Regulation of CTP:phosphocholine cytidylyltransferase activity and subcellular location by phosphorylation in Chinese hamster ovary cells. The effect of phospholipase C treatment

Membrane Lipids Assist Catalysis by CTP: Phosphocholine Cytidylyltransferase

While most of the articles in this issue review the workings of integral membrane enzymes, in this review, we describe the catalytic mechanism of an enzyme that contains a soluble catalytic domain but appears to catalyze its reaction on the membrane surface, anchored and assisted by a separate regulatory amphipathic helical domain and inter-domain linker. Membrane partitioning of CTP: phosphocholine cytidylyltransferase (CCT), a key regulatory enzyme of phosphatidylcholine metabolism, is regulated chiefly by changes in membrane phospholipid composition, and boosts the enzyme's catalytic efficiency >200-fold. Catalytic enhancement by membrane binding involves the displacement of an auto-inhibitory helix from the active site entrance-way and promotion of a new conformational ensemble for the inter-domain, allosteric linker that has an active role in the catalytic cycle. We describe the evidence for close contact between membrane lipid, a compact allosteric linker, and the CCT active site, and discuss potential ways that this interaction enhances catalysis.

Differential dephosphorylation of CTP:phosphocholine cytidylyltransferase upon translocation to nuclear membranes and lipid droplets

CTP:phosphocholine cytidylyltransferase-alpha (CCTα) and CCTβ catalyze the rate-limiting step in phosphatidylcholine (PC) biosynthesis. CCTα is activated by association of its α-helical M-domain with nuclear membranes, which is negatively regulated by phosphorylation of the adjacent P-domain. To understand how phosphorylation regulates CCT activity, we developed phosphosite-specific antibodies for pS319 and pY359+pS362 at the N- and C-termini of the P-domain, respectively. Oleate treatment of cultured cells triggered CCTα translocation to the nuclear envelope (NE) and nuclear lipid droplets (nLDs) and rapid dephosphorylation of pS319. Removal of oleate led to dissociation of CCTα from the NE and increased phosphorylation of S319. Choline depletion of cells also caused CCTα translocation to the NE and S319 dephosphorylation. In contrast, Y359 and S362 were constitutively phosphorylated during oleate addition and removal, and CCTα-pY359+pS362 translocated to the NE and nLDs of oleate-treated cells. Mutagenesis revealed that phosphorylation of S319 is regulated independently of Y359+S362, and that CCTα-S315D+S319D was defective in localization to the NE. We conclude that the P-domain undergoes negative charge polarization due to dephosphorylation of S319 and possibly other proline-directed sites and retention of Y359 and S362 phosphorylation, and that dephosphorylation of S319 and S315 is involved in CCTα recruitment to nuclear membranes.

Lipids and Membrane Microdomains: The Glycerolipid and Alkylphosphocholine Class of Cancer Chemotherapeutic Drugs

Synthetic antitumor lipids are metabolically stable lysophosphatidylcholine derivatives, encompassing a class of non-mutagenic drugs that selectively target cancerous cells. In this chapter we review the literature as relates to the clinical efficacy of these antitumor lipid drugs and how our understanding of their mode of action has evolved alongside key advances in our knowledge of membrane structure, organization, and function. First, the history of the development of this class of drugs is described, providing a summary of clinical outcomes of key members including edelfosine, miltefosine, perifosine, erufosine, and erucylphosphocholine. A detailed description of the biophysical properties of these drugs and specific drug-lipid interactions which may contribute to the selectivity of the antitumor lipids for cancer cells follows. An updated model on the mode of action of these lipid drugs as membrane disorganizing agents is presented. Membrane domain organization as opposed to targeting specific proteins on membranes is discussed. By altering membranes, these antitumor lipids inhibit many survival pathways while activating pro-apoptotic signals leading to cell demise.

Membrane lipid compositional sensing by the inducible amphipathic helix of CCT

The amphipathic helical (AH) membrane binding motif is recognized as a major device for lipid compositional sensing. We explore the function and mechanism of sensing by the lipid biosynthetic enzyme, CTP:phosphocholine cytidylyltransferase (CCT). As the regulatory enzyme in phosphatidylcholine (PC) synthesis, CCT contributes to membrane PC homeostasis. CCT directly binds and inserts into the surface of bilayers that are deficient in PC and therefore enriched in lipids that enhance surface charge and/or create lipid packing voids. These two membrane physical properties induce the folding of the CCT M domain into a ≥60 residue AH. Membrane binding activates catalysis by a mechanism that has been partially deciphered. We review the evidence for CCT compositional sensing, and the membrane and protein determinants for lipid selective membrane-interactions. We consider the factors that promote the binding of CCT isoforms to the membranes of the ER, nuclear envelope, or lipid droplets, but exclude CCT from other organelles and the plasma membrane. The CCT sensing mechanism is compared with several other proteins that use an AH motif for membrane compositional sensing. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

CTP:phosphocholine cytidylyltransferase: Function, regulation, and structure of an amphitropic enzyme required for membrane biogenesis

CTP:phosphocholine cytidylyltransferase (CCT) catalyzes a rate-limiting and regulated step in the CDP-choline pathway for the synthesis of phosphatidylcholine (PC) and PC-derived lipids. Control of CCT activity is multi-layered, and includes direct regulation by reversible membrane binding involving a built-in lipid compositional sensor. Thus CCT contributes to phospholipid compositional homeostasis. CCT also modifies the curvature of its target membrane. Knowledge of CCT structure and regulation of its catalytic function are relatively advanced compared to many lipid metabolic enzymes, and are reviewed in detail. Recently the genetic origins of two human developmental and lipogenesis disorders have been traced to mutations in the gene for CCTα.

The intrinsically disordered nuclear localization signal and phosphorylation segments distinguish the membrane affinity of two cytidylyltransferase isoforms

Membrane phosphatidylcholine homeostasis is maintained in part by a sensing device in the key regulatory enzyme, CTP:phosphocholine cytidylyltransferase (CCT). CCT responds to decreases in membrane phosphatidylcholine content by reversible membrane binding and activation. Two prominent isoforms, CCTα and -β2, have nearly identical catalytic domains and very similar membrane binding amphipathic helical (M) domains but have divergent and structurally disordered N-terminal (N) and C-terminal phosphorylation (P) regions. We found that the binding affinity of purified CCTβ2 for anionic membranes was weaker than CCTα by more than an order of magnitude. Using chimeric CCTs, insertion/deletion mutants, and truncated CCTs, we show that the stronger affinity of CCTα can be attributed in large part to the electrostatic membrane binding function of the polybasic nuclear localization signal (NLS) motif, present in the unstructured N-terminal segment of CCTα but lacking in CCTβ2. The membrane partitioning of CCTβ2 in cells enriched with the lipid activator, oleic acid, was also weaker than that of CCTα and was elevated by incorporation of the NLS motif. Thus, the polybasic NLS can function as a secondary membrane binding motif not only in vitro but in the context of cell membranes. A comparison of phosphorylated, dephosphorylated, and region P-truncated forms showed that the in vitro membrane affinity of CCTβ2 is more sensitive than CCTα to phosphorylation status, which antagonizes membrane binding of both isoforms. These data provide a model wherein the primary membrane binding motif, an amphipathic helical domain, works in collaboration with other intrinsically disordered segments that modulate membrane binding strength. The NLS reinforces, whereas the phosphorylated tail antagonizes the attraction of domain M for anionic membranes.

The rate-limiting enzyme in phosphatidylcholine synthesis is associated with nuclear speckles under stress conditions

Phosphatidylcholine (PtdCho) is the most abundant phospholipid in eukaryotic membranes and its biosynthetic pathway is generally controlled by CTP:Phosphocholine Cytidylyltransferase (CCT), which is considered the rate-limiting enzyme. CCT is an amphitropic protein, whose enzymatic activity is commonly associated with endoplasmic reticulum (ER) translocation; however, most of the enzyme is intranuclearly located. Here we demonstrate that CCTα is concentrated in the nucleoplasm of MDCK cells. Confocal immunofluorescence revealed that extracellular hypertonicity shifted the diffuse intranuclear distribution of the enzyme to intranuclear domains in a foci pattern. One population of CCTα foci colocalised and interacted with lamin A/C speckles, which also contained the pre-mRNA processing factor SC-35, and was resistant to detergent and salt extraction. The lamin A/C silencing allowed us to visualise a second more labile population of CCTα foci that consisted of lamin A/C-independent foci non-resistant to extraction. We demonstrated that CCTα translocation is not restricted to its redistribution from the nucleus to the ER and that intranuclear redistribution must thus be considered. We suggest that the intranuclear organelle distribution of CCTα is a novel mechanism for the regulation of enzyme activity.

