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PubMed1992

Li+ increases accumulation of inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate in cholinergically stimulated brain cortex slices in guinea pig, mouse and rat. The increases require inositol supplementation in mouse and rat but not in guinea pig.

Li+, beginning at a concentration as low as 1 mM, produced a time- and dose-dependent increase in accumulation of [3H]Ins(1,4,5)P3 and [3H]Ins(1,3,4,5)P4 in acetylcholine (ACh)-stimulated guinea-pig brain cortex slices prelabelled with [3H]inositol and containing 1 mM-inositol in the final incubation period. Similar results were obtained by mass measurement of samples incubated with 10 mM-Li+ by using a receptor-binding assay, although the percentage stimulation of Ins(1,4,5)P3 accumulation by Li+ was somewhat less by this assay. The increase in accumulation of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 by Li+ was absolutely dependent on the presence of ACh. In the absence of added inositol, 1-5 mM-Li+ produced smaller increases in Ins(1,4,5)P3, but the Li(+)-dependent increase in Ins(1,3,4,5)P4 was not as affected by inositol omission. In previous studies with cholinergically stimulated rat and mouse brain cortex slices, Li+ inhibited accumulation of Ins(1,4,5)P3 in rat and inhibited Ins(1,3,4,5)P4 accumulation in rat and mouse [Batty & Nahorski (1987) Biochem. J. 247, 797-800; Whitworth & Kendall (1988) J. Neurochem. 51, 258-265]. We found that Li+ inhibited both Ins(1,4,5)P3 and Ins(1,3,4,5)P4 accumulation in these species, but we could reverse this inhibition by adding 10-30 mM-inositol; we then observed a Li(+)-induced increase in Ins(1,4,5)P3 and Ins(1,3,4,5)P4. The species differences observed in the absence of supplemented inositol were explained by the fact that a much higher concentration of inositol was required to bring the Li(+)-elevated levels of CDP-diacylglycerol (CDPDG) down to baseline in the rat and mouse. These data suggest that inositol is more rate-limiting for phosphatidylinositol synthesis in the presence of Li+ in rat and mouse, which can account for the previous reports of inhibition of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 accumulation by this ion in these species. Thus, in all species examined. Li+ could be shown to increase accumulation of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 in cholinergically stimulated brain cortex slices if the slices were supplemented with sufficient inositol to bring the Li(+)-elevated level of CDPDG down to near baseline, as seen in the absence of Li+. In guinea-pig brain cortex slices, increases in Ins(1,4,5)P3 and Ins(1,3,4,5)P4 accumulation could then be seen at Li+ concentrations as low as 1 mM, which falls within the therapeutic range of plasma concentrations in the treatment of manic-depressive disorders. These observations may have therapeutic implications.

Animals↗
PubMed2003

Hybridomas expressing gammadelta T-cell receptors respond to cardiolipin and beta2-glycoprotein 1 (apolipoprotein H).

Hybridomas expressing murine gammadelta T-cell receptors were found to produce cytokines in response to cardiolipin (CL) and structurally related anionic phospholipids. This response required serum at concentrations related to the amount of CL in cultures. The purified serum factor, beta2-glycoprotein 1 (beta2-GP1) (apolipoprotein H), supported the CL response alone, whereas several other serum proteins and ovalbumin did not. beta2-GP1 is known to form complexes with anionic phospholipids, particularly CL, which are often recognized by pathological autoantibodies. We speculate that gammadelta T cells also recognize such complexes and that the hybridoma response reported here reflects this specificity.

Animals↗
PubMed1996

Inhibition of neuroblastoma cell phosphatidylinositol 3-kinase by CDP-diacylglycerol and phosphatidate.

Phosphatidylinositol (PI) 3-kinase is activated by a variety of agents, including various growth factors, and has been proposed to play a role in initiation of cell growth, proliferation, and differentiation. We here investigate the effect of various membrane lipids on PI 3-kinase immunopurified from human SH-SY5Y neuroblastoma cells. CDP-diacylglycerol (CDP-DAG) inhibited PI 3-kinase activity with an IC50 of 6 microM. Phosphatidate (PA) was also inhibitory (IC50 - 38 microM) as was lysophosphatidate. Neither DAG nor any of the other phospholipids examined affected PI 3-kinase activity. The results offer the possibility that CDP-DAG or PA at critical membrane sites may exert functionally significant metabolic regulation at the point of convergence of the PI 3-kinase-directed and the PI 4-kinase-directed phosphoinositide signal transduction pathways.

Cytidine Diphosphate Diglycerides↗
PubMed1996

Carbachol inhibits insulin-stimulated phosphatidylinositol 3-kinase activity in SH-SY5Y neuroblastoma cells.

