Deletion and alanine mutation analyses for the formation of active homo- or hetero-dimer complexes of mouse choline kinase-α and -β

Choline kinase (CK) is the first-step regulatory enzyme for the biosynthesis of phosphatidylcholine in all mammalian cells. It exists as at least three isoforms (alpha1, alpha2 and beta) that are encoded by two separate genes termed ck-alpha and ck-beta. The active enzyme has been proposed to consist of either their homo- or hetero-dimeric forms. Here, we report on the identification of several essential domains and amino acid residues involved in their active dimer formation. Full-length cDNAs or their truncated or alanine-mutated versions for mouse CK-alpha1 and CK-beta tagged with either HA or Myc at their N-termini were expressed in COS-7 cells. Each dimer formation was analyzed by immuno-precipitation followed by Western blotting. Kinetic analysis for CK reaction was performed with different expression products. Both the N-terminal domain-1 and C-terminal portions (E424-K430 for CK-alpha1 and Q379-K385 for CK-beta) were shown to be critical for the formation of active homo- or hetero-dimer complex. Interestingly, D320 in the CK-motif of CK-alpha1 was found to be essential for alpha1/alpha1 homo-dimerization but not for alpha1/beta hetero-dimerization. A mutation of the corresponding D276 of CK-beta to A276 did not show any effect on either its homo- or hetero-dimerization but it caused a strong inhibition of CK activity in either case.

Choline Kinase Is a Novel Oncogene that Potentiates RhoA-Induced Carcinogenesis

Choline kinase is overexpressed in human breast, lung, colorectal, and prostate tumors, a finding that suggests the involvement of this enzyme in carcinogenesis. Here we show that overexpression of choline kinase induce oncogenic transformation of human embryo kidney fibroblasts and canine epithelial Madin-Darby canine kidney cells. Choline kinase lays downstream of RhoA signaling and is activated through ROCK kinase, one of the best-characterized RhoA effectors. In keeping with this, coexpression of RhoA and choline kinase potentiates both anchorage independent growth and tumorigenesis. Finally, choline kinase-mediated transformation is sensitive to MN58b, a well-characterized specific choline kinase inhibitor. These results provide the definitive evidence that choline kinase has oncogenic properties and that choline kinase inhibition constitutes a novel valid antitumor strategy.

Regulatory enzymes of phosphatidylcholine biosynthesis: a personal perspective

Phosphatidylcholine is a prominent constituent of eukaryotic and some prokaryotic membranes. This Perspective focuses on the two enzymes that regulate its biosynthesis, choline kinase and CTP:phosphocholine cytidylyltransferase. These enzymes are discussed with respect to their molecular properties, isoforms, enzymatic activities, and structures, and the possible molecular mechanisms by which they participate in regulation of phosphatidylcholine levels in the cell.

Choline Kinase Activation Is a Critical Requirement for the Proliferation of Primary Human Mammary Epithelial Cells and Breast Tumor Progression

Breast cancer is still one of the most important tumors among women in industrialized countries. Improvement in both understanding the molecular events associated with the disease and the development of new additional treatments is still an important goal to be achieved. Choline kinase (ChoK) is increased in human mammary tumors with high incidence, and this activation is associated with clinical variable indicators of greater malignancy. Here, we have investigated the role of ChoK in the development of breast cancer and found that ChoK is both necessary and sufficient for growth factor-induced proliferation in primary human mammary epithelial cells and an absolute requirement for the specific mitogenic response to heregulin in breast tumor-derived cells. These results demonstrate that ChoK plays an essential role in both normal human mammary epithelial cell proliferation and breast tumor progression. Furthermore, inhibition of ChoK shows a strong in vivo antitumor activity against human breast cancer xenografts. Thus, ChoK constitutes a novel bona fide molecular target for the treatment of breast cancer patients.

