Inositol 1,4,5-trisphosphate binding to porcine tracheal smooth muscle aldolase

Aldolase sequesters WASP and affects WASP/Arp2/3-stimulated actin dynamics

In addition to its roles in sugar metabolism, fructose-1,6-bisphosphate aldolase (aldolase) has been implicated in cellular functions independent from these roles, termed "moonlighting functions." These moonlighting functions likely involve the known aldolase-actin interaction, as many proteins with which aldolase interacts are involved in actin-dependent processes. Specifically, aldolase interacts both in vitro and in cells with Wiskott-Aldrich Syndrome Protein (WASP), a protein involved in controlling actin dynamics, yet the function of this interaction remains unknown. Here, the effect of aldolase on WASP-dependent processes in vitro and in cells is investigated. Aldolase inhibits WASP/Arp2/3-dependent actin polymerization in vitro. In cells, knockdown of aldolase results in a decreased rate of cell motility and cell spreading, two WASP-dependent processes. Expression of exogenous aldolase rescues these defects. Whether these effects of aldolase on WASP-dependent processes were due to aldolase catalysis or moonlighting functions is tested using aldolase variants defective in either catalytic or actin-binding activity. While the actin-binding deficient aldolase variant is unable to inhibit actin polymerization in vitro and is unable to rescue cell motility defects in cells, the catalytically inactive aldolase is able to perform these functions, providing evidence that aldolase moonlighting plays a role in WASP-mediated processes.

Targeting of several glycolytic enzymes using RNA interference reveals aldolase affects cancer cell proliferation through a non-glycolytic mechanism

In cancer, glucose uptake and glycolysis are increased regardless of the oxygen concentration in the cell, a phenomenon known as the Warburg effect. Several (but not all) glycolytic enzymes have been investigated as potential therapeutic targets for cancer treatment using RNAi. Here, four previously untargeted glycolytic enzymes, aldolase A, glyceraldehyde 3-phosphate dehydrogenase, triose phosphate isomerase, and enolase 1, are targeted using RNAi in Ras-transformed NIH-3T3 cells. Of these enzymes, knockdown of aldolase causes the greatest effect, inhibiting cell proliferation by 90%. This defect is rescued by expression of exogenous aldolase. However, aldolase knockdown does not affect glycolytic flux or intracellular ATP concentration, indicating a non-metabolic cause for the cell proliferation defect. Furthermore, this defect could be rescued with an enzymatically dead aldolase variant that retains the known F-actin binding ability of aldolase. One possible model for how aldolase knockdown may inhibit transformed cell proliferation is through its disruption of actin-cytoskeleton dynamics in cell division. Consistent with this hypothesis, aldolase knockdown cells show increased multinucleation. These results are compared with other studies targeting glycolytic enzymes with RNAi in the context of cancer cell proliferation and suggest that aldolase may be a useful target in the treatment of cancer.

Protein tyrosine nitration of aldolase in mast cells: a plausible pathway in nitric oxide-mediated regulation of mast cell function

NO is a short-lived free radical that plays a critical role in the regulation of cellular signaling. Mast cell (MC)-derived NO and exogenous NO regulate MC activities, including the inhibition of MC degranulation. At a molecular level, NO acts to modify protein structure and function through several mechanisms, including protein tyrosine nitration. To begin to elucidate the molecular mechanisms underlying the effects of NO in MCs, we investigated protein tyrosine nitration in human MC lines HMC-1 and LAD2 treated with the NO donor S-nitrosoglutathione. Using two-dimensional gel Western blot analysis with an anti-nitrotyrosine Ab, together with mass spectrometry, we identified aldolase A, an enzyme of the glycolytic pathway, as a target for tyrosine nitration in MCs. The nitration of aldolase A was associated with a reduction in the maximum velocity of aldolase in HMC-1 and LAD2. Nuclear magnetic resonance analysis showed that despite these changes in the activity of a critical enzyme in glycolysis, there was no significant change in total cellular ATP content, although the AMP/ATP ratio was altered. Elevated levels of lactate and pyruvate suggested that S-nitrosoglutathione treatment enhanced glycolysis. Reduced aldolase activity was associated with increased intracellular levels of its substrate, fructose 1,6-bisphosphate. Interestingly, fructose 1,6-bisphosphate inhibited IgE-mediated MC degranulation in LAD2 cells. Thus, for the first time we report evidence of protein tyrosine nitration in human MC lines and identify aldolase A as a prominent target. This posttranslational nitration of aldolase A may be an important pathway that regulates MC phenotype and function.