SH-SY5Y human neuroblastoma cells express muscarinic M3 receptors as well as insulin receptors, thus offering the opportunity to investigate possible cross-talk following activation of two distinct intracellular signal transduction pathways that convert the precursor phosphatidylinositol (PI) to its 3' phosphate or its 4' phosphate, respectively. In this study, the effect of carbachol on insulin-stimulated PI 3-kinase (PI3K) activity was examined in SH-SY5Y cells. Insulin addition to the cell medium induced a 10-26-fold increase in anti-phosphotyrosine-immunoprecipitable PI3K activity. Preincubation with 1 mM carbachol inhibited the insulin-stimulated PI3K activity in a time-dependent manner, with half-maximal and maximal inhibition times of 4 and 15 min, respectively. Atropine blocked the inhibitory effect of carbachol. Although carbachol did not change the amount of 85-kDa subunit protein regulatory unit associated with tyrosine-phosphorylated proteins, either in control or in insulin-stimulated cells, it appears to decrease the amount of associated 110-kDa catalytic subunit protein in the latter instance. Because PI3K activity from SH-SY5Y cells has been shown to be inhibited in vitro in the presence of cytidine diphosphodiacylglycerol (CDP-DAG) or phosphatidate (PA), we examined the presence of these lipids in SH-SY5Y cells that had been treated with carbachol. Formation of both lipids was increased in a time-dependent manner following carbachol addition, and their increased levels are proposed to account for the observed in vivo inhibition of PI3K. Addition of the cell-permeable homologue didecanoyl-CDP-DAG to intact cells inhibited insulin-stimulated PI3K activity up to 75%, with an IC50 of 0.5 microM, a result that further supports a proposed lipid-mediated inhibition of PI3K. Exogenously added didecanoyl-PA, however, did not affect PI3K activity. The possibility that stimulation of the PI 4-kinase-mediated signal transduction pathway leads to down-regulation of the PI3K-mediated signal transduction pathway in vivo, via inhibition of PI3K by CDP-DAG or by other consequences of phosphoinositidase C-linked receptor activation, is discussed.

Carbachol↗
PubMed1998

Regulation of phosphatidylglycerophosphate synthase levels in Saccharomyces cerevisiae.

The PGS1 gene of Saccharomyces cerevisiae encodes phosphatidylglycerophosphate (PG-P) synthase. PG-P synthase activity is regulated by factors affecting mitochondrial development and through cross-pathway control by inositol. The molecular mechanism of this regulation was examined by using a reporter gene under control of the PGS1 gene promoter (PPGS1-lacZ). Gene expression subject to carbon source regulation was monitored both at steady-state level and during the switch between different carbon sources. Cells grown in a non-fermentable carbon source had beta-galactosidase levels 3-fold higher than those grown in glucose. A shift from glucose to lactate rapidly raised the level of gene expression, whereas a shift back to glucose had the opposite effect. In either a pgs1 null mutant or a rho mutant grown in glucose, PPGS1-lacZ expression was 30-50% of the level in wild type cells. Addition of inositol to the growth medium resulted in a 2-3-fold reduction in gene expression in wild type cells. In ino2 and ino4 mutants, gene expression was greatly reduced and was not subject to inositol regulation consistent with inositol repression being dependent on the INO2 and INO4 regulatory genes. PPGS1-lacZ expression was elevated in a cds1 null mutant in the presence or absence of inositol, indicating that the capacity to synthesize CDP-diacylglycerol affects gene expression. Lack of cardiolipin synthesis (cls1 null mutant) had no effect on reporter gene expression.

Carbon↗
PubMed2000

Regulation of the DPP1-encoded diacylglycerol pyrophosphate (DGPP) phosphatase by inositol and growth phase. Inhibition of DGPP phosphatase activity by CDP-diacylglyceron and activation of phosphatidylserine synthase activity by DGPP.