Structure and function of choline kinase isoforms in mammalian cells

Choline kinase (CK) catalyzes the first phosphorylation reaction in the CDP-choline pathway for the biosynthesis of phosphatidylcholine (PC), yielding phosphocholine (P-Cho) from choline and ATP in the presence of Mg(2+). This enzyme exists in mammalian cells as at least three isoforms that are encoded by two separate genes termed ck-alpha and ck-beta. Each isoform is not active in its monomeric form. The active enzyme consists of either their homo- or hetero-dimeric (or oligomeric) forms. In recent years, the roles of CK in cell growth and cell stress/defense mechanisms have been intensely investigated. These functions of CK do not seem to be directly related to the net PC biosynthesis but predict another important role of this enzyme in certain cell physiology. This review summarizes briefly the recent progress of mammalian CK study which will include the gene structure of each isoform and its possible transcriptional regulation, the active configuration of the enzyme, induction of the particular isoform in chemically induced cell stress, and the possible role of this enzyme as well as of its reaction product, P-Cho, in cell growth and other cellular physiology.

A CDP-choline pathway for phosphatidylcholine biosynthesis in Treponema denticola

The genomes of Treponema denticola and Treponema pallidum contain a gene, licCA, which is predicted to encode a fusion protein containing choline kinase and CTP:phosphocholine cytidylyltransferase activities. Because both organisms have been reported to contain phosphatidylcholine, this raises the possibility that they use a CDP-choline pathway for the biosynthesis of phosphatidylcholine. This report shows that phosphatidylcholine is a major phospholipid in T. denticola, accounting for 35-40% of total phospholipid. This organism readily incorporated [14C]choline into phosphatidylcholine, indicating the presence of a choline-dependent biosynthetic pathway. The licCA gene was cloned, and recombinant LicCA had choline kinase and CTP:phosphocholine cytidylyltransferase activity. The licCA gene was disrupted in T. denticola by erythromycin cassette mutagenesis, resulting in a viable mutant. This disruption completely blocked incorporation of either [14C]choline or 32Pi into phosphatidylcholine. The rate of production of another phospholipid in T. denticola, phosphatidylethanolamine, was elevated considerably in the licCA mutant, suggesting that the elevated level of this lipid compensated for the loss of phosphatidylcholine in the membranes. Thus it appears that T. denticola does contain a licCA-dependent CDP-choline pathway for phosphatidylcholine biosynthesis.

Multiple isoforms of choline kinase from Caenorhabditis elegans: cloning, expression, purification, and characterization

Choline kinase is the first enzymatic step in the CDP-choline pathway for phosphatidylcholine biosynthesis. The genome of the nematode, Caenorhabditis elegans, contains seven genes that appear likely to encode choline and/or ethanolamine kinases. We cloned five and expressed four of these genes, and purified or partially purified three of the encoded enzymes. All expressed proteins had choline kinase activity; those that most closely resemble the mammalian choline kinases were the most active. CKA-2, a very active form, was purified to near homogeneity. The K(m) values for CKA-2 were 1.6 and 2.4 mM for choline and ATP, respectively, and k(cat) was 74 s(-1). CKA-2 was predominantly a homodimer as assessed by glycerol gradient sedimentation and dynamic light scattering. CKB-2, which was less similar to mammalian choline kinases, had K(m) values for choline and ATP of 13 and 0.7 mM, and k(cat) was 3.8 s(-1). Both of these highly purified enzymes required magnesium, had very alkaline pH optima, and were much more active with choline as substrate than with ethanolamine. These results provide a foundation for future studies on the structure and function of choline kinases, as well as studies on the genetic analysis of the function of the multiple isoforms in this organism.