Use of phosphatide precursors to promote synaptogenesis

New brain synapses form when a postsynaptic structure, the dendritic spine, interacts with a presynaptic terminal. Brain synapses and dendritic spines, membrane-rich structures, are depleted in Alzheimer's disease, as are some circulating compounds needed for synthesizing phosphatides, the major constituents of synaptic membranes. Animals given three of these compounds, all nutrients-uridine, the omega-3 polyunsaturated fatty acid docosahexaenoic acid, and choline-develop increased levels of brain phosphatides and of proteins that are concentrated within synaptic membranes (e.g., PSD-95, synapsin-1), improved cognition, and enhanced neurotransmitter release. The nutrients work by increasing the substrate-saturation of low-affinity enzymes that synthesize the phosphatides. Moreover, uridine and its nucleotide metabolites activate brain P2Y receptors, which control neuronal differentiation and synaptic protein synthesis. A preparation containing these compounds is being tested for treating Alzheimer's disease.

Synapse formation is enhanced by oral administration of uridine and DHA, the circulating precursors of brain phosphatides

OBJECTIVE

The loss of cortical and hippocampal synapses is a universal hallmark of Alzheimer's disease, and probably underlies its effects on cognition. Synapses are formed from the interaction of neurites projecting from "presynaptic" neurons with dendritic spines projecting from "postsynaptic" neurons. Both of these structures are vulnerable to the toxic effects of nearby amyloid plaques, and their loss contributes to the decreased number of synapses that characterize the disease. A treatment that increased the formation of neurites and dendritic spines might reverse this loss, thereby increasing the number of synapses and slowing the decline in cognition.

DESIGN SETTING, PARTICIPANTS, INTERVENTION, MEASUREMENTS AND RESULTS

We observe that giving normal rodents uridine and the omega-3 fatty acid docosahexaenoic acid (DHA) orally can enhance dendritic spine levels (3), and cognitive functions (32). Moreover this treatment also increases levels of biochemical markers for neurites (i.e., neurofilament-M and neurofilament-70) (2) in vivo, and uridine alone increases both these markers and the outgrowth of visible neurites by cultured PC-12 cells (9). A phase 2 clinical trial, performed in Europe, is described briefly.

DISCUSSION AND CONCLUSION

Uridine and DHA are circulating precursors for the phosphatides in synaptic membranes, and act in part by increasing the substrate-saturation of enzymes that synthesize phosphatidylcholine from CTP (formed from the uridine, via UTP) and from diacylglycerol species that contain DHA: the enzymes have poor affinities for these substrates, and thus are unsaturated with them, and only partially active, under basal conditions. The enhancement by uridine of neurite outgrowth is also mediated in part by UTP serving as a ligand for neuronal P2Y receptors. Moreover administration of uridine with DHA activates many brain genes, among them the gene for the m-1 metabotropic glutamate receptor [Cansev, et al, submitted]. This activation, in turn, increases brain levels of that gene's protein product and of such other synaptic proteins as PSD-95, synapsin-1, syntaxin-3 and F-actin, but not levels of non-synaptic brain proteins like beta-tubulin. Hence it is possible that giving uridine plus DHA triggers a neuronal program that, by accelerating phosphatide and synaptic protein synthesis, controls synaptogenesis. If administering this mix of phosphatide precursors also increases synaptic elements in brains of patients with Alzheimer 's disease, as it does in normal rodents, then this treatment may ameliorate some of the manifestations of the disease.

Oral administration of circulating precursors for membrane phosphatides can promote the synthesis of new brain synapses

Although cognitive performance in humans and experimental animals can be improved by administering omega-3 fatty acid docosahexaenoic acid (DHA), the neurochemical mechanisms underlying this effect remain uncertain. In general, nutrients or drugs that modify brain function or behavior do so by affecting synaptic transmission, usually by changing the quantities of particular neurotransmitters present within synaptic clefts or by acting directly on neurotransmitter receptors or signal-transduction molecules. We find that DHA also affects synaptic transmission in mammalian brain. Brain cells of gerbils or rats receiving this fatty acid manifest increased levels of phosphatides and of specific presynaptic or postsynaptic proteins. They also exhibit increased numbers of dendritic spines on postsynaptic neurons. These actions are markedly enhanced in animals that have also received the other two circulating precursors for phosphatidylcholine, uridine (which gives rise to brain uridine diphosphate and cytidine triphosphate) and choline (which gives rise to phosphocholine). The actions of DHA aere reproduced by eicosapentaenoic acid, another omega-3 compound, but not by omega-6 fatty acid arachidonic acid. Administration of circulating phosphatide precursors can also increase neurotransmitter release (acetylcholine, dopamine) and affect animal behavior. Conceivably, this treatment might have use in patients with the synaptic loss that characterizes Alzheimer's disease or other neurodegenerative diseases or occurs after stroke or brain injury.

Regulation of Human Cytidine Triphosphate Synthetase 1 by Glycogen Synthase Kinase 3

Cytidine triphosphate synthetase (CTPS) catalyzes the rate-limiting step in the de novo synthesis of CTP, and both the yeast and human enzymes have been reported to be regulated by protein kinase A or protein kinase C phosphorylation. Here, we provide evidence that stimulation or inhibition of protein kinase A and protein kinase C does not alter the phosphorylation of endogenous human CTPS1 in human embryonic kidney 293 cells under the conditions tested. Unexpectedly, we found that low serum conditions increased phosphorylation of endogenous CTPS1 and this phosphorylation was inhibited by the glycogen synthase kinase 3 (GSK3) inhibitor indirubin-3'-monoxime and GSK3beta short interfering RNAs, demonstrating the involvement of GSK3 in phosphorylation of endogenous human CTPS1. Separating tryptic peptides from [(32)P]orthophosphate-labeled cells and analyzing the phosphopeptides by mass spectrometry identified Ser-574 and Ser-575 as phosphorylated residues. Mutation of Ser-571 demonstrated that Ser-571 was the major site phosphorylated by GSK3 in intact human embryonic kidney 293 cells by GSK3 in vitro. Furthermore, mutation of Ser-575 prevented the phosphorylation of Ser-571, suggesting that phosphorylation of Ser-575 was necessary for priming the GSK3 phosphorylation of Ser-571. Low serum was found to decrease CTPS1 activity, and incubation with the GSK3 inhibitor indirubin-3'-monoxime protected against this decrease in activity. Incubation with an alkaline phosphatase increased CTPS1 activity in a time-dependent manner, demonstrating that phosphorylation inhibits CTPS1 activity. This is the first study to investigate the phosphorylation and regulation of human CTPS1 in human cells and suggests that GSK3 is a novel regulator of CTPS activity.

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.

Increased Phosphatidylcholine Production but Disrupted Glycogen Metabolism in Fetal Type II Cells of Mice That Overexpress CTP:Phosphocholine Cytidylyltransferase

CTP:phosphocholine cytidylyltransferase (CCT) is a rate-determining enzyme in the de novo synthesis of phosphatidylcholine (PtdCho). Alveolar type II cells synthesize large quantities of disaturated PtdCho, the surface-active agent of pulmonary surfactant, particularly at late gestation when the lung prepares itself for postnatal air breathing. To clarify the role of CCTalpha in lung surfactant maturation, we overexpressed CCTalpha(1-367) using the surfactant protein-C promoter. Lungs of transgenic mice were analyzed at day 18 of gestation (term = 19 days). Overexpression of CCTalpha(1-367) increased the synthesis and content of PtdCho in fetal type II cells isolated from the transgenic mice. Also, PtdCho content of fetal lung fluid was increased. No changes in surfactant protein content were detected. Interestingly, fetal type II cells of transgenic mice contained more glycogen than control cells. Incorporation studies with [U-(14)C]glucose demonstrated that overexpression of CCTalpha(1-367) in fetal type II cells increased glycogen synthesis without affecting glycogen breakdown. To determine which domain contributes to this glycogen phenotype, two additional transgenes were created overexpressing either CCTalpha(1-239) or CCTalpha(239-367). Glycogen synthesis and content were increased in fetal type II cells expressing CCTalpha(239-367) but not CCTalpha(1-239)(.) We conclude that overexpression of CCTalpha increases surfactant PtdCho synthesis without affecting surfactant protein levels but that it disrupts glycogen metabolism in differentiating type II cells via its regulatory domain.