The regulation of the Saccharomyces cerevisiae DPP1-encoded diacylglycerol pyrophosphate (DGPP) phosphatase by inositol supplementation and growth phase was examined. Addition of inositol to the growth medium resulted in a dose-dependent increase in the level of DGPP phosphatase activity in both exponential and stationary phase cells. Activity was greater in stationary phase cells when compared with exponential phase cells, and the inositol- and growth phase-dependent regulations of DGPP phosphatase were additive. Analyses of DGPP phosphatase mRNA and protein levels, and expression of beta-galactosidase activity driven by a P(DPP1)-lacZ reporter gene, indicated that a transcriptional mechanism was responsible for this regulation. Regulation of DGPP phosphatase by inositol and growth phase occurred in a manner that was opposite that of many phospholipid biosynthetic enzymes. Regulation of DGPP phosphatase expression by inositol supplementation, but not growth phase, was altered in opi1Delta, ino2Delta, and ino4Delta phospholipid synthesis regulatory mutants. CDP-diacylglycerol, a phospholipid pathway intermediate used for the synthesis of phosphatidylserine and phosphatidylinositol, inhibited DGPP phosphatase activity by a mixed mechanism that caused an increase in K(m) and a decrease in V(max). DGPP stimulated the activity of pure phosphatidylserine synthase by a mechanism that increased the affinity of the enzyme for its substrate CDP-diacylglycerol. Phospholipid composition analysis of a dpp1Delta mutant showed that DGPP phosphatase played a role in the regulation of phospholipid metabolism by inositol, as well as regulating the cellular levels of phosphatidylinositol.

Amino Acid Sequence↗
PubMed2004

Regulation of phospholipid synthesis in the yeast cki1Delta eki1Delta mutant defective in the Kennedy pathway. The Cho1-encoded phosphatidylserine synthase is regulated by mRNA stability.

In the yeast Saccharomyces cerevisiae, the most abundant phospholipid phosphatidylcholine is synthesized by the complementary CDP-diacylglycerol and Kennedy pathways. Using a cki1Delta eki1Delta mutant defective in choline kinase and ethanolamine kinase, we examined the consequences of a block in the Kennedy pathway on the regulation of phosphatidylcholine synthesis by the CDP-diacylglycerol pathway. The cki1Delta eki1Delta mutant exhibited increases in the synthesis of phosphatidylserine, phosphatidylethanolamine, and phosphatidylcholine via the CDP-diacylglycerol pathway. The increase in phospholipid synthesis correlated with increased activity levels of the CDP-diacylglycerol pathway enzymes phosphatidylserine synthase, phosphatidylserine decarboxylase, phosphatidylethanolamine methyltransferase, and phospholipid methyltransferase. However, other enzyme activities, including phosphatidylinositol synthase and phosphatidate phosphatase, were not affected in the cki1Delta eki1Delta mutant. For phosphatidylserine synthase, the enzyme catalyzing the committed step in the pathway, activity was regulated by increases in the levels of mRNA and protein. Decay analysis of CHO1 mRNA indicated that a dramatic increase in transcript stability was a major component responsible for the elevated level of phosphatidylserine synthase. These results revealed a novel mechanism that controls phospholipid synthesis in yeast.

Biochemical Phenomena↗
PubMed2004

Regulation of phospholipid synthesis in Saccharomyces cerevisiae by zinc.

Zinc is an essential nutrient required for the growth and metabolism of eukaryotic cells. In this work, we examined the effects of zinc depletion on the regulation of phospholipid synthesis in the yeast Saccharomyces cerevisiae. Zinc depletion resulted in a decrease in the activity levels of the CDP-diacylglycerol pathway enzymes phosphatidylserine synthase, phosphatidylserine decarboxylase, phosphatidylethanolamine methyltransferase, and phospholipid methyltransferase. In contrast, the activity of phosphatidylinositol synthase was elevated in response to zinc depletion. The level of Aut7p, a marker for the induction of autophagy, was also elevated in zinc-depleted cells. For the CHO1-encoded phosphatidylserine synthase, the reduction in activity in response to zinc depletion was controlled at the level of transcription. This regulation was mediated through the UAS(INO) element and by the transcription factors Ino2p, Ino4p, and Opi1p that are responsible for the inositol-mediated regulation of UAS(INO)-containing genes involved in phospholipid synthesis. Analysis of the cellular composition of the major membrane phospholipids showed that zinc depletion resulted in a 66% decrease in phosphatidylethanolamine and a 29% increase in phosphatidylinositol. A zrt1Delta zrt2Delta mutant (defective in the plasma membrane zinc transporters Zrt1p and Zrt2p) grown in the presence of zinc exhibited a phospholipid composition similar to that of wild type cells depleted for zinc. These results indicated that a decrease in the cytoplasmic levels of zinc was responsible for the alterations in phospholipid composition.

Autophagy-Related Protein 8 Family↗
PubMedDate not supplied

Identification of minor metabolites of phospholipid signal molecules in [(32)P]P(i)-labelled platelets.