The gene encoding muscle-type carnitine palmitoyltransferase I: comparison of the 5'-upstream region of human and rodent genes

Muscle-type carnitine palmitoyltransferase I (M-CPTI) is the key enzyme for fatty acid beta-oxidation in heart and skeletal muscles and in adipose tissue. So far, M-CPTI mRNA has been detected in white adipocytes from epididymal fat pads of rats and humans, but not in mouse adipocytes. To characterize the gene expression of M-CPTI in mice, we isolated the genomic DNA encoding mouse M-CPTI and determined its transcription initiation site. As a result, the mouse M-CPTI gene seemed to have multiple initiation sites, as in the case of the rat and human genes. Furthermore, the conserved nucleotide sequence of the response element for peroxisome proliferators was shown to exist in the upstream of the mouse gene as in that of the rat and human genes. From these observations, we suggest that the anomalous expression of M-CPTI in mouse adipocytes reported previously may be regulated by factors other than peroxisome proliferators. Previously, we reported that there were transcripts containing regions of both CK/EK-beta and M-CPTI genes in humans. In this study, we found that such transcripts also exist in rodents and that the amounts of the transcripts containing regions of both of these genes did not depend on the expression level of CK/EK-beta.

Bacteriophage gene targeting vectors generated by transplacement

A rate-determining step in gene targeting is the generation of the targeting vector. We have developed bacteriophage gene targeting vectorology, which shortens the timeline of targeting vector construction. Using retro-recombination screening, we can rapidly isolate targeting vectors from an embryonic stem cell genomic library via integrative and excisive recombination. We have demonstrated that recombination can be used to introduce specific point mutations or unique restriction sites into gene targeting vectors via transplacement. Using the choline/ethanolamine kinase alpha and beta genes as models, we demonstrate that transplacement can also be used to introduce specifically a neo resistance cassette into a gene targeting phage. In our experience, the lambdaTK gene targeting system offers considerable flexibility and efficiency in TV construction, which makes generating multiple vectors in one week's time possible.

Phosphorylation of Saccharomyces cerevisiae choline kinase on Ser30 and Ser85 by protein kinase A regulates phosphatidylcholine synthesis by the CDP-choline pathway

The Saccharomyces cerevisiae CKI-encoded choline kinase is phosphorylated on a serine residue and stimulated by protein kinase A. We examined the hypothesis that amino acids Ser(30) and Ser(85) contained in a protein kinase A sequence motif in choline kinase are target sites for protein kinase A. The synthetic peptides SQRRHSLTRQ (V(max)/K(m) = 10.8 microm(-1) nmol min(-1) mg(-1)) and GPRRASATDV (V(max)/K(m) = 0.15 microm(-1) nmol min(-1) mg(-1)) containing the protein kinase A motif for Ser(30) and Ser(85), respectively, within the choline kinase protein were substrates for protein kinase A. Choline kinase with Ser(30) to Ala (S30A) and Ser(85) to Ala (S85A) mutations were constructed alone and in combination by site-directed mutagenesis and expressed in a cki1Delta eki1Delta double mutant that lacks choline kinase activity. The mutant enzymes were expressed normally, but the specific activity of choline kinase in cells expressing the S30A, S85A, and S30A,S85A mutant enzymes was reduced by 44, 8, and 60%, respectively, when compared with the control. In vivo labeling experiments showed that the extent of phosphorylation of the S30A, S85A, and S30A,S85A mutant enzymes was reduced by 70, 17, and 83%, respectively. Phosphorylation of the S30A, S85A, and S30A,S85A mutant enzymes by protein kinase A in vitro was reduced by 60, 7, and 96%, respectively, and peptide mapping analysis of the mutant enzymes confirmed the phosphorylation sites in the enzyme. The incorporation of (3)H-labeled choline into phosphocholine and phosphatidylcholine in cells bearing the S30A, S85A, and S30A,S85A mutant enzymes was reduced by 56, 27, and 81%, respectively, and by 58, 33, and 84%, respectively, when compared with control cells. These data supported the conclusion that phosphorylation of choline kinase on Ser(30) and Ser(85) by protein kinase A regulates PC synthesis by the CDP-choline pathway.