Effects of Clostridium perfringens phospholipase C in mammalian cells

Clostridium perfringens phospholipase C (Cp-PLC), the major virulence factor in the pathogenesis of gas gangrene, is a Zn(2+) metalloenzyme with lecithinase and sphingomyelinase activities. Its structure shows an N-terminal domain containing the active site, and a C-terminal Ca(2+) binding domain required for membrane interaction. Although the knowledge of the structure of Cp-PLC and its interaction with aggregated phospholipids has advanced significantly, an understanding of the effects of Cp-PLC in mammalian cells is still incomplete. Cp-PLC binds to artificial bilayers containing cholesterol and sphingomyelin or phosphatidylcholine (PC) and degrades them, but glycoconjugates present in biological membranes influence its binding or positioning toward its substrates. Studies with Cp-PLC variants harboring single amino-acid substitutions have revealed that the active site, the Ca(2+) binding region, and the membrane interacting surface are required for cytotoxic and haemolytic activity. Cp-PLC causes plasma membrane disruption at high concentrations, whereas at low concentrations it perturbs phospholipid metabolism, induces DAG generation, PKC activation, Ca(2+) mobilization, and activates arachidonic acid metabolism. The cellular susceptibility to Cp-PLC depends on the composition of the plasma membrane and the capacity to up-regulate PC synthesis. The composition of the plasma membrane determines whether Cp-PLC can bind and acquire its active conformation, and thus the extent of phospholipid degradation. The capacity of PC synthesis and the availability of precursors determine whether the cell can replace the degraded phospholipids. Whether the perturbations of signal transduction processes caused by Cp-PLC play a role in cytotoxicity is not clear. However, these perturbations in endothelial cells, platelets and neutrophils lead to the uncontrolled production of intercellular mediators and adhesion molecules, which inhibits bacterial clearance and induces thrombotic events, thus favouring bacterial growth and spread in the host tissues.

Purification and kinetic characterization of CTP:phosphocholine cytidylyltransferase from Saccharomyces cerevisiae

CTP:phosphocholine cytidylyltransferase (CCT) regulates the biosynthesis of phosphatidylcholine in mammalian cells. In order to understand the mechanism by which this enzyme controls phosphatidylcholine synthesis, we have initiated studies of CCT from the model genetic system, the yeast Saccharomyces cerevisiae. The yeast CCT gene was isolated from genomic DNA using the polymerase chain reaction and was found to encode tyrosine at position 192 instead of histidine, as originally reported. Levels of expression of yeast CCT activity in Escherichia coli or in the yeast, Pichia pastoris, were somewhat low. Expression of yeast CCT in a baculovirus system as a 6x-His-tag fusion protein was higher and was used to purify yeast CCT by a procedure that included delipidation. Kinetic characterization revealed that yeast CCT was activated approximately 20-fold by 20 microM phosphatidylcholine:oleate vesicles, a level 5-fold lower than that necessary for maximal activation of rat CCT. The k(cat) value was 31.3 s(-1) in the presence of lipid and 1.5 s(-1) in the absence of lipid. The K(m) values for the substrates CTP and phosphocholine did not change significantly upon activation by lipids; K(m) values in the presence of lipid were 0.80 mM for phosphocholine and 1.4 mM for CTP while K(m) values in the absence of lipid were 1.2 mM for phosphocholine and 0.8 mM for CTP. Activation of yeast CCT, therefore, appears to be due to an increase in the k(cat) value upon lipid binding.

Lipid activation of CTP: phosphocholine cytidylyltransferase alpha: characterization and identification of a second activation domain

The CTP:phosphocholine cytidylyltransferase (CCT) governs the rate of phosphatidylcholine (PtdCho) biosynthesis, and its activity is governed by interaction with membrane lipids. The carboxy-terminus was dissected to delineate the minimum sequences required for lipid responsiveness. The helical domain is recognized as a site of lipid interaction, and all three tandem alpha-helical repeats from residues 257 through 290 were found to be required for regulation of enzymatic activity by this domain. Truncation of the carboxy-terminus to remove one or more of the alpha-helical repeats yielded catalytically compromised proteins that were not responsive to lipids but retained sufficient activity to accelerate PtdCho biosynthesis when overexpressed in vivo. The role of the helical region in lipid-activation was tested further by excising residues 257 through 309 to yield a protein that retained a 57-residue carboxy terminal domain fused to the catalytic core. This construct tested the hypothesis that the helical region inhibits activity in the absence of lipid rather than activates the enzyme in the presence of lipid. This hypothesis predicts constitutive activity for CCTalpha[Delta257-309]; however, this protein was tightly regulated by lipid with activities comparable to the full-length CCTalpha, in both the absence and presence of lipid. Activation of CCTalpha[Delta257-309] was dependent exclusively on anionic lipids, whereas full-length CCTalpha responded to either anionic or neutral lipids. Phosphatidic acid delivered in Triton X-100 micelles was the preferred activator of the second lipid-activation domain. These data demonstrate that CCTalpha can be regulated by lipids by two independent domains: (i) the three amphipathic alpha-helical repeats that interact with both neutral and anionic lipid mixtures and (ii) the last 57 residues that interact with anionic lipids. The results show that both domains are inhibitory in the absence of lipid and activating in the presence of lipid. Removal of both domains results in a nonresponsive, dysregulated enzyme with reduced activity. The data also demonstrate for the first time that the 57-residue carboxy-terminal domain in CCTalpha participates in lipid-mediated regulation and is sufficient for maximum activation of enzyme activity.

Regulation of mammalian cell membrane biosynthesis

This review explores current information on the interrelationship between phospholipid biochemistry and cell biology. Phosphatidylcholine is the most abundant phospholipid and it biosynthesis has been studied extensively. The choline cytidylyltransferase regulates phosphatidylcholine production, and recent advances in our understanding of the mechanisms that govern cytidylyltransferase include the discovery of multiple isoforms and a more complete understanding of the lipid regulation of enzyme activity. Similarities between phosphatidylcholine formation and the phosphatidylethanolamine and phosphatidylinositol biosynthetic pathways are discussed, together with current insight into control mechanisms. Membrane phospholipid doubling during cell cycle progression is a function of periodic biosynthesis and degradation. Membrane homeostasis is maintained by a phospholipase A-mediated degradation of excess phospholipid, whereas insufficient phosphatidylcholine triggers apoptosis in cells.

Distribution of CTP:phosphocholine cytidylyltransferase (CCT) isoforms. Identification of a new CCTbeta splice variant

CTP:phosphocholine cytidylyltransferase is a major regulator of phosphatidylcholine biosynthesis. A single isoform, CCTalpha, has been studied extensively and a second isoform, CCTbeta, was recently identified. We identify and characterize a third cDNA, CCTbeta2, that differs from CCTbeta1 at the carboxyl-terminal end and is predicted to arise as a splice variant of the CCTbeta gene. Like CCTalpha, CCTbeta2 is heavily phosphorylated in vivo, in contrast to CCTbeta1. CCTbeta1 and CCTbeta2 mRNAs were differentially expressed by the human tissues examined, whereas CCTalpha was more uniformly represented. Using isoform-specific antibodies, both CCTbeta1 and CCTbeta2 localized to the endoplasmic reticulum of cells, in contrast to CCTalpha which resided in the nucleus in addition to associating with the endoplasmic reticulum. CCTbeta2 protein has enzymatic activity in vitro and was able to complement the temperature-sensitive cytidylyltransferase defect in CHO58 cells, just as CCTalpha and CCTbeta1 supporting proliferation at the nonpermissive conditions. Overexpression experiments did not reveal discrete physiological functions for the three isoforms that catalyze the same biochemical reaction; however, the differential cellular localization and tissue-specific distribution suggest that CCTbeta1 and CCTbeta2 may play a role that is distinct from ubiquitously expressed CCTalpha.

