From the
Institute for Cellular Signal Transduction and the
Institute for Cell Biochemistry and Clinical Neurobiology, University Hospital Hamburg-Eppendorf, Martinistrasse 52, 20246 Hamburg, Germany
Received for publication, October 29, 2002 , and in revised form, March 19, 2003.
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ABSTRACT |
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INTRODUCTION |
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In this study we highlighted possible special roles of IP3K-C in the cellular and in particular in nuclear inositol phosphate metabolism. A rat homologue of HsIP3K-C, termed rat Ins(1,4,5)P3 3-kinase C (RnIP3K-C) was identified. Its mRNA expression pattern in different tissues, the enzymatic properties and the allosteric regulation of bacterially expressed enzyme, and the nucleocytoplasmic shuttling of EGFP fusion proteins derived from RnIP3K-C expressed in mammalian cells were examined. The most interesting findings of our study are that the RnIP3K-C enzyme is strongly Ca2+-CaM-activated in a substrate concentration-dependent manner, is enzymatically optimized for an efficient conversion of basal cellular concentrations of Ins(1,4,5)P3 in presence of Ins(1,3,4,5)P4, and can convert Ins(2,4,5)P3 to Ins(2,4,5,6)P4 under these conditions. Furthermore, RnIP3K-C undergoes active nucleocytoplasmic shuttling as a result of a hitherto unidentified nuclear localization activity and an identified nuclear export signal (NES) both residing in the N-terminal domain of the enzyme. Active transport of proteins between the nucleus and cytoplasm is mediated mainly by the intensively studied canonical nuclear localization signals (NLS) and by the recently characterized nuclear export signal (both reviewed in Refs. 11 and 12). Leptomycin B (LMB), an antifungal antibiotic and an inhibitor of the cell cycle in mammalian and fission yeast cells (13), binds to CRM1 (for chromosome region maintenance), an NES receptor in the nuclear pore complex (14), and thereby inhibits NES-dependent nuclear export (15). By employing LMB inhibition and deleting the canonical NES in the N-terminal domain of RnIP3K-C, we could prove the functionality of this site and the nuclear shuttling activities of the enzyme when expressed in cells.
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EXPERIMENTAL PROCEDURES |
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5'-RACEmRNA, purified by oligo(dT) columns (Amersham Biosciences) from total RNA prepared from rat brain (strain Wistar) as described (17), was used for 5'-RACE employing the MarathonTM kit (Clontech) according to the instructions from the manufacturer. For reverse transcription, carried out for1hat42 °C, a gene-specific primer (5'-GGGCCTCGGCTGGCAATG-3') was used in a reaction containing 3.5 µg of mRNA. With the generated double-stranded cDNA, a first round of PCR was performed using a gene-specific primer (5'-CGGCACGGGCAGCGCCTCATAC-3') and primer AP1 (MarathonTM kit) with an annealing temperature of 56 °C. On the diluted PCR product, nested PCR was performed using a gene-specific primer (5'-CTTCCCCTTTAAATCCTACAAG-3') and primer AP2 (MarathonTM kit) and an annealing temperature of 56 °C. A second round of RACE PCR was done on the original cDNA template with a gene-specific primer (5'-GCTCCAGGCCGCTCGGATATGTAG-3') and primer AP1 using an annealing temperature of 56 °C. From this reaction a nested PCR was performed with a gene-specific primer (5'-TATGTAGGGGTTAACCGATACTC-3') and primer AP2 at 59 °C annealing temperature.
Northern Blot AnalysisA Northern blot containing 2 µg of poly(A)+ RNA/lane from eight different rat tissues was purchased and hybridized according to the instructions from the supplier (Clontech). The 3.3-kb DNA fragment representing the full-length RnIP3K-C cDNA labeled with [32P]dCTP by random priming was used for hybridization in ExpressHyb solution at 68 °C for 90 min, followed by washing two times in 0.1x SSC, 0.1% SDS for 40 min at 50 °C.
To compare specifically RnIP3K-C mRNA in tongue and taste epithelium with other tissues, 15 µg of glyoxylated total RNA/lane was separated by agarose gel electrophoresis, transferred onto a Hybond N membrane (Amersham Biosciences) and hybridized with the radioactive RnIP3K-C probe described above. Hybridization in ULTRAHyb solution at 42 °C overnight and washing in 0.2x SSC, 0.1% SDS at 45 °C were carried out according to the instructions from the manufacturer (Ambion, Austin, TX). As control for RNA loading and integrity, the blot was stripped and hybridized with a 32P-labeled actin probe under the same conditions. Blots were exposed to a phosphorimaging screen for up to 24 h and analyzed in a Fujix Bio-Imaging Analyzer (BAS 2000).
Reverse Transcription-PCR1 µg of total RNA from various rat tissues including enzymatically prepared tongue and taste epithelium was reverse transcribed using Superscript II reverse transcriptase (Invitrogen) and random hexamer primers (Amersham Biosciences). A pair of RnIP3K-C-specific primers (forward, 5'-TTCACAGACCTGACCTCC-3'; reverse, 5'-GAAGCCATCGGTGTCATGTG-3') was designed to amplify a 271-bp fragment from 1/20 of the reverse transcribed total RNA using the Taq PCR Master Mix (Qiagen). PCR was performed in 25 cycles (94 °C, 1 min for the initial denaturation, 94 °C, 30 s; 55 °C, 30 s; 72 °C, 30 s for each cycle and 72 °C, 10 min for the final elongation). Following the same protocol, a hypoxanthine-guanine-phosphoribosyltransferase fragment of 526 bp was amplified as control. PCR fragments were visualized with ethidium bromide after separation of equal volumes of each reaction on a 1.8% agarose gel.