Regulation of choline kinase activity by Ras proteins involves Ral-GDS and PI3K

Ras proteins are molecular switches that control signaling pathways critical in the onset of a variety of human cancers. The signaling pathways activated by Ras proteins are those controlled by its direct effectors such as the serine-threonine protein kinase Raf-1, the exchange factor for other GTPases Ral-GDS, and the lipid kinase PI3K. As a consequence of Ras activation, a number of additional enzymes are affected, including several members of the serine-threonine intracellular proteins kinases as well as enzymes related to phospholipid metabolism regulation such as phospholipases A2 and D, and choline kinase. The precise mechanisms by which ras oncogenes impinge into these later molecules and their relevance to the onset of the carcinogenic process is still not fully understood. Here we have investigated the mechanism of regulation of choline kinase by Ras proteins and found no direct link between PLD and choline kinase activation. We provide evidence that Ras proteins regulate the activity of choline kinase through its direct effectors Ral-GDS and PI3K, while the Raf pathways seems to be not relevant in this process. The importance of Ras-dependent activation of choline kinase is discussed.

Pea choline kinase: purification, properties and isolation of a cDNA

Choline kinase has been partially purified from pea seedlings and its properties studied. Using sequence information from soya bean and other choline kinases, we have also isolated a cDNA encoding the enzyme. It encodes a protein of 343 amino acids (calculated molecular mass of 39785 Da), which shows 82% homology with the soya bean choline kinase. The protein has been expressed in Escherichia coli with very good activity and high expression levels.

Novel expression of equivocal messages containing both regions of choline/ethanolamine kinase and muscle type carnitine palmitoyltransferase I

For characterization of the detailed gene structure of human muscle type carnitine palmitoyltransferase I (M-CPTI), we analyzed the 5'-upstream region of the M-CPTI transcripts. As a result, we found a cDNA clone containing a nucleotide sequence unexpected from the reported M-CPTI gene structure in the upstream region of its 5' end. Comparison of this nucleotide sequence with that of genomic DNA showed that this sequence was derived from the 3'-untranslated region of the gene encoding choline/ethanolamine kinase-beta (CK/EK-beta) located upstream of the M-CPTI gene. Southern blot analysis showed that there was no other region homologous to the CK/EK-beta gene in the whole human genome. Thus, the overlapping transcript was concluded to be produced from the functional genes of CK/EK-beta and M-CPTI. Furthermore, cDNAs containing both exons of these genes were detected by the polymerase chain reaction using the cDNA of human heart M-CPTI obtained by specific reverse transcription from its 3'-untranslated region as a template. From these results, the production and organization of these overlapping transcripts are discussed.

Expression of human choline kinase in NIH 3T3 fibroblasts increases the mitogenic potential of insulin and insulin-like growth factor I

In mammalian cells, growth factors, oncogenes, and carcinogens stimulate phosphocholine (PCho) synthesis by choline kinase (CK), suggesting that PCho may regulate cell growth. To validate the role of PCho in mitogenesis, we determined the effects of insulin, insulin-like growth factor I (IGF-I), and other growth factors on DNA synthesis in NIH 3T3 fibroblast sublines highly expressing human choline kinase (CK) without increasing phosphatidylcholine synthesis. In serum-starved CK expressor cells, insulin and IGF-I stimulated DNA synthesis, p70 S6 kinase (p70 S6K) activity, phosphatidylinositol 3-kinase (PI3K) activity, and activating phosphorylation of p42/p44 mitogen-activated protein kinases (MAPK) to greater extents than in the corresponding vector control cells. Furthermore, the CK inhibitor hemicholinium-3 (HC-3) inhibited insulin- and IGF-I-induced DNA synthesis in the CK overexpressors, but not in the vector control cells. The results indicate that high cellular levels of PCho potentiate insulin- and IGF-I-induced DNA synthesis by MAPK- and p70 S6K-regulated mechanisms.

