InsP(3) dynamics and allows for paired pulse facilitation of calcium release. Calcium oscillations produce oscillatory responses in parts of the metabolic network and are in turn temporally modulated by the metabolism of InsP(3).

The human homologue of yeast ArgRIII protein is an inositol phosphate multikinase with predominantly nuclear localization

The function of the transcription regulator ArgRIII in the expression of several genes involved in the metabolism of arginine in yeast has been well studied. It was previously reported that it is also an inositol phosphate multikinase and an important factor of the mRNA export pathway [reviewed by Shears (2000) Bioessays 22, 786-789]. In the present study we report the cloning of a full-length 1248-bp cDNA encoding a human inositol phosphate multikinase (IPMK). This protein has a calculated molecular mass of 47.219 kDa. Functionally important motifs [inositol phosphate-binding site, ATP-binding site, catalytically important SSLL (Ser-Ser-Leu-Leu) domain] are conserved between the human IPMK and yeast ArgRIII. Bacterially expressed protein demonstrated an inositol phosphate multikinase activity similar to that of yeast ArgRIII. Ins(1,4,5)P3 is phosphorylated at positions 3 and 6 up to Ins(1,3,4,5,6)P5. The human IPMK fused with a fluorescent protein tag is localized predominantly in the nucleus when transiently expressed in mammalian cells. A basic cluster in the protein's C-terminus is positively involved in nuclear targeting. These findings are consistent with the concept of a nuclear inositol phosphate signalling and phosphorylation pathway in mammalian cells.

US2, a human cytomegalovirus-encoded type I membrane protein, contains a non-cleavable amino-terminal signal peptide

The human cytomegalovirus US2 gene product targets major histocompatibility class I molecules for degradation in a proteasome-dependent fashion. Degradation requires interaction between the endoplasmic reticulum (ER) lumenal domains of US2 and class I. While ER insertion of US2 is essential for US2 function, US2 lacks a cleavable signal peptide. Radiosequence analysis of glycosylated US2 confirms the presence of the NH(2) terminus predicted on the basis of the amino acid sequence, with no evidence for processing by signal peptidase. Despite the absence of cleavage, the US2 NH(2)-terminal segment constitutes its signal peptide and is sufficient to drive ER translocation of chimeric reporter proteins, again without further cleavage. The putative US2 signal peptide c-region is responsible for the absence of cleavage, despite the presence of a suitable -3,-1 amino acid motif for signal peptidase recognition. In addition, the US2 signal peptide affects the early processing events of the nascent polypeptide, altering the efficiency of ER insertion and subsequent N-linked glycosylation. To our knowledge, US2 is the first example of a membrane protein that does not contain a cleavable signal peptide, yet otherwise behaves like a type I membrane glycoprotein.

Molecular profiling of behavioural development: differential expression of mRNAs for inositol 1,4,5-trisphosphate 3-kinase isoforms in naive and experienced honeybees (Apis mellifera)

In seeking genetic factors that may control the extended behavioural maturation of adult honeybees we found that inositol 1,4,5-trisphosphate (IP(3)) 3-kinase, a key enzyme in the IP(3)-mediated signalling cascade, is differentially expressed in brains of naive, newly emerged bees and experienced foragers. DNA sequencing yielded a contig of 21.5 kb spanning the honeybee IP(3)K locus and a 3' flanking gene similar to a transcription factor NFR-kappa-B. The IP(3)K locus gives rise to three differentially expressed major transcripts produced by alternative splicing that encode proteins with identical, highly conserved C-termini and distinct, non-conserved N-terminal domains. The type A transcript is dominant in the adult brain and its level of expression increases threefold during the first 4 days of adult development. The type B message is expressed in brains of naive bees, but is also found in the thorax and abdomen, whereas transcript C is expressed largely in non-neural tissues and in the antenna. In contrast to type A message, the brain levels of transcript B decrease during the first 4 days of adult life. Our data are evaluated in the context of the contrasting behavioural phenotypes of immature and experienced worker honeybees.