Bacterial Expression of a RnIP3K-C Fragment Comprising the CaM Binding and the Catalytic Domain (CBD-RnIP3K-C) and a Fragment Comprising Only the Catalytic Domain (CBD-RnIP3K-C)A fragment of the protein coding region of RnIP3K-C (termed CBDRnIP3K-C) comprising CaM binding and catalytic domain (amino acids 366678) was amplified with the following primer pair (5'-GAGGATGACCATATGGCTGGGGGCGGAGGTACCAG-3'; 5'-TCCTCACTAGTGAGCCTAGCAGTGGCAGCTG-3') in a PCR (25 cycles) using 2.5 units of Pfu polymerase (Promega) in a 100-µl standard reaction mixture according to the manufacturer (annealing temperature of 65 °C and 4-min elongation at 72 °C/cycle). The PCR product was first cloned into the pGEM T-Easy vector (Promega). The cloned PCR product was cleaved out with NdeI and SpeI and re-ligated into pET17b vector (Novagen) previously cut with the same restriction enzymes. The recombinant fragment of RnIP3K-C was expressed in E. coli BL21(DE3)pRIL cells (Novagen) transformed with the resulting expression vector and purified by phosphocellulose and CaM affinity chromatography essentially as described previously for chicken IP3K (9).
A fragment of RnIP3K-C (termed CBD-RnIP3K-C) comprising only the catalytic domain (amino acids 410678) was amplified with the following primer pair (5'-AACAGCTATGACCATGATTACGCC-3';5'-ACGGCGCCTCCGGACATGCTGGGAACTTCC-3') in a PCR as described above. The PCR product cloned into the pGEM T-Easy vector was cleaved out with EheI and NsiI and re-ligated into pET17b vector (Novagen). The fragment of RnIP3K-C was expressed in E. coli BL21(DE3)pLysS,pREP cells (Novagen) transformed with the resulting expression vector and purified by phosphocellulose chromatography essentially as described previously for chicken IP3K (9). No addition of lysozyme to the cells was necessary, because of the lysozyme expression by the bacteria. The destruction of DNA by DNase treatment was replaced by a sonification step.
Enzymatic Analysis of CBD-RnIP3K-C and CBD-RnIP3K-CEnzymatic activities were measured under various substrate, product, activator, and inhibitor concentrations using a coupled enzymatic optical assay essentially as described previously (9). A Lambda 20 UV-visible spectrometer (PerkinElmer Life Sciences) equipped with thermostatted cuvette holder and numerical data storage and derivation device (program UV Winlab, PerkinElmer Life Sciences) was employed. The standard reaction conditions used in the assays were: 10 mM triethanolamine, pH 7.5, 5 mM MgCl2, 30 mM KCl, 1 mM dithiothreitol, 0.2 mM NADH, 1 mM phospho(enol) pyruvate, 0.5 mM ATP, 5 units/ml lactate dehydrogenase, 2.5 units/ml PK in the presence of 6 mM ammonium sulfate. All assays were performed at 30 °C after a 15-min preincubation of the reaction mixture. After following background NADH consumption in presence of PK, lactate dehydrogenase, and IP3K for 510 min, the IP3K assay was started with inositol phosphate substrate. All assays were performed in absence and presence of 0.1 µM Ca2+-CaM. In the latter case, 20 µM Ca2+ was present. The dependence of enzymatic activity on the concentration of Ins(1,4,5)P3 (bought from Alexis, Woburn, MA) was assayed by single transients. They were started at differing initial InsP3 concentrations (varying between 1 and 35 µM,) and followed in the optical assay until InsP3 was completely consumed. The Km for ATP was assayed by varying the initial ATP concentration at a fixed initial Ins(1,4,5)P3 concentration of 5 µM and measuring initial activities. Activation by Ca2+-CaM was also assayed by measuring initial activities at 5 µM initial Ins(1,4,5)P3; the concentration of Ca2+ was 20 µM, and that of CaM was varied. Activation or inhibition by Ins(1,3,4,5)P4 was assayed by determining the initial enzyme activity at 1 µM Ins(1,4,5)P3 and increasing concentrations of Ins(1,3,4,5)P4 present before starting the optical assays. In case of the enzyme form
CBD-RnIP3K-C, the true Km for Ins(1,4,5)P3 of this enzyme form was determined from the Km(app) values of a series of single transients started at different initial Ins(1,3,4,5)P4 concentrations by plotting these parameters against Ins(1,3,4,5)P4 concentration. The y intercept of the resulting linear regression curve corresponds to Km, the x intercept to Ki for Ins(1,3,4,5)P4. The reaction products generated by conversion of Ins(2,4,5)P3 (pure synthetic Ins(2,4,5)P3 was a gift from Barry V. L. Potter) and from assays of the activation of IP3K-C by Ins(1,3,4,5)P4 were analyzed by metal dye detection (MDD)HPLC (18, 19, 20). For that, aliquots from the optical assay mixture were trichloroacetic acid-precipitated and charcoal-treated to remove ATP and NAD(H) as described (18, 19, 20).