Structure and characterization of the genes for murine choline/ethanolamine kinase isozymes alpha and beta

Choline/ethanolamine kinase (CK/EK) is the first enzyme in phosphatidylcholine/phosphatidylethanolamine biosynthesis in all animal cells. The highly purified CKs from mammalian sources and their recombinant gene products so far were all shown to have EK activity also, indicating that both activities reside on the same protein. CK/EK in most animal cells exists as several isoforms, for two of which (alpha and beta) their cDNAs have been cloned from both the rat and mouse, and they are found to be separate gene products. The physiological significance for the existence of more than one CK/EK enzyme, however, remains to be clarified. In this study, we isolated mouse genes encoding both types of CK/EK isozyme and determined their entire structure. The 5'-flanking promoter regions were found to have quite different features from each other, indicating that their expression could be under distinct control. Comparison of the nucleotide sequence between the corresponding coding exons showed the best homology (75%) residing on exon VIII. A search of the database resulted in the possible existence of 17 different origins of eukaryotic CK and/or EK, each of which presumably contained the entire amino acid sequence. Multialignment of their putative amino acid sequences led to an identification of the novel consensus sequence possibly required for the expression of either CK or EK activity, which corresponded to the sequence within exons VII and VIII of CK/EK-alpha and -beta genes from the mouse. This sequence was localized in close proximity to the C-terminal region of the general (Brenner's) phosphotransferase concensus sequence which was also completely conserved in all of the putative eukaryotic CK/EK proteins. The results demonstrated that, while both CK/EK-alpha and -beta genes were composed of 11 major exons, the size of their genes was quite different: 40 kb for CK/EK-alpha, whereas it was only 3.5 kb for CK/EK-beta.

Increased choline kinase activity in 1,2-dimethylhydrazine-induced rat colon cancer

Cancer cells acquire particular characteristics that benefit their proliferation. We previously reported that human colon cancers examined had increased choline kinase activity and phosphocholine levels. The elevated phosphocholine levels were in part due to both activation of choline kinase and increased choline kinase alpha protein levels. In this report, we analyzed choline kinase, which catalyzes the phosphorylation of choline to produce phosphocholine, in rat 1,2-dimethylhydrazine (DMH)-induced colon cancer. This study is the first to demonstrate increased choline kinase alpha enzymatic activity, protein levels, and mRNA levels in DMH-induced colon cancer as well as human colon cancer, although phosphocholine was not increased in DMH-induced rat cancer. The increase in the mRNA level was partly due to an increase in the transcription of the choline kinase alpha gene. The increased choline kinase activity may be a specific characteristic acquired by cancer cells that benefits their proliferation.

Molecular cloning of mouse choline kinase and choline/ethanolamine kinase: their sequence comparison to the respective rat homologs

Complementary DNAs homologous to a rat 42-kDa choline/ethanolamine kinase [C. Aoyama et al., Biochim. Biophys. Acta 1390 (1998) 1-7] and to a 50-kDa choline kinase [T. Uchida and S. Yamashita, J. Biol. Chem. 267 (1992) 10156-10162] were isolated from a 17-day post coitum mouse embryo cDNA library and their sequences were compared with the two murine species, respectively. The nucleotide sequence homology (within the coding sequence) between mouse and rat 50-kDa choline kinases (96.0%) was considerably higher than that between their 42-kDa choline/ethanolamine kinases (92.4%). Northern blot and RT-PCR studies on several rat tissues demonstrated that both isozymes are expressed ubiquitously with the highest level in testis.