Cloning and expression of a full-length cDNA encoding human inositol 1,4,5-trisphosphate 3-kinase B

Inositol 1,4,5-trisphosphate (InsP(3)) 3-kinase catalyzes the phosphorylation of InsP(3) to inositol 1,3,4,5-tetrakisphosphate (InsP(4)). cDNAs encoding three isoenzymes of InsP(3) 3-kinase (3-kinases A, B, and C) have been previously reported; however, a demonstrably full-length cDNA encoding human InsP(3) 3-kinase B was still lacking. Here we report the cloning of a full-length 2841-bp cDNA encoding human InsP(3) 3-kinase B. Northern blot analysis shows the presence of an ubiquitous transcript of approximately 7.2 kb in a large number of human tissues. InsP(3) 3-kinase activity measured in COS-7 cells transfected with InsP(3) 3-kinase B shows an activity that was 8-fold increased upon the addition of Ca(2+)/calmodulin in the assay mixture.

Inositol 1,4,5-trisphosphate 3-kinase A associates with F-actin and dendritic spines via its N terminus

The consequences of the rapid 3-phosphorylation of inositol 1,4,5-trisphosphate (IP(3)) to produce inositol 1,3,4,5-tetrakisphosphate (IP(4)) via the action of IP(3) 3-kinases involve the control of calcium signals. Using green fluorescent protein constructs of full-length and truncated IP(3) 3-kinase isoform A expressed in HeLa cells, COS-7 cells, and primary neuronal cultures, we have defined a novel N-terminal 66-amino acid F-actin-binding region that localizes the kinase to dendritic spines. The region is necessary and sufficient for binding F-actin and consists of a proline-rich stretch followed by a predicted alpha-helix. We also localized endogenous IP(3) 3-kinase A to the dendritic spines of pyramidal neurons in primary hippocampal cultures, where it is co-localized postsynaptically with calcium/calmodulin-dependent protein kinase II. Our experiments suggest a link between inositol phosphate metabolism, calcium signaling, and the actin cytoskeleton in dendritic spines. The phosphorylation of IP(3) in dendritic spines to produce IP(4) is likely to be important for modulating the compartmentalization of calcium at synapses.

A monomer is the minimum functional unit required for channel and ATPase activity of the cystic fibrosis transmembrane conductance regulator

The cystic fibrosis transmembrane conductance regulator (CFTR) normally functions as a phosphorylation-regulated chloride channel on the apical surface of epithelial cells, and lack of this function is the primary cause for the fatal disease cystic fibrosis (CF). Previous studies showed that purified, reconstituted CFTR can function as a chloride channel and, further, that its intrinsic ATPase activity is required to regulate opening and closing of the channel gate. However, these previous studies did not identify the quaternary structure required to mediate conduction and catalysis. Our present studies show that CFTR molecules may self-associate in CHO and Sf9 membranes, as complexes close to the predicted size of CFTR dimers can be captured by chemical cross-linking reagents and detected using nondissociative PAGE. However, CFTR function does not require a multimeric complex for function as we determined that purified, reconstituted CFTR monomers are sufficient to mediate regulated chloride conduction and ATPase activity.

Back in the water: the return of the inositol phosphates

Following the discovery of inositol-1,4,5-trisphosphate as a second messenger, many other inositol phosphates were discovered in quick succession, with some understanding of their synthesis pathways and a few guesses at their possible functions. But then it all seemed to go comparatively quiet, with an explosion of interest in the inositol lipids. Now the water-soluble phase is once again becoming a focus of interest. Old and new data point to a new vista of inositol phosphates, with functions in many diverse aspects of cell biology, such as ion-channel physiology, membrane dynamics and nuclear signalling.

Inositol phosphates: Does IP(4) run a protection racket?

The phosphorylation of IP(3) by IP(3) 3-kinase leads to a number of physiological events, most of which are poorly understood. Recent findings about a hitherto unsuspected action of the IP(3) 3-kinase product, IP(4), suggest that the evolution of IP(3) 3-kinase may have even more far-reaching consequences than we thought.