Construction of Fusion Genes and Fusion Gene DerivatesThe EGFP fusion genes N-tagfull and C-tagfull were created by PCR techniques. The open reading frame of RnIP3K-C was amplified using primer pairs (N-tagfull: 5'-GTCGACATGAGGCGCTGCCCGTGCCG-3' and 5'-GGATCCTAGCTCTGGGCCAGGCCCTGAA-3'; C-tagfull: 5'-AGCGCTATGAGGCGCTGCCCGTGCCG-3' and 5'-CTCGAGGCTCTGGGCCAGGCCCTGAA-3'). The PCR products were initially cloned into the pGEM T-Easy vector (Promega, Mannheim, Germany). The open reading frame was completely sequenced, and then the fragment was subcloned by the introduced restriction sites (SalI/BamHI for N-tagfull, Eco47III/XhoI for C-tagfull) into the expression vectors pEGFP-C1 and pEGFP-N1 (Clontech), respectively.
The fusion gene derivates (N-tagNterm and C-tagNterm) containing an N-terminal fragment (aa 1379) of RnIP3K-C were created by the same PCR techniques as the fusion genes using the following primer pairs (N-tagNterm: 5'-GTCGACATGAGGCGCTGCCCGTGCCG-3' and 5'-GGATCCTATCCAGATCTGTCCTCAGGATCGCT-3'; C-tagNterm: 5'-AGCGCTATGAGGCGCTGCCCGTGCCG-3' and 5'-CTCGAGTCCAGATCTGTCCTCAGGATCGCT-3'). The deletion mutant C-tagfull,NES (aa 318326 deleted) and the point mutation C-tagfull,mNES (L318CPV mutated to ACPA) were created by using modified QuikChange site-directed mutagenesis (21). The C-tagfull fusion gene was used as template. The following primer pairs were used for mutagenesis (C-tagfull,
NES: 5'-TCCCACCTGGAGTGCAGCTCCACCTCAGAGTCTCCTGAGCCT-3' and 5'-AGGCTCAGGAGACTCTGAGGTGGAGCTGCACTCCAGGTGGGA-3'; C-tagfull,mNES: 5'-TCCCACCTGGAGTGCAGCTCCGCGTGTCCTGCGCCCCGCCTTATCATCACCTCA-3' and 5'-TGAGGTGATGATAAGGCGGGGCGCAGGACACGCGGAGCTGCACTCCAGGTGGGA-3').
Cell Culture, Transient Gene Expression, and Fluorescence MicroscopyThe cell lines were purchased from Cell Lines Service (Heidelberg, Germany). NRK 52E cells were grown in Dulbecco's modified Eagle's medium containing 10% fetal calf serum (Invitrogen). PC12 cells were grown in RPMI medium with Glutamax I supplemented with 10% horse serum and 5% fetal calf serum (Invitrogen). Both cell lines were cultured at 37 °C in a humidified atmosphere in the presence of 5% CO2.
Cell transfections were performed using the Metafectene method according to the instructions from the manufacturer (Biontex Laboratories, Munich, Germany). Fixation, staining with 4',6-diamidino-2-phenylindole dihydrochloride, and examination of the intracellular localization by fluorescence microscopy were carried out as described (22). A minimum of 100 undamaged cells/experiment were examined. LMB was purchased from Sigma and added to the complete medium to a final concentration of 11 ng/ml (20 nM).
Statistics and Enzyme Kinetic Data EvaluationUnpaired t test was performed using GraphPad InStat version 3.05 (GraphPad Software, San Diego, CA). A value of p < 0.05 was considered statistically significant.
Evaluation of enzyme kinetic parameters was performed from primary UV Winlab data sets as follows. In each assay data set (NADH consumption versus time), a correction for the initially determined background ATPase activity (coming from the indicator enzymes employed), a smoothing of corrected NADH versus time data, and a final derivation of specific activity versus substrate concentration data (v versus S data) was performed by exporting the data sets into Excel and employing special macro routines. For all enzymatic parameter and goodness of fit estimates performed with the final v versus S data, non-linear least squares techniques were employed. For that, the standard non-linear fitting and initial parameter definition techniques implied in the enzyme kinetics program GraphPad PRISM version 2.1 were used. Special kinetic models employed were formulated as described by Segel (47), and corresponding novel functions were defined in the non-linear function fitting module.
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RESULTS |
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Tissue Distribution of RnIP3K-CNorthern blot analysis showed that RnIP3K-C is expressed in a tissue-specific fashion at relatively low abundance indicated by the fact that only poly(A)+ RNA blots produced reasonable strong signals after overnight exposure. A single transcript of 3.4 kb was present predominantly in heart, brain, and testis; at lower levels in lung, liver, and kidney; and almost undetectably in spleen and skeletal muscle (Fig. 2A). The size of the transcript is consistent with the length of the cloned cDNA. The expression pattern of RnIP3K-C mRNA in rat tissues differs remarkably from that found in human organs (10). In humans, skeletal muscle showed highest expression, whereas brain and kidney contained almost no specific mRNA. Specific rat tissues such as tongue and taste epithelium exhibited amounts of RnIP3K-C mRNA that were considerably higher than those found in, e.g., rat brain (Fig. 2B). In these epithelial tissues, the 3.4-kb transcript could be detected on Northern blots even when using total RNA. The relatively high abundance of RnIP3K-C mRNA in taste and tongue epithelium compared with other tissues was confirmed by semiquantitative RT-PCR with a set of cDNAs prepared separately (Fig. 2C). The RT-PCR data correlate exactly with the results of both Northern blot experiments.