Fructose metabolism in the cerebellum

Under normal physiological conditions, the brain utilizes only a small number of carbon sources for energy. Recently, there is growing molecular and biochemical evidence that other carbon sources, including fructose, may play a role in neuro-energetics. Fructose is the number one commercial sweetener in Western civilization with large amounts of fructose being toxic, yet fructose metabolism remains relatively poorly characterized. Fructose is purportedly metabolized via either of two pathways, the fructose-1-phosphate pathway and/or the fructose-6-phosphate pathway. Many early metabolic studies could not clearly discriminate which of these two pathways predominates, nor could they distinguish which cell types in various tissues are capable of fructose metabolism. In addition, the lack of good physiological models, the diet-induced changes in gene expression in many tissues, the involvement of multiple genes in multiple pathways involved in fructose metabolism, and the lack of characterization of some genes involved in fructose metabolism have complicated our understanding of the physiological role of fructose in neuro-energetics. A recent neuro-metabolism study of the cerebellum demonstrated fructose metabolism and co-expression of the genes specific for the fructose 1-phosphate pathway, GLUT5 (glut5) and ketohexokinase (khk), in Purkinje cells suggesting this as an active pathway in specific neurons? Meanwhile, concern over the rapid increase in dietary fructose, particularly among children, has increased awareness about how fructose is metabolized in vivo and what effects a high fructose diet might have. In this regard, establishment of cellular and molecular studies and physiological characterization of the important and/or deleterious roles fructose plays in the brain is critical. This review will discuss the status of fructose metabolism in the brain with special reference to the cerebellum and the physiological roles of the different pathways.

Aldolase potentiates DIDS activation of the ryanodine receptor in rabbit skeletal sarcoplasmic reticulum

DIDS (4,4'-di-isothiocyanostilbene-2,2'-disulfonate), an anion channel blocker, triggers Ca2+ release from skeletal muscle SR (sarcoplasmic reticulum). The present study characterized the effects of DIDS on rabbit skeletal single Ca2+-release channel/RyR1 (ryanodine receptor type 1) incorporated into a planar lipid bilayer. When junctional SR vesicles were used for channel incorporation (native RyR1), DIDS increased the mean P(o) (open probability) of RyR1 without affecting unitary conductance when Cs+ was used as the charge carrier. Lifetime analysis of single RyR1 activities showed that 10 microM DIDS induced reversible long-lived open events (P(o)=0.451+/-0.038) in the presence of 10 microM Ca2+, due mainly to a new third component for both open and closed time constants. However, when purified RyR1 was examined in the same condition, 10 microM DIDS became considerably less potent (P(o)=0.206+/-0.025), although the caffeine response was similar between native and purified RyR1. Hence we postulated that a DIDS-binding protein, essential for the DIDS sensitivity of RyR1, was lost during RyR1 purification. DIDS-affinity column chromatography of solubilized junctional SR, and MALDI-TOF (matrix-assisted laser-desorption ionization-time-of-flight) MS analysis of the affinity-column-associated proteins, identified four major DIDS-binding proteins in the SR fraction. Among them, aldolase was the only protein that greatly potentiated DIDS sensitivity. The association between RyR1 and aldolase was further confirmed by co-immunoprecipitation and aldolase-affinity batch-column chromatography. Taken together, we conclude that aldolase is physically associated with RyR1 and could confer a considerable potentiation of the DIDS effect on RyR1.