Shuttling of CTP:Phosphocholine cytidylyltransferase between the nucleus and endoplasmic reticulum accompanies the wave of phosphatidylcholine synthesis during the G(0) --> G(1) transition

The transition from quiescence (G(0)) into the cell division cycle is marked by accelerated phospholipid turnover. We examined the rates of phosphatidylcholine (PC) synthesis and the activity, membrane affinity, and intracellular localization of the rate-limiting enzyme in the synthesis of PC, CTP:phosphocholine cytidylyltransferase (CT) during this transition. The addition of serum to quiescent IIC9 fibroblasts resulted in a wave of PC synthesis beginning at approximately 10 min, peaking at approximately 3 h with a >10-fold increase in rate, and declining to near basal rates by 10 h. CT activity, monitored in situ, was elevated approximately 3-fold between 1 and 2 h postserum. Neither CT mass nor its phosphorylation state changed during the surge in PC synthesis and CT activity. On the other hand, the ratio of particulate/soluble CT surged and then receded in concert with the wave of PC synthesis. During quiescence, CT was confined to the nucleus, as assessed by indirect immunofluorescence. Within 10 min after serum stimulation, a portion of the CT fluorescence appeared in the cytoplasm, where it intensified until approximately 4 h postserum. Thereafter, the cytoplasmic CT signal waned, while the nuclear signal increased, and by 8 h CT was once again predominantly nuclear. The dynamics of CT's apparent translocation in and out of the nucleus paralleled the wave of PC synthesis and the solubility changes of CT. Cytoplasmic CT co-localized with BiP, a resident endoplasmic reticulum protein, in a double labeling experiment. These data suggest that the wave of PC synthesis that accompanies the G(0) --> G(1) transition is regulated by the coordinated changes in CT activity, membrane affinity, and intracellular distribution. We describe for the first time a redistribution of CT from the nucleus to the ER that correlates with an activation of the enzyme. We propose that this movement is required for the stimulation of PC synthesis during entry into the cell cycle.

Expression analysis of glycogen synthase kinase-3 in human tissues

Human glycogen synthase kinase-3 (GSK-3) is a multisubstrate, proline-directed kinase that phosphorylates tau, beta-amyloid and neurofilaments. In this study, the expression levels of the two GSK-3 isoforms, alpha and beta, RNA and proteins in different human tissues were examined. Northern analysis demonstrated that GSK-3alpha is encoded by a 2.6-kb mRNA and GSK-3beta by 8.3- and 2.8-kb mRNAs. The two GSK-3beta mRNA species were variably expressed in different tissues. Northern and quantitative polymerase chain reaction demonstrated that both GSK-3alpha and GSK-3beta mRNA were prominently expressed in testis, thymus, prostate and ovary but were low in adult lung and kidney. Western blot analysis showed that the 51-kDa GSK-3alpha protein was highly expressed in lung, ovary, kidney and testis, whereas the 46-kDa GSK-3beta protein was highly expressed in lung, kidney and brain. The differential expression of GSK-3alpha and GSK-3beta mRNA and proteins and the lack of relationship between transcription and translation in some tissues indicate that GSK-3alpha and GSK-3beta are subject to different means of regulation.

CTP:phosphocholine cytidylyltransferase: insights into regulatory mechanisms and novel functions

A key regulatory enzyme in phosphatidylcholine biosynthesis, CTP:cholinephosphate cytidylyltransferase (CCT), catalyzes the formation of CDP-choline. This review discusses the essential features of CCT and addresses intriguing new insights into the catalytic and regulatory properties of this complex enzyme. Characterization of a lipid-binding segment in rat CCT is described and the role of lipids in CCT activation is discussed. An analysis of the phosphorylation domain is presented and possible physiological rationales for reversible phosphorylation of CCT are discussed. The nuclear localization of CCT is examined in the context of multiple CCT isoforms, as is recent evidence establishing a potential link between CCT activity and vesicular transport.

Phosphorylation and regulation of choline kinase from Saccharomyces cerevisiae by protein kinase A

The CKI1-encoded choline kinase (ATP:choline phosphotransferase, EC 2.7.1.32) from Saccharomyces cerevisiae was phosphorylated in vivo on multiple serine residues. Activation of protein kinase A activity in vivo resulted in a transient increase in the phosphorylation of choline kinase. This phosphorylation was accompanied by a stimulation in choline kinase activity. In vitro, protein kinase A phosphorylated choline kinase on a serine residue with a stoichiometry (0.44 mol of phosphate/mol of choline kinase) consistent with one phosphorylation site/choline kinase subunit. The major phosphopeptide derived from the enzyme phosphorylated in vitro by protein kinase A was common to one of the major phosphopeptides derived from the enzyme phosphorylated in vivo. Protein kinase A activity was dose- and time-dependent and dependent on the concentrations of ATP (Km 2.1 microM) and choline kinase (Km 0.12 microM). Phosphorylation of choline kinase with protein kinase A resulted in a stimulation (1.9-fold) in choline kinase activity whereas alkaline phosphatase treatment of choline kinase resulted in a 60% decrease in choline kinase activity. The mechanism of the protein kinase A-mediated stimulation in choline kinase activity involved an increase in the apparent Vmax values with respect to ATP (2.6-fold) and choline (2.7-fold). Overall, the results reported here were consistent with the conclusion that choline kinase was regulated by protein kinase A phosphorylation.

Analysis of choline and phosphorylcholine content in human neutrophils stimulated by f-Met-Leu-Phe and phorbol myristate acetate: contribution of phospholipase D and C

ABSTRACT. We analysed changes in choline (CHO) and phosphorylcholine (PCHO) content of stimulated human polymorphonuclear leukocytes (PMNs) by a chemiluminescence assay to further examine the relative contributions of phospholipase D (PLD) and PLC to phosphatidylcholine (PC) breakdown. PLD activation was also analysed by measuring tritiated phosphatidic acid (PA) and diglycerides (GDs) in PMNs labelled with tritiated alkyl-lyso PC. Stimulation of PMNs with formyl-methionyl-leucyl-phenylalanine fMLP; 0.1 microM induced a weak elevation of mass choline (+25% of basal level) that was strongly potentiated in PMNs primed with cytochalasin B (+350% relative to the control value of 657+/-53 pmol/10(7) cells). CHO production was rapid and transient, peaking within 1 min, and ran parallel to that of tritiated PA. Thereafter, the amount of tritiated PA declined strongly (40% of maximum by 3 min), whereas the elevated choline content induced by fMLP plateaued for at least 5 min. Phorbol myristate acetate (PMA) sustained the formation of CHO for as long as 20 min, which correlated with that of [3H]PA in a time- and concentration-dependent manner. PCHO content of resting PMN leukocytes (1560 +/- 56 pmol/10(7) cells) was not modified after stimulation of PMNs with fMLP or PMA for at least 10 min, which argues against breakdown of phosphatidylcholine by PLC. For longer treatment (10-20 min), fMLP stimulated a significant enhancement of PCHO level, which occurred concomitantly with a decrease in CHO level, suggesting that choline kinase rather than PLC may be activated. Unlike fMLP, PMA stimulated a fall in PCHO between 10 and 15 min after PMN stimulation, pointing to different regulatory mechanisms of PCHO level. These data indicate that DG formation from PC in PMNs is mediated by PLD but not by PLC and show that chemiluminescence measurement of choline is a reliable index of PLD activation.