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Enzymatic Activities and Calmodulin Regulation of Bacterially Expressed RnIP3K-C FragmentsMeasurements of the enzymatic activity of a recombinant RnIP3K-C fragment comprising the catalytic and calmodulin binding domains (CBDRnIP3K-C) revealed an unexpected behavior of this enzyme regarding its dependence on the substrate concentration. The enzymatic activity of the fragment is already very high at low Ins(1,4,5)P3 concentrations (<100 nM), displays a maximum at 0.3 µM, and decreases with higher substrate concentrations. This substrate inhibition effect is particularly evident in the absence of the activator Ca2+-CaM, whereas its presence leads to a higher relative activity at high substrate concentrations, i.e. a lower relative substrate inhibition. This has the consequence that the degree of Ca2+-CaM activation increases from
4-fold at 0.3 µM Ins(1,4,5)P3 to
8-fold at 5 µM Ins(1,4,5)P3 (Fig. 3, A and B). These data sets can be fitted almost equally well with a simplified model for general non-competitive substrate inhibition or a model for an ordered Bi-Bi reaction (for parameters and explanations, see subscripts of Fig. 3 (A and B), and Ref. 47). Only in the case without Ca2+-CaM does the ordered Bi-Bi reaction model seem to fit slightly better to the data points (Fig. 3A). As compared with the known Km values of IP3Ks ranging from 0.7 to 3.1 µM (see Ref. 29 and citations therein), depending on the source and isoform examined, very low Km values for Ins(1,4,5)P3 of
0.2 µM (0.15 or 0.27 µM, depending on the model employed for parameter derivation) in absence and
0.18 µM (almost independent on the model employed) in presence of Ca2+-CaM were derived. Apparent Vmax values of 2.53.0 units/mg in absence and 12 units/mg in presence of Ca2+-CaM (observed at
0.3 and 0.5 µM Ins(1,4,5)P3, respectively) could be derived from inspection of the v versus S curves in Fig. 3(A and B, respectively). These maximum values are lower than the "true" Vmax values derived for the reaction models (see parameters to Fig. 3 (A and B)) as a result of the substrate inhibition phenomenon. A KD value for the binding of and activation by Ca2+-CaM of 9.4 nM was determined at 5 µM Ins(1,4,5)P3 and 0.5 mM ATP by the Ca2+-CaM-induced enzyme activation data (Fig. 3C). The Km value for ATP, determined at 5 µM Ins(1,4,5)P3, was 33 µM in the absence and 52 µM in the presence of 0.1 µM Ca2+-CaM (data not shown).
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Unusual is the effect that the product Ins(1,3,4,5)P4 exerted on the enzyme in absence of Ca2+-CaM. Although both IP3K-A and IP3K-B show a marked competitive inhibition by Ins(1,3,4,5)P4 with respect of the substrate Ins(1,4,5)P3 both in absence and in presence of Ca2+-CaM (our own data for IP3K-B (not shown); Ref. 29 for IP3K-A), the rat C-isoform displays a distinct activation by Ins(1,3,4,5)P4 only in the absence of Ca2+-CaM. The degree of activation by Ins(1,3,4,5)P4 determined at 1 µM Ins(1,4,5)P3 is more than 2-fold, and an activation plateau is reached at 3 µM Ins(1,3,4,5)P4. An apparent KD for the binding of and activation by Ins(1,3,4,5)P4 of 0.52 µM was derived from the data by assuming simple hyperbolic binding kinetics (Fig. 4A). MDD-HPLC analysis showed that no InsP5 product was formed in presence of Ins(1,3,4,5)P4 (data not shown); therefore, the product activation observed is a true allosteric activation of the conversion of Ins(1,4,5)P3. This allosteric effect is apparently abolished by Ca2+-CaM binding (Fig. 4B), which per se activates the enzyme.