Defective calcium homeostasis in the cerebellum in a mouse model of Niemann-Pick A disease

We recently demonstrated that calcium homeostasis is altered in mouse models of two sphingolipid storage diseases, Gaucher and Sandhoff diseases, owing to modulation of the activities of a calcium-release channel (the ryanodine receptor) and of the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) respectively, by the accumulating sphingolipids. We now demonstrate that calcium homeostasis is also altered in a mouse model of Niemann-Pick A disease, the acid sphingomyelinase (A-SMase)-deficient mouse (ASM-/-), with reduced rates of calcium uptake via SERCA in the cerebellum of 6-7-month-old mice. However, the mechanism responsible for defective calcium homeostasis is completely different from that observed in the other two disease models. Thus, levels of SERCA expression are significantly reduced in the ASM-/- cerebellum by 6-7 months of age, immediately before death of the mice, as are levels of the inositol 1,4,5-triphosphate receptor (IP3R), the major calcium-release channel in the cerebellum. Systematic analyses of the time course of loss of SERCA and IP3R expression revealed that loss of the IP3R preceeded that of SERCA, with essentially no IP3R remaining by 4 months of age, whereas SERCA was still present even after 6 months. Expression of zebrin II (aldolase C), a protein found in about half of the Purkinje cells in the adult mouse cerebellum, was essentially unchanged during development. We discuss possible pathological mechanisms related to calcium dysfunction that may cause Purkinje cell degeneration, and as a result, the onset of neuropathology in Niemann-Pick A disease.

Ca2+-dependent elevation of inositol 1,4,5-trisphosphate level induced by freezing or homogenization of tissues and cells

Various cells and tissues contain high basal levels of inositol 1,4,5-trisphosphate, raising questions about the functional significance of inositol 1,4,5-trisphosphate in some tissues such as the heart. We used intact tissue and isolated cells from heart and liver of adult rats to examine if different fixation procedures might artificially elevate the level of inositol 1,4,5-trisphosphate. The basal level of inositol 1,4,5-trisphosphate in intact, freeze-clamped cardiac tissue from adult rats was 10 times higher than in isolated, non-frozen cardiomyocytes, while freeze-clamped liver contained approximately 4 times higher inositol 1,4,5-trisphosphate levels than isolated, non-frozen hepatocytes. Stimulation with norepinephrine induced a significant increase in the inositol 1,4,5-trisphosphate level in isolated cardiomyocytes, whereas no significant increase was observed in freeze-clamped cardiac tissue. Freezing of isolated cardiomyocytes or hepatocytes before extraction increased basal inositol 1,4,5-trisphosphate levels 3 times. In cellular homogenates prepared in the presence of EGTA and stored at 4 degrees , readdition of calcium resulted in a time-dependent increase in inositol 1,4,5-trisphosphate mass and a decrease in the mass of phosphatidylinositol 4,5-bisphosphate (PIP(2)). The reaction was essentially complete within 30 sec. in homogenates from cardiomyocytes, while PIP(2) hydrolysis was slower in hepatocyte homogenates. Perfusion of intact rat hearts with EGTA present during the last 2 min. of perfusion, followed by freeze-clamping, resulted in basal inositol 1,4,5-trisphosphate levels comparable to those in isolated cardiomyocytes, and norepinephrine stimulation increased inositol 1,4,5-trisphosphate mass by approximately 80%. The presence of EGTA did not significantly affect PIP(2) levels in perfused hearts. The results suggest that freezing or homogenization of intact tissue and isolated cells may result in Ca(2+)-dependent activation of phospholipase C, leading to high basal inositol 1,4,5-trisphosphate levels that may mask agonist-induced changes.