Studies on the regulation of CTP:phosphocholine cytidylyltransferase using permeabilized HEP G2 cells: evidence that both active and inactive enzyme are membrane-bound

To obtain more insight into the mechanisms regulating CTP:phosphocholine cytidylyltransferase (CT), we determined the effect of oleate treatment on the rate of CT release from permeabilized Hep G2 cells and the distribution of the CT remaining in the permeabilized cells. When we permeabilized untreated cells in pH 7.5 buffer containing 0.15 M KCl, the rate of CT release was much slower than the release of lactate dehydrogenase. Oleate treatment caused a further decrease in CT release from cells. In untreated cells, 70-80% of the CT remaining in cells 10 min after permeabilization was recovered as soluble CT. Oleate treatment increased the amount of bound CT but over 50% of the CT in cells 10 min after permeabilization was recovered as soluble CT. In both control and oleate-treated cells, the increase in CT release with time correlated with a decrease in the amount of CT recovered from permeabilized cells as soluble CT. These results suggested that CT existed in a form that was not immediately available for release from permeabilized cells, but was recovered in the soluble fraction after cell disruption. When cells were permeabilized in 10 mM imidazole-20% glycerol-5 mM Mg2+ pH 6.5, over 80% of CT in control and over 90% of CT in oleate-treated cells was recovered bound to the particulate fraction. Essentially no CT was released from the cells. The recovery of CT in the particulate fraction required Mg2+ to be present when permeabilization was initiated. The addition of Mg2+, after cells were disrupted, did not increase CT in the particulate fraction. In untreated cells, 50% of bound CT was active. Oleate treatment increased the amount of active CT in the particulate fraction to over 70% of total. About 50% of particulate CT in untreated cells but only 15% in oleate-treated cells was extracted with 0.15 M KCl. Inactive CT was preferentially extracted by KCl. The bound CT was recovered in isolated nuclei. Overall, the results suggested that both inactive and active CT are bound to nuclear membranes, and that the activation of CT involves conversion of CT loosely bound to membrane to a form more tightly bound to membranes perhaps by hydrophobic interaction with phospholipids. This model does not involve translocation from a soluble pool.

Reversible translocation of CTP:phosphocholine cytidylyltransferase from cytosol to membranes in the adult bovine liver around parturition

The key regulatory enzyme of phosphatidylcholine (PC) synthesis, CTP:phosphocholine cytidylyltransferase (CT), is known to be activated in vitro by translocation from soluble to particulate fractions of the cell. In the present study the periparturient cow was chosen as a model to investigate whether translocation of CT can contribute to the regulation of PC synthesis in vivo. Between parturition and 1.5 weeks post-partum, the cytosolic CT activity in the liver of the adult animal decreased 1.9-fold, and this correlated with a 1.8-fold increase in microsomal CT activity. At that time, microsomal CT activity started to decline again whereas the cytosolic activity rose concomitantly until both activities reached their pre-partum values at 8 weeks post-partum. The activities of soluble and membrane-bound CTP:phosphoethanolamine cytidylyltransferase (ET), the analogous enzyme in the CDP-ethanolamine pathway, did not change significantly throughout this period. Whereas hepatic PC concentrations declined until about 2 weeks post-partum and thereafter gradually returned to pre-partum levels, the PC levels in very-low-density-lipoproteins, started to rise 2 weeks after the partus reaching a maximum of 219% of the original value at 8 weeks post-partum. These results strongly suggest that there is a reversible redistribution of CT between cytosol and membranes in a physiologically relevant animal model, supporting the concept that translocation of CT is occurring in vivo.

Cytidylyltransferase-binding protein is identical to transcytosis-associated protein (TAP/p115) and enhances the lipid activation of cytidylyltransferase

We previously identified a protein from rat liver that binds CTP:phosphocholine cytidylyltransferase (CT). We have now purified this protein (cytidylyltransferase-binding protein (CTBP)) from rat liver. The purification involved precipitation at pH 5 and extraction of the precipitate with buffer, followed by sequential chromatography on DEAE-Sepharose and butyl-agarose. Final purification was accomplished by either preparative electrophoresis or hydroxylapatite chromatography. Amino acid sequences from six peptides derived from pure CTBP matched sequences in transcytosis-associated protein (TAP) with 98% identity. Thus, CTBP was positively identified to be TAP. Purified CTBP increased the activity of purified CT measured with phosphatidylcholine (PC)/oleic acid. In the absence of PC/oleic acid, CTBP did not stimulate CT activity. Dilution of CT to reduce the Triton X-100 concentration produced a loss of CT activity. The lost activity was recovered by the addition of CTBP plus PC/oleic acid to the assay, but not by the addition of either PC/oleic acid or CTBP alone. Removal of CTBP from purified preparations by immunoprecipitation with CTBP antibodies eliminated the activation of CT. Both CT and CTBP were shown to bind to PC/oleic acid liposomes. The formation of complexes between CT and CTBP in the absence of PC/oleic acid liposomes could not be demonstrated. These results suggest that CTBP functions to modify the interaction of CT with PC/oleic acid liposomes, resulting in an increase in the catalytic activity perhaps by the formation of a ternary complex between CT, CTBP, and lipid. Overall, these results suggest that CTBP (TAP) may function to coordinate the biosynthesis of phosphatidylcholine with vesicle transport.

CTP:phosphocholine cytidylyltransferase

CTP:phosphocholine cytidylyltransferase (CCT) catalyzes the synthesis of CDP-choline and is regulatory for phosphatidylcholine biosynthesis. This review focuses on recent developments in understanding the catalytic and regulatory mechanisms of this enzyme. Evidence for the nuclear localization of the enzyme is discussed, as well as evidence suggesting cytoplasmic localization. A comparison of the catalytic domains of CCTs from a wide variety of organisms is presented, highlighting a large number of completely conserved residues. Work implying a role for the conserved HXGH sequence in catalysis is described. The membrane-binding domain in rat CCT has been defined, and the role of lipids in activating the enzyme is discussed. The identification of the phosphorylation domain is described, as well as approaches to understand the role of phosphorylation in enzyme activity. Other possible control mechanisms such as enzyme degradation and gene expression are presented.

Binding of CTP:phosphocholine cytidylyltransferase to lipid vesicles: diacylglycerol and enzyme dephosphorylation increase the affinity for negatively charged membranes

The regulation of membrane binding and activity of purified CDP:phosphocholine cytidylyl-transferase (CT) by lipid activators and enzyme dephosphorylation was examined. The binding of CT to membranes was analyzed using sucrose-loaded vesicles (SLVs). Binding to phosphatidylcholine vesicles was not detected even at a lipid:protein ratio of approximately 2000 (1 mM PC). CT bound to vesicles containing anionic lipids with apparent molar partition coefficients between 2 x 10(5) and 2 x 10(6), depending on the vesicle charge. The vesicle binding and activation of CT showed very similar sigmoidal dependencies on the lipid negative charge. In addition, diacylglycerol interacted synergistically with anionic phospholipids to stimulate both binding and activation at lower mole percent anionic lipid. These results demonstrate parallel requirements for binding and activity. Dephosphorylation of CT without destabilization was accomplished using the catalytic subunit of protein phosphatase 1. Dephosphorylated CT required a lower mole percent anionic phospholipid than phosphorylated CT for binding to and activation by SLVs. The combination of 10 mol % diacylglycerol and enzyme dephosphorylation shifted the mole percent phosphatidic acid required for half-maximal activation from 25% to 12%. These results suggest a mechanism whereby large changes in CT activity can result from changes in the phosphorylation state combined with small alterations in the membrane content of diacylglycerol. We propose a mechanism whereby dephosphorylation on the domain adjacent to the membrane binding domain increases the affinity of the latter for a negatively charged membrane surface.

Phosphorylation and regulation of CTP synthetase from Saccharomyces cerevisiae by protein kinase A

The phosphorylation and regulation of the URA7-encoded CTP synthetase (EC 6.3.4.2, UTP:ammonia ligase (ADP-forming)) from Saccharomyces cerevisiae by cAMP-dependent protein kinase (protein kinase A) were examined. Protein kinase A is the principal mediator of signals transmitted through the RAS/cAMP pathway in S. cerevisiae. The results of labeling experiments indicated that the phosphorylation of CTP synthetase was mediated by the RAS/cAMP pathway in vivo. In vitro, protein kinase A phosphorylated CTP synthetase at a serine residue with a stoichiometry consistent with one phosphorylation site per CTP synthetase subunit. Protein kinase A activity was dose- and time-dependent using CTP synthetase as a substrate. The dependence of protein kinase A activity on CTP synthetase was cooperative (n = 1.8) and the Km value for CTP synthetase was 73 nM. Phosphorylation of CTP synthetase with protein kinase A resulted in the stimulation (190%) of activity. The mechanism of this stimulation included an increase in the Vmax of the reaction with respect to UTP and ATP, a decrease in the Km for ATP, and a decrease in the cooperative kinetic behavior of the enzyme. Phosphorylated CTP synthetase was less sensitive to product inhibition by CTP. Protein kinase C also phosphorylates and activates CTP synthetase. Phosphorylation of CTP synthetase with protein kinases A and C together resulted in an increase in CTP synthetase activity that was slightly greater than that obtained when the enzyme was phosphorylated with either protein kinase alone.