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To test the hypothesis that the product activation that is not observed in presence of Ca2+-CaM is really a regulatory property residing in the CaM binding domain of IP3K-C and is caused by binding of Ins(1,3,4,5)P4 to this domain in the absence of Ca2+-CaM, we constructed a bacterial expression vector termed CBD-RnIP3K-C, lacking the cDNA sequence coding for the CaM binding domain. This enzyme showed a more than 10-fold increased maximal specific activity in absence of Ca2+-CaM as compared with the enzyme containing the CaM binding domain (Vmax = 31 (± 1.5) units/mg versus Vmax(app) = 2.5 (± 0.8) units/mg) and exhibited (i) no more substrate inhibition and (ii) no more activation by Ins(1,3,4,5)P4, but instead a competitive inhibition by this product. This inhibition by Ins(1,3,4,5)P4 instead of an activation can directly be seen from the data in Fig. 4B, where again (see above) initial enzyme activities at 1 µM Ins(1,4,5)P3 were measured in presence of increasing initial concentrations of Ins(1,3,4,5)P4. Furthermore, this truncated enzyme exhibited absolutely normal Michaelis-Menten type substrate kinetics with respect to Ins(1,4,5)P3 with and without Ins(1,3,4,5)P4 present (complete substrate kinetic data not shown). Its true Km value for Ins(1,4,5)P3, derived from linear extrapolations of Km(app) values determined at differing inhibiting Ins(1,3,4,5)P4 concentrations (see "Experimental Procedures" and data not shown) was 11-fold increased to 2.2 (± 0.3) µM as compared with the enzyme containing the CaM binding domain (0.21 (± 0.06) µM; see Fig. 3A) but uncomplexed with Ca2+-CaM. This Km(app) versus Ins(1,3,4,5)P4 replot resulted in a negative x intercept equaling KI(InsP4) of 4.3 (± 0.6) µM, which compares well with the apparent KD value of 4.18 µM derived for inhibition at 1 µM Ins(1,4,5)P3 (Fig. 4B). Apparently the presence of the CaM binding domain is not only directly responsible for the observed product activation but also for the strong substrate inhibition of the Ca2+-CaM free enzyme and an increased substrate affinity (see above). The obvious isosteric interaction between substrate and product at the substrate/product binding site of the catalytic domain devoid of the CaM binding domain is characterized by markedly lower apparent binding affinities for InsP4 (KI(InsP4) = 4.3 µM; see also the inhibition at 1 µM InsP3; Fig. 4B) and for Ins(1,4,5)P3 (Km(InsP3) = 2.2 µM) than the ones derived from product activation (KD(app) = 0.52 µM; see Fig. 4A) and substrate inhibition (KI = 0.09 (± 0.01) µM; see Fig. 3A) in the enzyme containing the CaM binding domain. The abolishment of Ins(1,3,4,5)P4-dependent product activation as well as the relief of the strong substrate inhibition by CaM binding to the enzyme are thus most likely a consequence of a direct competitive interaction between Ca2+-CaM and Ins(1,3,4,5)P4 and/or Ins(1,4,5)P3 at the CaM binding domain. The latter domain of IP3K-C thus is likely to be itself an allosteric substrate and product binding domain with higher affinities for both ligands than the catalytic domain devoid of the CaM binding domain.
Unexpectedly, this isoform is also able to phosphorylate a second biological InsP3 isomer, namely Ins(2,4,5)P3, with relatively high efficiency. In assays where we used Ins(2,4,5)P3 instead of Ins(1,4,5)P3 as a substrate, we were able to detect a low but consistent enzyme activity of 50 milliunits/mg (Fig. 5A). The maximal activity was twice as high in the presence of Ca2+-CaM as in the absence, but also the Km value increased from 1.5 to
5 µM under these conditions (Fig. 5B). Although these maximal activities are
50 and 100-fold lower, respectively, than the corresponding Vmax(app) values for Ins(1,4,5)P3 (see above), they are in the range of activities reported for the conversion of Ins(1,3,4)P3 to Ins(1,3,4,6)P4 and Ins(1,3,4,5)P4 by purified Ins(1,3,4)P3 5/6-kinase (46). To identify the product of this reaction, MDD-HPLC analysis of the reaction mixture after 0, 1, and 2 h of incubation with the enzyme was performed. The comparison of the inositol phosphates present in these reaction mixtures against a mixture of standard inositol phosphate isomers revealed a predominating reaction product co-eluting with Ins(2,4,5,6)P4 (Fig. 6). Because both the substrate and the product appear to be >95% pure, this clearly indicates the phosphorylation of Ins(2,4,5)P3 exclusively to Ins(2,4,5,6)P4. This Ins(2,4,5)P3 6-kinase thus is an authentic side activity of RnIP3K-C. Small peaks of Ins(1,3,4,5)P4 and Ins(3,4,5,6)P4 also detected by MDD-HPLC analysis after incubation of Ins(2,4,5)P3 with RnIP3K-C can be explained (i) by a small contamination of Ins(2,4,5)P3 with Ins(1,4,5)P3 (being converted to Ins(1,3,4,5)P4), as well as (ii) by a further contamination of Ins(2,4,5)P3 with a small amount of Ins(3,4,5)P3, the latter one apparently also phosphorylated, like Ins(2,4,5)P3, at the 6-hydroxy group to Ins(3,4,5,6)P4.
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To find out whether the ability of IP3K-C to convert Ins(2,4,5)P3 to Ins(2,4,5,6)P4 is an exclusive property of this isoform of IP3K, we expressed catalytic forms of rat IP3K-B and avian IP3K-A, both also containing the catalytic and the adjacent CaM binding domain but no N-terminal domain (data not shown). In both cases we found the same type of specific phosphorylation of this substrate at the hydroxyl group 6. In IP3K-A, which is only expressed in neurons, testes, and avian red blood cells (9, 27), a high Vmax of 382 milliunits/mg and a Km of 3.9 µM were derived in absence of CaM and similar values also in presence of 0.1 µM Ca2+-CaM. The more ubiquitously expressed IP3K-B (28) revealed Vmax and Km values of 11.5 milliunits/mg and 1.3 µM, respectively, in absence of CaM and of 24.5 milliunits/mg and 4.6 µM, respectively, in presence of 0.1 µM Ca2+-CaM (kinetics not shown). Among the two more ubiquitously expressed IP3K isoforms, IP3K-B and IP3K-C, the latter exhibits 4.5-fold higher specific activity toward Ins(2,4,5)P3 in absence and in presence of Ca2+-CaM, whereas the Km values for Ins(2,4,5)P3 are not significantly different between these two isoforms. We also tested whether the initial presence of activator Ins(1,3,4,5)P4 together with Ins(2,4,5)P3 or its addition during the reaction could further increase the specific activity of IP3K-C for this alternative substrate but no significant effect, either activation or inhibition, was observed (data not shown). However, in the other two isoforms, A and B, Ins(1,3,4,5)P4 exhibited a strong competitive inhibition of conversion of Ins(2,4,5)P3. IP3K-C thus is the isoform better suited to perform this reaction in tissues and cell types expressing both isoform B and C because of its 5-fold higher Vmax against this substrate and the absence of competitive inhibition by Ins(1,3,4,5)P4. We determined in different tissues and cell types by direct mass analysis of inositol phosphates using MDD-HPLC whether Ins(2,4,5,6)P4 is resent and is increased after phospholipase C stimulation and could confirm both phenomena (data not shown).