Expression of tetraspanins in peripheral blood leukocytes: a comparison between normal and infectious conditions

The role of tetraspanins is undefined, despite their detection in diverse cell types and functions. This study addresses the characterization of tetraspanin expression levels in normal peripheral blood leukocytes (PBL) and in patients with bacterial infection. Membranal and cytoplasmic expression of CD9, CD53, CD63, CD81, CD82 and CD151 in polymorphonuclears (PMN), monocytes, B and T lymphocytes was assessed using flow cytometry. Results suggested that for normal PBL, PMN are distinguished by dominant cytoplasmic CD63; monocytes and B cells prevailingly express CD53; CD82 is primarily expressed on T-cell membranes. However, a major trend of downregulation was demonstrated for the examined tetraspanins, except CD63, in all patients' PBL subtypes. Therefore, tetraspanin modulation in infections may be attributed to elevated leukocyte motility in immune reactions and this is compatible with the previous publications of tetraspanins as metastasis suppressors. This work represents the first comprehensive baseline of tetraspanin expression in normal PBL and in infectious disorders.

Snapshots of catalysis: the structure of fructose-1,6-(bis)phosphate aldolase covalently bound to the substrate dihydroxyacetone phosphate

Fructose-1,6-bis(phosphate) aldolase is an essential glycolytic enzyme found in all vertebrates and higher plants that catalyzes the cleavage of fructose 1,6-bis(phosphate) (Fru-1,6-P(2)) to glyceraldehyde 3-phosphate and dihydroxyacetone phosphate (DHAP). Mutations in the aldolase genes in humans cause hemolytic anemia and hereditary fructose intolerance. The structure of the aldolase-DHAP Schiff base has been determined by X-ray crystallography to 2.6 A resolution (R(cryst) = 0.213, R(free) = 0.249) by trapping the catalytic intermediate with NaBH(4) in the presence of Fru-1,6-P(2). This is the first structure of a trapped covalent intermediate for this essential glycolytic enzyme. The structure allows the elucidation of a comprehensive catalytic mechanism and identification of a conserved chemical motif in Schiff-base aldolases. The position of the bound DHAP relative to Asp33 is consistent with a role for Asp33 in deprotonation of the C4-hydroxyl leading to C-C bond cleavage. The methyl side chain of Ala31 is positioned directly opposite the C3-hydroxyl, sterically favoring the S-configuration of the substrate at this carbon. The "trigger" residue Arg303, which binds the substrate C6-phosphate group, is a ligand to the phosphate group of DHAP. The observed movement of the ligand between substrate and product phosphates may provide a structural link between the substrate cleavage and the conformational change in the C-terminus associated with product release. The position of Glu187 in relation to the DHAP Schiff base is consistent with a role for the residue in protonation of the hydroxyl group of the carbinolamine in the dehydration step, catalyzing Schiff-base formation. The overlay of the aldolase-DHAP structure with that of the covalent enzyme-dihydroxyacetone structure of the mechanistically similar transaldolase and KDPG aldolase allows the identification of a conserved Lys-Glu dyad involved in Schiff-base formation and breakdown. The overlay highlights the fact that Lys146 in aldolase is replaced in transaldolase with Asn35. The substitution in transaldolase stabilizes the enamine intermediate required for the attack of the second aldose substrate, changing the chemistry from aldolase to transaldolase.

Identification of casein kinase I substrates by in vitro expression cloning screening

Casein kinase I (CKI) is a widely expressed protein kinase family implicated in diverse processes including membrane trafficking, DNA repair, and circadian rhythm. Despite the large number of CKI genes, few biologically relevant substrates have been identified. As an approach to better defining the spectrum of CKI substrates, we extended a recently described in vitro expression cloning (IVEC) strategy. Polypeptides pools were screened for kinase-dependent electrophoretic mobility shifts. Ten putative CKI substrates were isolated from an initial sample of 3000 random cDNA clones. Candidate substrates include proteins involved in RNA metabolism (a putative RNA helicase, the nucleolar protein hNOP56, and hnRNP A1, and ribosomal proteins L4, L8, and L13), as well as keratin 17, a necdin-related protein, and the calcium-binding proteins desmoglein 2 and annexin II. The same pools were also screened with active ERK2, and four substrates identified: aldolase, NSD-like protein, uracil-DNA glycosylase, and HHR23A. IVEC is an effective method to identify novel protein kinase substrates.