Effects of elevated expression of inositol 1,4,5-trisphosphate 3-kinase B on Ca2+ homoeostasis in HeLa cells

Ins(1,4,5)P(3) 3-kinase (IP3K) phosphorylates the Ca(2+)-mobilizing second messenger Ins(1,4,5)P(3) to yield the putative second messenger Ins(1,3,4,5)P(4). A HeLa cell line was established expressing the rat B isoform of IP3K under the control of an inducible promoter. The IP3KB-transfected cell line possessed 23-fold greater IP3K activity than untransfected cells after induction of IP3KB expression, but only 0.23-fold greater activity when IP3KB expression was not induced. Elevating IP3KB expression significantly reduced levels of Ins(1,4,5)P(3) and increased levels of Ins(1,3,4,5)P(4) after stimulation of cells with histamine, but had no effect on basal levels. Histamine- and ATP-evoked cytosolic Ca(2+) responses were dramatically reduced upon elevation of IP3KB expression. On stimulation with a supramaximal dose of histamine, 67% of cells induced to express IP3KB gave no detectable elevation in cytosolic Ca(2+), compared with 3% of uninduced cells. The quantity of Ca(2+) within thapsigargin-sensitive and -insensitive stores was unaffected by elevation of IP3KB expression, as was capacitative Ca(2+) entry. These data suggest that IP3KB may play a significant role in the regulation of Ins(1,4,5)P(3) levels, and consequently in Ca(2+) responses following stimulation of cells with Ins(1,4,5)P(3)-elevating agonists.

Cloning and expression of a cDNA encoding human inositol 1,4,5-trisphosphate 3-kinase C

Inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)] 3-kinase catalyses the phosphorylation of Ins(1,4,5)P(3) to Ins(1,3,4,5)P(4). cDNAs encoding two isoenzymes of Ins(1,4,5)P(3) 3-kinase (3-kinases A and B) have been described previously. In the present study, we report the cloning of a full-length 2052 bp cDNA encoding a third human isoenzyme of the Ins(1,4,5)P(3) 3-kinase family, referred to as isoform C. This novel enzyme has a calculated molecular mass of 75. 207 kDa and a K(m) for Ins(1,4,5)P(3) of 6 microM. Northern-blot analysis showed the presence of a transcript of approx. 3.9 kb in various human tissues. Inositol trisphosphate 3-kinase C demonstrates enzymic activity when expressed in DH5alphaF' bacteria or COS-7 cells. Calcium alone decreases the Ins(1,4,5)P(3) 3-kinase activity of the 3-kinase C isoenzyme in transfected COS-7 cells. This inhibitory effect is reversed in the presence of calmodulin. The recombinant bacterial 3-kinase C can be adsorbed on calmodulin-Sepharose in the presence of calcium. The present data show that Ins(1,4,5)P(3) 3-kinase C: (i) shares a conserved catalytic domain of about 275 amino acids with the two other mammalian isoforms, (ii) could be purified on a calmodulin-Sepharose column and (iii) could be distinguished from the A and B isoenzymes by the effects of calcium and of calmodulin.

A single C-terminal peptide segment mediates both membrane association and localization of lysyl hydroxylase in the endoplasmic reticulum

Hydroxylation of lysyl residues is crucial for the unique glycosylation pattern found in collagens and for the mechanical strength of fully assembled extracellular collagen fibers. Hydroxylation is catalyzed in the lumen of the endoplasmic reticulum (ER) by a specific enzyme, lysyl hydroxylase (LH). The absence of the known ER-specific retrieval motifs in its primary structure and its association with the ER membranes in vivo have suggested that the enzyme is localized in the ER via a novel retention/retrieval mechanism. We have identified here a 40-amino acid C-terminal peptide segment of LH that is able to convert cathepsin D, normally a soluble lysosomal protease, into a membrane-associated protein. The same segment also markedly slows down the transport of the reporter protein from the ER into post-ER compartments, as assessed by our pulse-chase experiments. The retardation efficiency mediated by this C-terminal peptide segment is comparable with that of the intact LH but lower than that of the KDEL receptor-based retrieval mechanism. Within this 40-amino acid segment, the first 25 amino acids appear to be the most crucial ones in terms of membrane association and ER localization, because the last 15 C-terminal amino acids did not possess substantial retardation activity alone. Our findings thus define a short peptide segment very close to the extreme C terminus of LH as the only necessary determinant both for its membrane association and localization in the ER.