The effect of edelfosine on CTP:cholinephosphate cytidylyltransferase activity in leukemic cell lines

Analogs of ether phospholipids have been shown to have selective anti-neoplastic activity. The compounds are known to inhibit phospholipid biosynthesis. This paper examines the effect of the alkyl-lysophospholipid, edelfosine, on the rate-limiting enzyme, CTP:cholinephosphate cytidylyltransferase, in de novo phosphatidylcholine synthesis in sensitive and resistant leukemic cell lines. Enzyme activity was measured by the incorporation of 14C-phosphocholine into CDP-choline by lysates of HL60 and K562; cells demonstrated inhibition of incorporation of 14C-phosphocholine in HL60 cell lysates but no inhibition in K562 lysates. Partial purification of cytidylyltransferase by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting demonstrated similarity between the enzyme isolated from each cell line. Cloning and sequencing of cytidylyltransferase cDNA of HL60 cells was accomplished using a probe encoding the entire protein sequence of the K562 cytidylyltransferase gene. A substitution at nucleotide 751 from A in the HL60 cell cDNA clone to G in the K562 cDNA clone resulted in a change in amino acid number 251 from lysine (positively charged) in the HL60 enzyme to glutamic acid (negatively charged) in the K562 enzyme. This negative charge in the lipid-binding domain of the K562 enzyme may result in a weaker binding of edelfosine and the observed decrease in activity, as evidenced by resistance to edelfosine by K562 cells.

Substitution of serine for glycine-91 in the HXGH motif of CTP:phosphocholine cytidylyltransferase implicates this motif in CTP binding

The effect of mutations in the proposed catalytic domain of CTP:phosphocholine cytidylyltransferase was investigated by constructing the single mutants CT-S91 and CT-C114 from the double mutant CT-S91C114, previously shown to have 4-fold lower than wild-type activity [Walkey, C.R., Kalmar, G. B., & Cornell, R. B. (1994) J. Biol. Chem. 269, 5742-5749]. The constructs were overexpressed in COS cells. The mutation Gly-91 to Ser-91 was found to be responsible for the decreased activity, whereas Ser-114 to Cys-114 had no effect. An alanine substitution at position 91, CT-A91, had a lesser effect on cytidylyltransferase activity. CT-S91 and CT-WT were purified from COS cells, and their kinetic constants were determined. CT-S91 had a 4-fold lower Vmax, and a K(m) for CTP 25-fold higher than the wild-type enzyme, suggesting that substitution of Gly-91 with serine interferes with CTP binding. The K(m) for phosphocholine was not affected in the CT-S91 mutant. There was no difference in the chymotrypsin sensitivities of CT-S91 and CT-WT, indicating that the mutation did not cause a global change in protein structure. However, the CT-S91 activity was more susceptible to inhibition by the denaturant urea than that of CT-WT, indicative of a perturbation of the active site folding. Gly-91 resides in the local sequence HSGH, which has been proposed to be a CTP-binding motif in the novel cytidylyltransferase superfamily [Bork, P., Holm, L., Koonin, E.V., & Sander, C. (1995) Proteins: Struct., Funct., Genet. 22, 259-266]. Our results represent the first experimental validation of this hypothesis.

Evidence for phosphorylation of CTP:phosphocholine cytidylyltransferase by multiple proline-directed protein kinases

Reversible phosphorylation of CTP:phosphocholine cytidylyltransferase, the rate-limiting enzyme of phosphatidylcholine biosynthesis, is thought to play a role in regulating its activity. In the present study, the hypothesis that proline-directed kinases play a major role in phosphorylating cytidylyltransferase is substantiated using a c-Ha-ras-transfected clone of the human keratinocyte cell line HaCaT. Cellular extracts from epidermal growth factor-stimulated HaCaT cells and from ras-transfected HaCaT cells phosphorylated cytidylyltransferase much stronger as compared with extracts from quiescent HaCaT cells. The tryptic phosphopeptide pattern of cytidylyltransferase phosphorylated by cell-free extracts from ras-transfected HaCaT cells was similar compared with the patterns of cytidylyltransferase phosphorylated by p44mpkmitogen-activated protein kinase and p34cdc2 kinase in vitro, whereas in the case of casein kinase II the pattern was different. Furthermore, in c-Ha-ras-transfected HaCaT cells the in vivo phosphorylation state of cytidylyltransferase was 2-fold higher as compared with untransfected HaCaT cells. This higher phosphorylation of cytidylyltransferase in the ras-transfected clone was reduced to a level below the phosphorylation of cytidylyltransferase in untransfected cells, using olomoucine, a specific inhibitor of proline-directed kinases. The reduced phosphorylation of cytidylyltransferase in olomoucine-treated cells correlated with an enhanced stimulation of enzyme activity by oleic acid.

Stimulation of CTP:phosphocholine cytidylyltransferase by free cholesterol loading of macrophages involves signaling through protein dephosphorylation

Free cholesterol-loaded macrophages in atheromata synthesize excess phosphatidylcholine (PC), which may be an important adaptive response to the excess free cholesterol (FC) load. We have recently shown that FC loading of macrophages leads to 2-4-fold increases in PC mass and biosynthesis and to the post-translational activation of the membrane-bound form of CTP:phosphocholine cytidylyltransferase (CT), a key enzyme in PC biosynthesis. Herein, we explore further the mechanism of CT activation in FC-loaded macrophages. First, enrichment of membranes from control macrophages with FC in vitro did not increase CT activity, and PC biosynthesis in vivo is up-regulated by FC loading even when CT and FC appear to be mostly in different intracellular sites. These data imply that FC activates membrane-bound CT by a signaling mechanism. That the proposed signaling mechanism involves structural changes in the CT protein was suggested by data showing that two different antibodies against synthetic CT peptides showed increased recognition of membrane-bound CT from FC-loaded cells despite no increase in CT protein. Since CT is phosphorylated, two-dimensional maps of peptides from 32P-labeled control and FC-loaded macrophages were compared: six peptide spots from membrane-bound CT, but none from soluble CT, were dephosphorylated in the FC-loaded cells. Furthermore, incubation of FC-loaded macrophages with the phosphatase inhibitor, calyculin A, blocked increases in both PC biosynthesis and antipeptide-antibody recognition of CT. Last, treatment of membranes from control macrophages with lambda phage protein phosphatase in vitro increased both CT activity (2-fold) and antipeptide-antibody recognition of CT; soluble CT activity and antibody recognition were not substantially affected by phosphatase treatment. In summary, FC loading of macrophages leads to the partial dephosphorylation of membrane-bound CT, and possibly other cellular proteins, which appears to be important in CT activation. This novel regulatory action of FC may allow macrophages to adapt to FC loading in atheromata.