Identification of conserved promoter elements for aldB and isozyme specific residues in aldolase B

The comparison of three complete aldolase B genes-including known and putative regulatory elements-is presented. The third aldolase B gene was provided by the complete aldB gene sequence (14803 bp) encoding the rabbit aldolase B isozyme. The promoter sequence alignment included the nonmammalian chicken aldolase B gene and confirms the promoter sequence conservation of those elements where trans-factor binding has been demonstrated in rat aldB. Moreover, the alignment reveals conserved sequences that may represent previously unidentified promoter elements that are present in all aldBs or specifically in the mammalian aldB promoters. One remarkable feature is a poly-purine segment found between the CAAT and TATA elements. In the mammalian promoters, this is exclusively a 9-10 bp poly-dA stretch. The avian promoter has an additional stretch of eight dG-bases immediately upstream of the poly-dA. Alignment of a portion of intron 1 of the chicken, human, and rabbit aldB genes reveals conserved sequences that are likely candidates for a reported positive activation sequence. In addition, the amino acid sequences of all eight known aldolase B isozymes is compared to the other vertebrate aldolases. A number of aldolase B-specific residues are identified that cluster in the carboxyl-portion of the sequence. With the exception of residue C268, these residues are not found near the active site, although, they are likely to be responsible for the substrate specificity of aldolase B.

Effects of polyamines and calcium and sodium ions on smooth muscle cytoskeleton-associated phosphatidylinositol (4)-phosphate 5-kinase

In many different cell types, including smooth muscle cells (Baron et al., 1989, Am. J. Physiol., 256: C375-383; Baron et al., J. Pharmacol. Exp. Ther. 266: 8-15), phosphatidylinositol (4)-phosphate 5-kinase plays a critical role in the regulation of membrane concentrations of phosphatidylinositol (4,5)-bisphosphate and formation of inositol (1,4,5)-trisphosphate. In unstimulated porcine trachealis smooth muscle, 70% of total cellular phosphatidylinositol (4)-phosphate 5-kinase activity was associated with cytoskeletal proteins and only trace activity was detectable in isolated sarcolemma. Using two different preparations, we studied cytoskeleton-associated phosphatidyl inositol (4)-phosphate 5-kinase under conditions that attempted to mimic the ionic and thermal cytoplasmic environment of living cells. The cytoskeleton-associated enzyme, studied using phosphatidylinositol (4)-phosphate substrate concentrations that produced phosphatidylinositol 4,5-bisphosphate at about 10% of the maximal rate, was sensitive to free [Mg2+], had an absolute requirement for phosphatidylserine, phosphatidic acid, or phosphatidylinositol, and included type I isoforms. At 0.5 mM free [Mg2+], physiological spermine concentrations, 0.2-0.4 mM, increased phosphatidylinositol (4)-phosphate 5-kinase activity two to four times compared to controls run without spermine. The EC50 for spermine-evoked increases in activity was 0.17 +/- 0.02 mM. Spermine-evoked enzyme activity was a function of both free [Mg2+] and substrate concentration. Cytoskeleton-associated phosphatidylinositol (4)-phosphate 5-kinase was inhibited by free [Ca2+] over a physiological range for cytoplasm--10(-8) to 10(-5) M, an effect independent of the presence of calmodulin. Na+ over the range 20 to 50 mM also inhibited this enzyme activated by 5 mM Mg2+ but had no effect on spermine-activated enzyme. Na+, Ca2+, and spermine appear to be physiological modulators of smooth muscle cytoskeleton-bound phosphatidylinositol (4)-phosphate 5-kinase.