We performed several optical assays to confirm the reversibility and the degree of Ca2+-CaM activation of the enzyme. One of these experiments is shown in Fig. 7. The reactions were started at high Ins(1,4,5)P3 in the presence of 20 µM free Ca2+, but in absence of CaM. After 12 min, when the enzyme exhibited basal substrate inhibited activity (see above), 0.1 µM CaM was added and the reaction followed. Thus, both the degree of CaM activation (7-fold in the example shown in Fig. 7) and the product activation and decrease of substrate inhibition could be directly demonstrated (see the activity derivations plotted together with the optically assayed decrease of NADH, which was converted into an equimolar decrease of Ins(1,4,5)P3 in Fig. 7). The addition of 125 µM EGTA apparently led to the dissociation of CaM and the observed re-decrease of activity. Finally, by addition of 100 µM Ca2+ a second (incomplete) CaM activation was induced and activity was followed until the substrate was completely consumed. After the second Ca2+ addition, the degree of Ca2+-CaM activation was incomplete as a result of only partial replacement of the complexed Ca2+.
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RnIP3K-C Tagged with EGFP Shuttles Actively between the Nucleus and CytoplasmThe intracellular localization of IP3K-C has been investigated by Dewaste et al. (10). In transfected COS-7 cells the enzyme seems to be localized predominantly in the cytoplasm, but no cellular targeting mechanisms are known so far. To further investigate the intracellular localization of RnIP3K-C, the coding region of its full-length cDNA was fused with an N-terminal EGFP tag (N-tagfull) and a C-terminal EGFP tag (C-tagfull), respectively. These fusion genes were transiently expressed in NRK 52E cells, and the localization of the fusion proteins was determined by inspection using fluorescence microscopy. (Typical images of the different types of localization are shown in Fig. 8A.) Both arrangements of protein sequences in the fusion protein were used to examine steric effects of the fused fluorescent protein influencing the localization of the fusion protein. In most of the transfected cells, the fusion proteins were detected evenly distributed between nucleus and cytoplasm or exclusively in the cytoplasm. In a small minority of cells, they were localized predominantly in the nucleus (Fig. 8B; N-tagfull, C-tagfull in Table I). This localization pattern was not an artifact of EGFP tagging, because the arrangement of protein sequences had no obvious impact on the localization pattern (N-tagfull versus C-tagfull in Table I). In control experiments where we expressed EGFP alone, all transfected NRK 52E cells showed the same localization, namely an even distribution between nucleus and cytoplasm (data not shown), ruling out a targeting activity of EGFP itself. Small proteins can enter the nucleus of a cell by passive diffusion, but proteins larger than 40 kDa require an NLS for active translocation through the nuclear pore complex (11). The molecular weight of the fusion protein (101.4 kDa) clearly exceeds this limit. This obviously active translocation of IP3K-C into the nucleus seems to be in contradiction to the exclusively cytoplasmic localization described by Dewaste et al. (10). Therefore, we examined the possibility of an additional nuclear export activity by incubation of NRK 52E cells expressing the fusion protein N-tagfull with the export inhibitor LMB. After 6 h of LMB treatment, no cells with an exclusively cytoplasmic localization of the fusion protein were observed (Fig. 8B; N-tagfull/no LMB versus N-tagfull/+LMB in Table I). The increase of cells showing an even distribution and the decrease of cells showing an exclusively cytoplasmic localization of the fusion protein are extremely significant (p < 0.001). Furthermore, the proportion of cells with a predominantly nuclear localization seems to be increased (0.05< p < 0.1). Therefore, the fusion protein is obviously transported out of the nucleus by an active, LMB-sensitive mechanism. Similar results were also obtained by additional transfection experiments using PC12 cells (data not shown). In summing up, the RnIP3K-C seems to be a nucleocytoplasmic shuttling protein with both nuclear import and nuclear export activity.