The association of lipid activators with the amphipathic helical domain of CTP:phosphocholine cytidylyltransferase accelerates catalysis by increasing the affinity of the enzyme for CTP

The biochemical mechanism for the regulation of enzyme activity by lipid modulators and the role of the amphipathic alpha-helical domain of CTP:phosphocholine cytidylyltransferase (CT) was investigated by analyzing the kinetic properties of the wild-type protein and two truncation mutants isolated from a baculovirus expression system. The CT[delta 312-367] mutant protein lacked the carboxyl-terminal phosphorylation domain and retained high catalytic activity along with both positive and negative regulation by lipid modulators. The CT[delta 257-367] deletion removed in addition the region containing three consecutive amphipathic alpha-helical repeats. The CT[delta 257-367] mutant protein exhibited a significantly lower specific activity compared to CT or CT[delta 312-367] when expressed in either insect or mammalian cells; however, CT[delta 257-367] activity was refractory to either stimulation or inhibition by lipid regulators. Lipid activators accelerated CT activity by decreasing the Km for CTP from 24.7 mM in their absence to 0.7 mM in their presence. The Km for phosphocholine was not affected by lipid activators. The activity of CT[delta 257-367] was comparable to the activity of wild-type CT in the absence of lipid activators and the CTP Km for CT[delta 257-367] was 13.9 mM. The enzymatic properties of the CT[delta 231-367] mutant were comparable to those exhibited by the CT[257-367] mutant indicating that removal of residues 231 through 257 did not have any additional influence on the lipid regulation of the enzyme. Thus, the region between residues 257 and 312 was required to confer lipid regulation on CT, and the association of activating lipids with this region of the protein stimulated catalysis by increasing the affinity of the enzyme for CTP.

Identification of an inhibitory domain of CTP:phosphocholine cytidylyltransferase

The function of the putative amphipathic helices between residues 236 and 314 of CTP:phosphocholine cytidylyltransferase was examined by constructing two truncation mutants; CT314 was missing the entire phosphorylation segment, whereas CT236 was missing both the region with the putative amphipathic helices and the phosphorylation segment. Stable cells lines expressing these truncation mutants in Chinese hamster ovary 58 cells were isolated and characterized. CT314 was predominantly soluble in control cells but became membrane-associated in cells treated with oleate, which also causes translocation of wild-type cytidylyltransferase. CT236 was found to be soluble both in control cells and in cells treated to cause translocation. These results strongly suggest that the membrane-binding site is located within residues 237-314. When assayed for activity in vitro, the mutant forms were catalytically active in the presence of exogenous lipids. CT236, moreover, was as active in the absence of lipids as in their presence, whereas CT314 required lipids for activity. The rate of phosphatidylcholine synthesis in cells expressing CT236 was considerably higher than in wild-type cells, consistent with the enzyme being constitutively active in the cells. These results indicate that residues 237-314 constitute an inhibitory segment; when this segment is removed from the catalytic domain by truncation or by binding to membranes, an inhibitory constraint is removed and cytidylyltransferase is activated.

Effects of altered phosphorylation sites on the properties of CTP:phosphocholine cytidylyltransferase

To investigate the role of phosphorylation and dephosphorylation in modulating the activity and location of CTP:phosphocholine cytidylyltransferase, we used site-directed mutagenesis to construct four mutant forms of cytidylyltransferase. These forms were 5SP-->AP, in which five of the seven Ser-Pro sequences were converted to Ala-Pro; 7SP-->AP, in which all of the seven Ser-Pro sequences converted to Ala-Pro; 16S-->A, in which all sixteen Ser residues that can be phosphorylated in wild type cytidylyltransferase were converted to Ala; and 16S-->E, in which all sixteen Ser residues were converted to Glu. The mutant enzymes were expressed in the strain 58 Chinese hamster ovary cell line, which is temperature-sensitive for growth and cytidylyltransferase activity. All mutant enzyme forms were enzymatically as active as the wild type when assayed under optimal conditions. In untreated cells, more of the Ser-->Ala mutants were membrane-associated than in cells expressing wild type enzyme, consistent with the phosphorylation state of the enzyme affecting its affinity for membranes. About half of the 16S-->A mutant remained soluble, however, indicating that dephosphorylation alone does not trigger membrane association. Although the amount of membrane-associated enzyme in the 16S-->A mutant was about 10-fold greater than that of wild type, phosphatidylcholine synthesis was increased by only about 75%, suggesting that membrane association does not necessarily cause full activation. All mutant forms, including the 16S-->E mutant, translocated to the particulate fraction upon oleate treatment, indicating that a high negative charge in the phosphorylation region does not preclude association of cytidylyltransferase with membranes. All mutant enzymes were able to support growth of strain 58 at 40 degrees C, and the rate of phosphatidylcholine synthesis was not greatly altered in the cell lines expressing mutant cytidylyltransferase forms. These results are consistent with a role for phosphorylation in the equilibrium distribution of cytidylyltransferase but suggest that changes in enzyme activity and location are not triggered exclusively by changes in the phosphorylation state.

Lipid activation of CTP:phosphocholine cytidylyltransferase is regulated by the phosphorylated carboxyl-terminal domain

The role of the phosphorylated carboxyl-terminal domain of CTP:phosphocholine cytidylyltransferase (CT) in the regulation of enzyme activity was investigated by comparing the catalytic properties of wild-type CT to two mutant proteins with altered carboxyl-terminal phosphorylation domains. CT isolated from a baculovirus expression system was extensively phosphorylated at multiple sites in the carboxyl-terminal domain. The CT[S315A] mutant lacked a major CT phosphorylation site, and the carboxyl-terminal deletion mutant, CT[delta 312-367], was not phosphorylated. The higher activities of CT[delta 312-367] and CT[S315A] relative to CT were attributed to differences in the sensitivities of the enzymes to lipid activators. The rank order of the apparent Km values for activation by either phosphatidylcholine/oleic acid or phosphatidylcholine/diacylglycerol was CT > CT[S315A] > CT[delta 312-367]. In addition, CT exhibited negative cooperativity in its activation by phosphatidylcholine/oleic acid (nH = 0.64) and phosphatidylcholine/diacylglycerol (nH = 0.74) vesicles, whereas CT[delta 312-367] and CT[S315A] did not. These data support the concept that the phosphorylation of the CT carboxyl-terminal domain interferes with the activation of CT by lipid regulators.

Phosphorylation of CTP synthetase from Saccharomyces cerevisiae by protein kinase C

Phosphorylation of CTP synthetase (EC 6.3.4.2, UTP:ammonia ligase (ADP-forming)) from Saccharomyces cerevisiae protein kinase C was examined. Using pure CTP by synthetase as a substrate, protein kinase C activity was dose- and time-dependent and required calcium, diacylglycerol, and phosphatidylserine for full activation. Protein kinase C activity was also dependent on the concentration of CTP synthetase. Protein kinase C phosphorylated CTP synthetase on serine and threonine residues in vitro whereas the enzyme was primarily phosphorylated on serine residues in vivo. Phosphopeptide mapping analysis of CTP synthetase phosphorylated in vitro and in vivo indicated that the enzyme was phosphorylated on more than one site. Most of the phosphopeptides derived from CTP synthetase phosphorylated in vivo were the same as those derived from CTP synthetase phosphorylated by protein kinase C in vitro. The stoichiometry of the phosphorylation of native CTP synthetase was 0.4 mol of phosphate/mol of enzyme whereas the stoichiometry of the phosphorylation of alkaline phosphatase-treated CTP synthetase was 2.2 mol of phosphate/mol of enzyme. This indicated that CTP synthetase was purified in a phosphorylated state. Phosphorylation of CTP synthetase resulted in a 3-fold activation in enzyme activity whereas alkaline phosphatase treatment of CTP synthetase resulted in a 5-fold decrease in enzyme activity. Overall, the results reported here were consistent with the conclusion that CTP synthetase was regulated by protein kinase C phosphorylation.

Phosphatidylcholine turnover in activated human neutrophils. Agonist-induced cytidylyltransferase translocation is subsequent to phospholipase D activation

Phosphatidylcholine synthesis and degradation are tightly regulated to assure a constant amount of the phospholipid in cellular membranes. The chemotactic peptide fMLP and the phorbol ester, phorbol 12-myristate 13-acetate, are known to stimulate phosphatidylcholine degradation by phospholipase D in human neutrophils. fMLP alone triggered phosphatidylcholine breakdown into phosphatidic acid, but did not stimulate phosphatidylcholine synthesis or activation of the rate-limiting enzyme CTP:phosphocholine cytidylyltransferase. Adding cytochalasin B to fMLP led to some conversion of phosphatidic acid into diglyceride, and fMLP was then able to trigger choline incorporation into phosphatidylcholine, and cytidylyltransferase translocation from cytosol to membranes. Inhibition of phosphatidyl-choline-phospholipase D activation with tyrphostin led to inhibition of choline incorporation. Therefore, phosphatidic acid-derived diglyceride but not phosphatidic acid alone was effective to promote cytidylyltransferase translocation. With phorbol 12-myristate 13-acetate as agonist, and by selective labeling of phosphatidylinositol and phosphatidylcholine, we demonstrated that only phosphatidylcholine-derived diglyceride participated in cytidylyltransferase translocation. Oleic acid stimulated phosphatidylcholine synthesis, but induced a weak increase in diglyceride and a slight cytidylyltransferase translocation, and did not stimulate phospholipase D activity. Our data established that only diglyceride derived from phosphatidylcholine degradation by the phospholipase D/phosphatidate phosphatase pathway are required for agonist-induced cytidylyltransferase translocation and subsequent choline incorporation into phosphatidylcholine.