Smooth muscle aldolase C-bound inositol 1,4,5-trisphosphate studied in vitro under physiological conditions

Our goal was to quantitate inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) binding to aldolase C tetramer (aldolase4) and its displacement by inositol 1,3,4-trisphosphate (Ins(1,3,4)P3) under conditions which approximated the in vivo state. Anions were found to have major effects. Decreasing [KCl] from 100 to 10mM, at 0 degrees C and pH 7.0, increased maximal Ins(1,4,5)P3 binding to 1.0 to 2.4mol per mol aldolase4. At 10 and 30mEq/l [Cl-], an additional high affinity site was detected (Kds = 0.43 and 0.86 microM, respectively). Increasing concentrations of other anions (SO42-, propanoate-, HCO3-, acetate-) also inhibited binding, but effects would be minimal at concentrations of these anions present in the cytoplasm of living cells. Ins(1,3,4)P3 displacement of aldolase C-bound Ins(1,4,5)P3 was sensitive to [Cl-]; at 30mEq/l [Cl-] and 37 degrees C, Ins(1,3,4)P3 released 20% of bound Ins(1,4,5)P3 at concentrations of 100nM. Changing temperature from 0 to 37 degrees C increased Kds for Ins(1,4,5)P3 binding. Changes in free [Ca2+], [Mg2+], [Na+] and [K+] and changes in osmolality had no effect on Ins(1,4,5)P3 binding to aldolase C. In vivo Ins(1,4,5)P3-aldolase4 binding at 30mEq/l [Cl-] and 37 degrees C were calculated for different [Ins(1,4,5)P3]free over the range 0.2 to 1.0 microM. For different cytoplasmic [Ins(1,4,5)P3]free. Ins(1,4,5)P3 binding to aldolase4 was sufficient, if acutely released, to nearly double cytoplasmic [Ins(1,4,5)P3]free. We proposed a schema whereby release of aldolase C-bound Ins(1,4,5)P3 evoked by Ins(1,3,4)P3 amplifies effects of phospholipase C-formed Ins(1,4,5)P3.

Identification of neuronal isozyme specific residues by comparison of goldfish aldolase C to other aldolases

A 2061 bp cDNA encoding a goldfish (Carassius auratus) aldolase was isolated from a goldfish brain library. The deduced 362 amino acid sequence is more similar to vertebrate brain (aldolase C) and muscle aldolases (aldolase A) than to the liver isozymes (aldolase B). Northern blot analysis indicates strong expression of the mRNA in brain but not in liver or muscle, which indicates that this is aldolase C rather than aldolase A. Analysis of all known vertebrate aldolase amino acid sequences reveals five residues; Leu-57, Arg-314, Thr-324, Glu-332, and Gly-350 that are present exclusively in aldolase Cs. The goldfish clone possesses all five residues. The residues are primarily located in the carboxyl-terminal region of the enzyme and may play a role in determining the neuronal isozyme-specific properties of the enzyme. Furthermore, the existence of an aldolase C in a teleost fish has implications with respect to the timing of genome duplication events that are thought to have been critical in vertebrate evolution.

Smooth muscle sarcolemma-associated phospholipase C-beta 2; agonist-evoked translocation

About 25% of the total cellular PLC beta 2 content was found to be associated with a sarcolemmal fraction (SARC) isolated from unstimulated porcine trachealis smooth muscle. SARC-associated PLC beta 2 was located within two compartments, a detergent-extractable compartment and a nondetergent extractable compartment. SARC PLC beta 2 was measured after extraction with 0.6 M KCI; therefore, PLC beta 2 was not bound solely by electrostatic forces within either of these compartments. PLC beta 2 was shown to translocate from cytosol to SARC during a 20-sec activation of intact muscle with a muscarinic agonist, carbachol (CARB); i.e., cytosolic total PLC beta 2 content decreased significantly to 73 +/- 7% of control and SARC total PLC beta 2 content increased to 180 +/- 15% of control value. This translocation was maintained at 5 min of CARB. CARB-evoked translocation occurred into the detergent-extractable SARC fraction, and PLC beta 2 content in this fraction increased 300% compared with that in unstimulated muscle. After CARB, SARC PLC beta 2 content accounted for > 50% of total cellular PLC beta 2 content. CARB-evoked increase in PLC activity in SARC paralleled the increase in PLC beta 2 content. CARB-induced translocations of PLC beta 2 from the cytosol to SARC were of a similar magnitude as occurred with phorbol ester-induced translocations of PKC alpha.