Calcium-calmodulin-dependent protein kinase II and protein kinase C-mediated phosphorylation and activation of D-myo-inositol 1,4, 5-trisphosphate 3-kinase B in astrocytes

D-myo-Inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) 3-kinase catalyzes the production of D-myo-inositol 1,3,4,5-tetrakisphosphate from the second messenger Ins (1,4,5)P3. Transient and okadaic acid-sensitive activation of Ins(1,4,5)P3 3-kinase by 8-10-fold is observed in homogenates prepared from rat cortical astrocytes after incubation with either carbachol or UTP. 12-O-Tetradecanoylphorbol-13-acetate provokes the activation of Ins(1,4,5)P3 3-kinase by 2-fold in both cell systems. The kinase was purified by calmodulin-Sepharose from the two cell systems. Enzyme activity corresponding to the silver-stained 88-kDa protein could be regenerated after SDS-polyacrylamide gel electrophoresis. Antibodies to two distinct peptides chosen in the primary structure of human Ins(1,4,5)P3 3-kinase B recognized the astrocytic native isoform. In [32P]orthophosphate-preincubated cells, a major phosphorylated 88-kDa enzyme could be purified and identified in cells in response to receptor activation or 12-O-tetradecanoylphorbol-13-acetate treatment. Calmodulin kinase II inhibitors (i.e. KN-93 and KN-62) and a protein kinase C inhibitor (i.e. calphostin C) prevented the phosphorylation of the 88-kDa isoenzyme. In addition to enzyme activation, a redistribution of Ins(1,4,5)P3 3-kinase from soluble to particulate fraction of astrocytes was observed. In vitro phosphorylation of the purified enzyme by calmodulin kinase II and protein kinase C added together resulted in a maximal 60-70-fold activation.

Tissue-specific expression and endogenous subcellular distribution of the inositol 1,3,4,5-tetrakisphosphate-binding proteins GAP1(IP4BP) and GAP1(m)

GAP1(IP4BP) and GAP1(m) belong to the GAP1 family of Ras GTPase-activating proteins that are candidate InsP4 receptors. Here we show they are ubiquitously expressed in human tissues and are likely to have tissue-specific splice variants. Analysis by subcellular fractionation of RBL-2H3 rat basophilic leukemia cells confirms that endogenous GAP1(IP4BP) is primarily localised to the plasma membrane, whereas GAP1(m) appears localised to the cytoplasm (cytosol and internal membranes) but not the plasma membrane. Subcellular fractionation did not indicate a specific co-localisation between membrane-bound GAP1(m) and several Ca2+ store markers, consistent with the lack of co-localisation between GAP1(m) and SERCA1 upon co-expression in COS-7 cells. This difference suggests that GAP1(m) does not reside at a site where it could regulate the ability of InsP4 to release intracellular Ca2+. As GAP1(m) is primarily localised to the cytosol of unstimulated cells it may be spatially regulated in order to interact with Ras at the plasma membrane.

Distinct subcellular localisations of the putative inositol 1,3,4,5-tetrakisphosphate receptors GAP1IP4BP and GAP1m result from the GAP1IP4BP PH domain directing plasma membrane targeting

Inositol 1,3,4,5-tetrakisphosphate (IP4), is a ubiquitous inositol phosphate that has been suggested to function as a second messenger. Recently, we purified and cloned a putative IP4 receptor, termed GAP1(IP4BP)[1], which is also a member of the GAP1 family of GTPase-activating proteins for the Ras family of GTPases. A homologue of GAP1(IP4BP), called GAP1(m), has been identified [2] and here we describe the cloning of a GAP1(m) cDNA from a human circulating-blood cDNA library. We found that a deletion mutant of GAP1(m), in which the putative phospholipid-binding domains (C2A and C2B) have been removed, binds to IP4 with a similar affinity and specificity to that of the corresponding GAP1(IP4BP) mutant. Expression studies of the proteins in either COS-7 or HeLa cells showed that, whereas GAP1(IP4BP) is located solely at the plasma membrane, GAP1(m) seems to have a distinct perinuclear localisation. By mutational analysis, we have shown that the contrast in subcellular distribution of these two closely related proteins may be a function of their respective pleckstrin homology (PH) domains. This difference in localisation has fundamental significance for our understanding of the second messenger functions of IP4.