Expression, purification, and characterization of choline kinase, product of the CKI gene from Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, choline kinase (ATP:choline phosphotransferase, EC 2.7.1.32) is the product of the CKI gene. Choline kinase catalyzes the committed step in the synthesis of phosphatidylcholine by the CDP-choline pathway. The yeast enzyme was overexpressed 106-fold in Sf-9 insect cells and purified 71.2-fold to homogeneity from the cytosolic fraction by chromatography with concanavalin A, Affi-Gel Blue, and Mono Q. The N-terminal amino acid sequence of purified choline kinase matched perfectly with the deduced sequence of the CKI gene. The minimum subunit molecular mass (73 kDa) of purified choline kinase was in good agreement with the predicted size (66.3 kDa) of the CKI gene product. Native choline kinase existed in oligomeric structures of dimers, tetramers, and octomers. The amounts of the tetrameric and octomeric forms increased in the presence of the substrate ATP. Antibodies were raised against the purified enzyme and were used to identify choline kinase in insect cells and in S. cerevisiae. Maximum choline kinase activity was dependent on Mg2+ ions (10 mM) at pH 9.5 and at 30 degrees C. The equilibrium constant (0.2) for the reaction indicated that the reverse reaction was favored in vitro. The activation energy for the reaction was 6.26 kcal/mol, and the enzyme was labile above 30 degrees C. Choline kinase exhibited saturation kinetics with respect to choline and positive cooperative kinetics with respect to ATP (n = 1.4-2.3). Results of the kinetic experiments indicated that the enzyme catalyzes a sequential Bi Bi reaction. The Vmax for the reaction was 138.7 micromol/min/mg, and the Km values for choline and ATP were 0.27 mM and 90 microM, respectively. The turnover number per choline kinase subunit was 153 s-1. Ethanolamine was a poor substrate for the purified choline kinase, and it was also poor inhibitor of choline kinase activity. ADP inhibited choline kinase activity (IC50 = 0.32 mM) in a positive cooperative manner (n = 1.5), and the mechanism of inhibition with respect to ATP and choline was complex. The regulation of choline kinase activity by ATP and ADP may be physiologically relevant.

Complementary DNA sequence for a 42 kDa rat kidney choline/ethanolamine kinase

By means of peptide sequence information, several cDNA clones encoding a 42 kDa choline/ethanolamine kinase were isolated from a rat kidney cDNA library. Eight clones were sequenced with all of them resulting in identical overlapping nucleotide sequences. Four of them possessed entire open reading frame which could encode 394 amino acids with a calculated molecular size of 45 100. The predicted amino acid sequence contained all of the internal peptide fragment sequences derived from the purified 42 kDa enzyme. When the open reading frame was introduced into pGEX-2T vector and transfected into E. coli cells, a significant choline/ethanolamine kinase activity did appear in the cell lysate. A homology comparison with the previously reported choline kinase cDNAs (CKR1 and CKR2) from rat liver showed 66%-68% in entire nucleotide sequences and 57%-59% in amino acid sequences, indicating that the cloned cDNA here must be a novel CK/EK gene product. (c) 1998 Elsevier Science B. V.

Choline kinase from yeast

Choline kinase, the initial enzyme of the CDP-choline pathway, mediates the conversion of choline to phosphorylcholine and is localized in the supernatant fraction of cells. The enzyme also catalyzes the phosphorylation of ethanolamine, functioning as the initial enzyme of the CDP-ethanolamine pathway as well. Yeast choline kinase is encoded by a single structural gene, CKI, which was cloned by the genetic complementation of the choline kinase mutation cki. The deduced sequence comprises 582 amino acid residues with a molecular mass of 66316 Da and bears local sequence similarity to various protein kinases and bacterial antibiotic phosphotransferases. The expression of yeast choline kinase is transcriptionally repressed by myo-inositol and choline in a coordinate manner with other phospholipid-synthesizing enzymes in yeast.

Construction of a plasmid carrying both CTP synthetase and a fused gene formed from cholinephosphate cytidylyltransferase and choline kinase genes and its application to industrial CDP-choline production: enzymatic production of CDP-choline from orotic acid (Part II)