Inositol-1,4,5-trisphosphate mass content in isolated perfused rat heart during alpha-1-adrenoceptor stimulation

Inositol-1,4,5-trisphosphate (IP3) has been proposed to be a second messenger in response to alpha-1-adrenoceptor stimulation also in myocardial cells. We studied the effect of alpha-1-adrenoceptor stimulation (5 x 10(-5) mol/l phenylephrine or 5 x 10(-5) mol/l noradrenaline both in the presence of 10(-6) mol/l timolol) on IP3 mass content in isolated perfused rat hearts. IP3 content was determined by a specific receptor-binding assay-kit (TRK 1000, Amersham) after validating the method. For comparison also the effect of muscarinic stimulation (10(-4) mol/l carbachol in the presence of 10(-6) mol/l timolol) on IP3 content was measured in corresponding preparations. A basal IP3 level of about 75 pmol/mg protein was found. There were no prominent effects of alpha-1-adrenoceptor stimulation on total IP3 content in isolated perfused rat hearts. Phenylephrine gave a statistically significant increase of about 40% at 1/4 min and a statistically significant decrease of about 25% at 4 min after start of exposure. Noradrenaline, however, gave no statistically significant change of IP3 at the time-points studied. Muscarinic stimulation caused a slight, statistically insignificant, increase of IP3 at 1/4 min. The results are compatible with an assumption that agonist stimulation evokes a localized increase of IP3 which may be masked by a relatively high total IP3 mass content. The IP3 peak after phenylephrine coincided with the early positive inotropic phase of the response reported earlier in perfused rat hearts for alpha-1-adrenoceptor stimulation by phenylephrine. Although this might be compatible with a role for IP3 in this early and transient phase, a mediator function of IP3 in the inotropic response is not established.

Isolation of inositol 1,3,4-trisphosphate 5/6-kinase, cDNA cloning and expression of the recombinant enzyme

Inositol 1,3,4-trisphosphate 5/6-kinase was purified 12,900-fold from calf brain using chromatography on heparin-agarose and affinity elution with inositol hexakisphosphate. The final preparation contained proteins of 48 and 36-38 kDa. All of these proteins had the same amino-terminal sequence and were enzymatically active. The smaller species represent proteolysis products with carboxyl-terminal truncation. The Km of the enzyme for inositol 1,3,4-trisphosphate was 80 nM with a Vmax of 60 nmol of product/min/mg of protein. The amino acid sequence of the tryptic peptide HSKLLARPAGGLVGERTCNAXP matched the protein sequence encoded by a human expressed sequence tag clone (GB T09063) at 16 of 22 residues. The expressed sequence tag clone was used to screen a human fetal brain cDNA library to obtain a cDNA clone of 1991 base pairs (bp) that predicts a protein of 46 kDa. The clone encodes the amino-terminal amino acid sequence obtained from the purified calf brain preparation, suggesting that it represents its human homologue. The cDNA was expressed as a fusion protein in Escherichia coli and was found to have inositol 1,3,4-trisphosphate 5/6-kinase activity. Remarkably, both the purified calf brain and recombinant proteins produced both inositol 1,3,4,6-tetrakisphosphate and inositol 1,3,4,5-tetrakisphosphate as products in a ratio of 2.3-5:1. This finding proves that a single kinase phosphorylates inositol in both the D5 and D6 positions. Northern blot analysis identified a transcript of 3.6 kilobases in all tissues with the highest levels in brain. The composite cDNA isolated contains 3054 bp with a poly(A) tail, suggesting that 500-600 bp of 5' sequence remains to be identified.

Inositol phosphates - whither bound? Intracellular signalling

Many proteins are being found to bind inositol phosphates with varying degrees of specificity; the variety of domains that can bind inositol phosphates suggests convergent evolution, but the functions of most of the binding sites are not yet clear.