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The N-terminal Domain Determines the Nucleocytoplasmic Shuttling of RnIP3K-COur results (see above) indicate that RnIP3K-C is a nucleocytoplasmic shuttling protein, but the positions of potential sites possessing nuclear import and export activity, respectively, are unknown. The N-terminal domains of the three IP3K isoforms are highly diverse and mainly uncharacterized in their function(s), whereas the other parts of the enzymes are conserved and well studied (Fig. 1). Therefore, these domains are preferred candidates to determine the intracellular targeting of the different isoforms. Indeed, the IP3K isoform A is localized to F-actin and dendritic spines by its N terminus (24). To narrow down sites exhibiting nuclear import or export activity, we fused the N-terminal domain of RnIP3K-C (aa 1379) with an N-terminal EGFP tag (N-tagNterm) and a C-terminal EGFP tag (C-tagNterm), respectively. The localization pattern of both EGFP-tagged fragments is comparable with that of the full-length fusion proteins (N-tagNterm versus N-tagfull; C-tagNterm versus C-tagfull in Table I), as determined by fluorescence microscopy of transiently transfected NRK 52E cells. An effect of EGFP tagging can be ruled out, because the pattern is obviously not influenced by the arrangement of protein sequences in the fusion protein (N-tagNterm versus C-tagNterm in Table I). The nuclear entry of the EGFP-tagged fragments still requires an active import, because their molecular size (67.2 kDa) rules out a passive diffusion into the nucleus. Thus, the N-terminal domain of RnIP3K-C tagged with EGFP seems to possess a nuclear import activity comparable with that of the full-length fusion protein, although no classical NLS (25) was revealed by consensus sequence search (data not shown). To further examine potential targeting mechanisms, NRK 52E cells expressing an EGFP-tagged fragment (N-tagNterm) were incubated with LMB. After an LMB incubation of 6 h, no cells with an exclusively cytoplasmic localization were observed (N-tagNterm/no LMB versus N-tagNterm/+LMB in Table I). Both the decrease of the cell number with an exclusively cytoplasmic localization and the increase of cells with an even distribution of the fragment are extremely significant (p < 0.001), whereas the proportion of cells showing a predominantly nuclear localization seems to be increased (0.05 < p < 0.10). Therefore, the EGFP-tagged N-terminal domain of RnIP3K-C demonstrates an LMB-sensitive nuclear export activity comparable with that of the full length fusion protein. To further narrow down potential sites possessing nuclear export activity, we analyzed the sequence of the N-terminal domain of RnIP3K-C. One candidate sequence (aa 318326) precisely fits the NES consensus (Fig. 9) (11, 12, 26) and is completely conserved between human and rat protein (data not shown). This putative NES was deleted in an EGFP-tagged full-length fusion gene to further investigate its role in the intracellular targeting of IP3K-C. The NES deletion mutant was transiently expressed in NRK 52E cells, and its localization was examined by fluorescence microscopy. Now almost all cells showed an even distribution of the fusion protein between nucleus and cytoplasm comparable with LMB-treated cells expressing EGFP-tagged full-length proteins (C-tagfull,NES in Fig. 8B and Table I). The deletion of an internal sequence is a drastic operation, which can lead to misfolding and thus changes in the three-dimensional structure of a protein. These changes can mask an NES without its direct deletion. A less drastic strategy to eliminate a putative NES is the mutation of conserved hydrophobic amino acids (Fig. 9) to alanine residues. Almost all cells expressing such an NES point mutant (L318CPV mutated to ACPA) showed an even distribution of the fusion protein between nucleus and cytoplasm comparable with the deletion mutant (C-tagfull,
NES versus C-tagfull,mNES in Fig. 8B and Table I). Thus, the identified motif is necessary for the observed active exclusion of the EGFP-tagged RnIP3K-C from the nucleus. Additional experiments with PC12 cells showed comparable results (data not shown). In summary, both an NES and an uncharacterized nuclear import activity are localized in the N-terminal domain of RnIP3K-C and seem to determine the nucleocytoplasmic shuttling of the enzyme.
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IP3K-C Can Phosphorylate Ins(2,4,5)P3 at Its 6-Hydroxy GroupAnother specific feature of the C isoform is its ability to bind and convert the alternative substrate Ins(2,4,5)P3 to Ins(2,4,5,6)P4. A synergistic control of Ca2+ mobilization from intracellular stores in mouse lymphoma cells by Ins(2,4,5)P3 and Ins(1,3,4,5)P4 has been reported (33). This isomer is possibly produced by enzymatic conversion of a cyclic inositol phosphate intermediate (cIns(1:2,4,5)P3), being formed as a side product of phospholipase C-catalyzed hydrolysis of phosphatidylinositol 4,5-bisphosphate (44, 45). In addition to the removal of once formed Ins(2,4,5)P3, this activity of rat IP3K-C may constitute an alternative route in the metabolism of higher inositol phosphates. Until now there has been no success in isolation and characterization of any enzyme activity that would perform the conversion of Ins(2,4,5)P3 to an InsP4 isomer in animal cells. This novel InsP4 isomer generated by IP3K-C may be crucial for the formation of InsP6, as in rat brain the main activity producing InsP6 from an InsP5 precursor relies on the Ins(1,2,4,5,6)P5 isomer (34) and because the specific activity of the recently discovered human Ins(1,3,4,5,6)P5 2-kinase is quite low and its expression in some tissues is apparently poor (37), although in each of these tissues InsP6 is present. Because of the 7-fold lower apparent affinity of IP3K-C for Ins(2,4,5)P3 than for Ins(1,4,5)P3, this enzyme will first convert all Ins(1,4,5)P3 to Ins(1,3,4,5)P4 in a cell with low Ca2+, and then, uninfluenced by the presence of this product, it will efficiently convert Ins(2,4,5)P3 to Ins(2,4,5,6)P4. IP3K-A and IP3K-B will not be able to convert Ins(2,4,5)P3 as well although exhibiting activity toward this substrate (our result), because in these isoforms the strong competitive inhibition by Ins(1,3,4,5)P4 once formed will prevent binding of this alternative substrate. In a number of tissues and cell lines, the presence of Ins(2,4,5,6)P4 could be confirmed; in OKT-3-stimulated Jurkat cells, a delayed formation of Ins(2,4,5,6)P4 after T cell receptor stimulation could be demonstrated by sensitive micro-MDD-HPLC analysis (data not shown). In these cells there is always a significant amount of Ins(1,2,4,5,6)P5 and/or its enantiomer Ins(2,3,4,5,6)P5 present, making it likely that there is a metabolic interrelationship between Ins(2,4,5,6)P4 formed by IP3K-C and one or both of these InsP5 isomers known to be convertible to InsP6. The low concentrations of Ins(2,4,5,6)P4 detected in cells and tissues imply that such enzyme converting Ins(2,4,5,6)P4 to InsP5 should have a high affinity/low Km for Ins(2,4,5,6)P4.