Lysophosphatidylcholine and 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine inhibit the CDP-choline pathway of phosphatidylcholine synthesis at the CTP:phosphocholine cytidylyltransferase step

The regulation of the CDP-choline pathway of phosphatidylcholine synthesis at the CTP:phosphocholine cytidylyltransferase (CT) step by lysophosphatidylcholine (LPC) and the nonhydrolyzable LPC analog, 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3), was investigated in a colony-stimulating factor 1-dependent murine macrophage cell line. LPC inhibited phosphatidylcholine synthesis in vivo and led to the accumulation of choline and phosphocholine coupled to the disappearance of CDP-choline pointing to CT as the intracellular target. LPC neither inhibited cell growth nor decreased the cellular content of CT or altered the distribution of CT between soluble and particulate subcellular fractions. The inhibition of phosphatidylcholine synthesis was specific for LPC since lysophospholipids lacking the choline headgroup were not inhibitors. ET-18-OCH3 was a more potent inhibitor of phosphatidylcholine synthesis than LPC and caused the translocation of CT from the soluble compartment to the particulate compartment. Both LPC and ET-18-OCH3 were inhibitors of CT activity in vitro and kinetic analysis showed competitive inhibition with respect to the lipid activator. These data point to LPC as a negative regulator of de novo phosphatidylcholine synthesis that acts at the CT step and establish the mechanism for the inhibition of phosphatidylcholine biosynthesis by antineoplastic phospholipids.

Cholecystokinin stimulates the down-regulation of CTP:phosphocholine cytidylyltransferase in pancreatic acinar cells

Stimulation of rat pancreatic acinar cells with cholecystokinin (CCK) is known to result in a significant inhibition of CTP:phosphocholine cytidylyltransferase (CT), a rate-limiting enzyme in phosphatidylcholine biosynthesis. Immunoprecipitation of CT from 32P-labeled acinar cells revealed that CCK treatment also caused a marked reduction in CT phosphate levels. The effects of CCK were maximal over 60 min and dependent on concentration, exhibiting an EC50 of 800 pM. Other calcium mobilizing secretagogues such as carbamylcholine (100 microM) and bombesin (10 nM) also reduced CT phosphate levels to 20 and 39% of control, respectively. Treatment of cells with thapsigargin and/or 12-O-tetradecanoyl-phorbol-13-acetate established that a combination of increased intracellular Ca2+ and protein kinase C activation was necessary to decrease phosphorylated CT content. Conversely, secretin (10 nM) or 8-(4-chlorophenylthio)-cAMP (100 microM) added alone had no effects. Use of the compound JMV-180 indicated CCK was acting through the low affinity state of the CCKA receptor to reduce CT phosphate levels. Further, the decrease in phosphorylated CT caused by CCK was blocked by the phosphatase inhibitors okadaic acid (3 microM) and calyculin A (100 nM). Finally, immunoblotting from whole cell lysates revealed CT was partially degraded in response to CCK, providing a novel mechanism by which the inhibition of CT enzyme activity occurs in response to the hormone. Moreover, this degradation was also blocked by a phosphatase inhibitor. These data suggest that the dephosphorylation of either CT itself or some other regulatory molecule(s) which mediates the CCK-induced protease activation may play a central role in reducing CT enzyme levels in acinar cells.

Identification of the nuclear localization signal of rat liver CTP:phosphocholine cytidylyltransferase

CTP:phosphocholine cytidylyltransferase (CT) is a major regulatory enzyme in phosphatidylcholine synthesis in mammalian cells. CT is found in both soluble and particulate forms, both of which are nuclear. We report here the identification of a 21-residue sequence at the amino terminus of CT, 8KVNSRKRRKEVPGPNGATEED28, which was sufficient to direct beta-galactosidase into the cell nucleus. Further deletions from either end of this sequence greatly reduced the nuclear localization of beta-galactosidase. Deletions of amino acids within the nuclear localization signal or of the entire signal disrupted CT nuclear localization, but CT was not completely excluded from the nucleus. Clones of stable transfectants of the nuclear localization signal-deficient CT expressed in Chinese hamster ovary (CHO) 58 cells, which is temperature-sensitive for growth and CT activity, were isolated and characterized. The deletion mutants were active under the same conditions as the wild-type enzyme. Despite the difference in subcellular location from wild-type CT, the nuclear localization mutants were fully able to complement the CT-deficient cell line CHO 58 for both growth and choline incorporation into phosphatidylcholine at the nonpermissive temperature. The mobility of the mutant enzymes on SDS gels was altered relative to the mobility of wild-type CT; however, the extent of phosphorylation of the mutant enzymes was decreased only slightly. Thus, the distribution of CT in both cytoplasm and nucleus, rather than exclusively nucleus, has little effect on the ability of CT to function in growing CHO cells.

Primary structure and expression of a human CTP:phosphocholine cytidylyltransferase

Human CTP:phosphocholine cytidylyltransferase (CT) cDNAs were isolated by PCR amplification of a human erythroleukemic K562 cell library. Initially two degenerate oligonucleotide primers derived from the sequence of the rat liver CT cDNA were used to amplify a centrally located 230 bp fragment. Subsequently overlapping 5' and 3' fragments were amplified, each using one human CT primer and one vector-specific primer. Two cDNAs encoding the entire translated domain were also amplified. The human CT (HCT) has close homology at the nucleotide and amino acid level with other mammalian CTs (from rat liver, mouse testis or mouse B6SutA hemopoietic cells and Chinese hamster ovary). The region which deviates most from the rat liver CT sequence is near the C-terminus, where 7 changes are clustered within 34 residues (345-359), of the putative phosphorylation domain. The region of the proposed catalytic domain (residues 75-235) is 100% identical with the rat liver sequence. Significant homology was observed between the proposed catalytic domain of CT and the Saccharomyces cerevisiae MUQ1 gene product, and between the proposed amphipathic alpha-helical membrane binding domains of CT and soybean oleosin, a phospholipid-binding protein. There are several shared characteristics of these amphipathic helices. An approx. 42,000 Da protein was over-expressed in COS cells using a pAX142 expression vector containing one of the full-length HCT cDNA clones. The specific activity of the HCT in COS cell homogenates was the same as that of analogously expressed rat liver CT. The activity of HCT was lipid dependent. The soluble form was activated 3 to 4-fold by anionic phospholipids and by oleic acid or diacylglycerol-containing PC vesicles.

Growth factors stimulate phosphorylation of CTP:phosphocholine cytidylyltransferase in HeLa cells

The effect of insulin and epidermal growth factor on the phosphorylation of CTP:phosphocholine cytidylyltransferase (EC 2.7.7.15) was investigated in HeLa cells. For the first time, cytidylyltransferase phosphorylation was shown to be influenced by growth factors in cell culture experiments. The rephosphorylation of cytidylyltransferase after an oleate-mediated dephosphorylation and translocation to membranes was increased after 2 min in the presence of insulin or epidermal growth factor by 99% and 76%, respectively, compared with controls. However, the increased phosphorylation of cytidylyltransferase did not have an effect on its subcellular distribution. Furthermore, purified cytidylyltransferase preincubated with alkaline phosphatase is a substrate for p44mapk, a member of the mitogen-activated protein (MAP) kinase family downstream of the growth factor receptors, in vitro. In accordance with the in vivo data, in vitro phosphorylation of cytidylyltransferase by p44mapk occurred after 2 min.