A new method for enzymatic production of cytidine diphosphate choline (CDP-choline) from orotic acid and choline chloride was developed. To establish an industrial manufacturing process, we constructed a plasmid, pCKG55, which simultaneously expressed in Escherichia coli the three following enzymes; CTP synthetase (encoded by the pyrG gene from E. coli), cholinephosphate cytidylyltransferase (encoded by the CCT gene from Saccharomyces cerevisiae), and choline kinase (encoded by the CKI gene from S. cerevisiae). CCT and CKI genes on pCKG55 were designed to be expressed as a single CCT/CKI fused protein. This CCT/CKI fused protein retained both activities and the thermal stability of its cholinephosphate cytidylyltransferase activity was nearly the same as the native CCT enzyme. Corynebacterium ammoniagenes KY13505 and E. coli MM294/pCKG55 were cultured in 5-liter jar fermentor independently. Equal volumes of each broth were mixed in a 2-liter jar fermentor, and then the enzymatic reaction was done using 47 mM orotic acid and 60 mM choline chloride as substrates. After 23 h of the reaction at 32 degrees C, 21.5 mM (11 g/liter) of CDP-choline was accumulated.

Incorporation of choline into Streptococcus pneumoniae cell wall antigens: evidence for choline kinase activity

The choline-containing teichoic and lipoteichoic acids play an important part in cell wall metabolism of Streptococcus pneumoniae. We propose that a choline kinase enzyme has a role in the synthesis of these antigens. The presence of this enzyme was demonstrated in cell free extracts of S. pneumoniae by measuring the fall in ATP concentration due to phosphorylation of choline. Genomic DNA of S. pneumoniae hybridised with a probe consisting of an internal fragment of the choline kinase gene of Saccharomyces cerevisiae and one consisting of the choline binding domain of lytA.

Characterization of soybean choline kinase cDNAs and their expression in yeast and Escherichia coli

An expressed sequence tag from Arabidopsis that displayed sequence homology to mammalian and yeast choline kinases was used to isolate choline kinase-like cDNAs from soybean (Glycine max L.). Two distinct cDNAs, designated GmCK1 and GmCK2, were recovered that possessed full-length reading frames, each sharing approximately 32% identity at the predicted amino acid level with the rat choline kinase. A third unique choline kinase-like cDNA, GmCK3, was also identified but was not full length. Heterologous expression of GmCK1 in yeast (Saccharomyces cerevisiae) and GmCK2 in both yeast and Escherichia coli demonstrated that each encodes choline kinase activity. In addition to choline, other potential substrates for the choline kinase enzyme include ethanolamine, monomethylethanolamine (MME), and dimethylethanolamine (DME). Both soybean choline kinase isoforms demonstrated negligible ethanolamine kinase activity. Competitive inhibition assays, however, revealed very distinct differences in their responses to DME and MME. DME effectively inhibited only the GmCK2-encoded choline kinase activity. Although MME failed to effectively inhibit either reaction, an unexpected enhancement of choline kinase activity was observed specifically with the GmCK1-encoded enzyme. These results show that choline kinase is encoded by a small, multigene family in soybean comprising two or more distinct isoforms that exhibit both similarities and differences with regard to substrate specificity.

Regulation of choline kinase R: analyses of alternatively spliced choline kinases and the promoter region

The previous report described the cloning kinase R1 cDNA [Uchida, T. and Yamashita, S. (1992) J. Biol. Chem. 267, 10156-10162]. A new cDNA, for choline kinase R2, was isolated from rat liver cDNA libraries. This transcript was thought to be generated by alternative splicing. Ribonuclease protection analysis revealed the presence of a third choline kinase. Three transcripts were detected in all of the examined rat tissues at different levels. Southern blot analysis demonstrated a single copy of choline kinase R gene. The genomic DNA containing the first exon and its flanking regions of choline kinase R gene was isolated and characterized. A number of transcription start sites, determined by ribonuclease protection and primer extension analyses, were found. The most 3' site, 193 base pairs upstream of the initiation codon and common in liver and testis, was the main site in liver. Some transcription start sites were detected only in testis. Choline kinase R gene showed features not only of a typical housekeeping gene but also of a gene regulated through a variety of putative cis-acting motifs. 3-Methylcholanthrene and carbon tetrachloride increased all of three transcripts to various levels, and enhanced transcription from the same start site, which scarcely gave a detectable product in normal liver.