IP3K-C Actively Shuttles between Cytoplasm and Nucleus as a Result of a Nuclear Import Activity and a Nuclear Export Activity in Its N-terminal DomainAs shown in this study, RnIP3K-C is not only localized in the cytoplasm, but can also be targeted to the nucleus of the cell. A phospholipase C-dependent inositol phosphate kinase pathway leading up to InsP6 in the nuclei of yeast cells has been described previously (35). Recent findings indicate that, in mammalian cells as well, inositol phosphate kinases in the nucleus act together with other enzymes of inositol phosphate metabolism (e.g. nuclear phospholipase C (Ref. 36)), forming a nuclear inositol phosphate signaling and phosphorylation pathway. The nuclear conversion of Ins(1,4,5)P3 to Ins(1,3,4,5)P4 can apparently be performed by inositol phosphate multikinase (22) and IP3K-C (our results), but no enzyme with nuclear localization additional to the inositol phosphate multikinase (22) is known to convert Ins(1,3,4,5)P4 to Ins(1,3,4,5,6)P5. This nuclear pathway may end up with highly phosphorylated inositol phosphates (e.g. InsP6 (Ref. 37)), part of which are pyrophosphorylated (38, 39) by nuclear inositol hexakisphosphate kinase isoform 2 (40). In this study, we have shown that a full-length RnIP3K-C fusion protein with EGFP readily enters the nuclei of mammalian cells and treatment with LMB or inactivation of a potential NES by deletion or point mutation promotes its nuclear accumulation. The size of the fusion protein rules out a nucleocytoplasmic shuttling by simple diffusion through the nuclear pore complexes (11). Therefore, active nucleocytoplasmic translocation mechanisms must exist. Our results suggest that the intracellular distribution of RnIP3K-C is regulated by two mechanisms, an uncharacterized active nuclear import mechanism additional to an LMB-sensitive nuclear export mechanism. The nuclear import could be either mediated by a non-classical NLS (41) or by a co-transport mechanism (42), because no classical NLS was identified. Intriguingly, both localization-determining activities are localized in the N-terminal domain of the protein (our results), and the association of IP3K-A with F-actin and dendritic spines is also mediated by a targeting domain in its N terminus (24). Therefore, it will be interesting to determine whether IP3K-B (28) is also targeted to specific intracellular regions (43) by its N-terminal domain. Furthermore, the targeting of IP3K-C may be cell type-dependent, because the intracellular localization observed in NRK 52E cells (our results) seems to differ from the distribution reported by Dewaste et al. using transfected COS-7 cells. In their study a nearly complete nuclear exclusion of the enzyme was revealed by activity determinations and Western blot analysis (10). Therefore, future experiments should focus on the identification of factors that influence the relative strength of nuclear import and nuclear export activities of IP3K-C in different cell types.
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FOOTNOTES |
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* This work was supported by Deutsche Forschungsgemeinschaft Grant MA 989/3-1 (to G. W. M. and U. B.), by the Graduiertenkolleg 336, and by the Fonds der Chemischen Industrie. This article is based on the doctoral thesis of M. M. N. in the Faculty of Chemistry at the University of Hamburg. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
¶ To whom correspondence should be addressed. Tel.: 49-40-42803-4639; Fax: 49-40-42803-6818; E-mail: mayr{at}uke.uni-hamburg.de.
1 Inositol phosphates are abbreviated according to the relaxed IUPAC nomenclature.
2 The abbreviations used are: IP3K, inositol 1,4,5-trisphosphate 3-kinase; EGFP, enhanced green fluorescent protein; CaM, calmodulin; NLS, nuclear localization signal; NES, nuclear export signal; MDD, metal dye detection; HsIP3K-C, human inositol 1,4,5-trisphosphate 3-kinase C; RnIP3K-C, rat inositol 1,4,5-trisphosphate 3-kinase C; LMB, leptomycin B; RACE, rapid amplification of cDNA ends; CBD, calmodulin binding domain; PK, pyruvate kinase; HPLC, high performance liquid chromatography; aa, amino acid(s); RT, reverse transcription.
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ACKNOWLEDGMENTS |
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