Incubation of blood platelets with (32)P-labelled inorganic phosphate for 60 min leads to incorporation of radioisotope mainly into phosphatidylinositol (PI), phosphatidylinositol-4-phosphate (PIP) and phosphatidyl-4,5-bisphosphate (PIP(2)) in resting platelets and into phosphatidic acid (PA) in activated platelets. Small amounts of other important phosphoinositide isomers also become labelled following platelet activation, among them the 3-phosphorylated derivatives. In addition, several other faintly labelled spots are visible on TLC separations. Three of these lipids have now been identified as lysophosphatidylinositol (lysoPI), lysophosphatidic acid (lysoPA) and CDP-diacylglcerol (CDP-DAG).[(32)P]LysoPI was present in resting and activated platelets, whereas [(32)P]lysoPA and [(32)P]CDP-DAG were observed only upon platelet activation. The phosphoinositide cycle turns over without accumulation of [(32)P]PA and [(32)P]CDP-DAG in resting platelets. A large increase (as much as 40-fold) in the steady-state level of [(32)P]PA is seen in thrombin-activated platelets. A slight increase in the steady-state levels of [(32)P]CDP-DAG is accompanied by a similar increase in [(32)P]PI and larger increases in [(32)P]PIP and [(32)P]PIP(2) (about 50%), which is indicative of a general increase in flux in the PPI cycle. Elevation of CDP-DAG levels is probably only a reflection of increased flux, whereas lysoPA and lysoPI have been reported to have diverse signalling functions in various cells.

Blood Platelets↗
PubMed1983

Cytidine metabolism in photoreceptor cells of the rat.

During brief (30-min) incubations, isolated rat retinas accumulated [3H]cytidine, converted it to cytidine triphosphate (CTP), and incorporated it into RNA and cytidine diphosphate-diacylglyceride (CDG), a phospholipid precursor of phosphatidylinositol (Pl). Labeled CTP, RNA, and CDG contents were found to be two- to three-fold higher in photoreceptor cells than in cells of the inner retina. Autoradiograms showed that, within photoreceptor cells, silver grains representing RNA were concentrated over the nuclei in dark and light, while silver grains representing CDG were concentrated over the inner segments only after incubation in dark. The formation of labeled CTP and the synthesis of RNA were enhanced in light, while labeled CDG levels became reduced in light concurrent with an increase in the incorporation of labeled inositol into Pl. The 3H-labeled CDG content, however, was increased two- to fourfold in light in the presence of actinomycin D, and autoradiograms show a heavy concentration of silver grains over the inner segments of photoreceptor cells. These findings establish a role for cytidine nucleotides in photoreceptor cell metabolism and in light-dependent increases in RNA and Pl synthesis. Furthermore, the observations indicate that a competition may exist in light for cytidine or CTP and suggest that availability of cytidine for CDG synthesis may have a regulatory role in Pl metabolism within the photoreceptor cells.

Animals↗
PubMed1995

Dependence of stimulus-transcription coupling on phospholipase D in agonist-stimulated pituitary cells.

Stimulation of phospholipase D activity is frequently observed during agonist activation of Ca(2+)-mobilizing receptors, but the cellular functions of this signaling pathway are not well defined. Pituitary gonadotrophs express Ca(2+)-mobilizing receptors for gonadotropin-releasing hormone (GnRH) and endothelin (ET), activation of which stimulates luteinizing hormone secretion and transient expression of c-fos. In pituitary cells and alpha T3-1 gonadotrophs, GnRH action was associated with both initial and sustained diacylglycerol (DG) production, whereas ET-1 induced only a transient DG response. Also, phospholipase D activity, estimated by the production of phosphatidylethanol from phosphatidylcholine in the presence of ethanol, was stimulated by GnRH but not ET-1. Such formation of phosphatidylethanol at the expense of phosphatidic acid (PA) during GnRH-induced activation of phospholipase D significantly reduced the production of PA, DG, and cytidine diphosphate diacylglycerol. Inhibition of PA-phosphohydrolase activity by propranolol also decreased GnRH-induced DG production and, in contrast to ethanol, increased PA and cytidine diphosphate diacylglycerol levels. The fall in DG production caused by ethanol and propranolol was accompanied by inhibition of GnRH-induced c-fos expression, whereas agonist-induced luteinizing hormone release was not affected. In contrast to their inhibitory actions on GnRH-induced early gene expression, neither ethanol nor propranolol affected ET-1-induced c-fos expression, or GnRH- and ET-1-induced inositol trisphosphate/Ca2+ signaling. These findings demonstrate that phospholipase D participates in stimulus-transcription but not stimulus-secretion coupling, and indicate that DG is the primary signal for this action.

Animals↗
PubMed1974

Phosphatidylglycerol synthesis in castor bean endosperm: kinetics, requirements, and intracellular localization.

The synthesis of phosphatidylglycerol in castor bean (Ricinus communis var. Hale) endosperm tissue was found to be located in both the endoplasmic reticulum and mitochondrial fractions separated on sucrose density gradients. The enzyme of both fractions attained maximum activity at 5 mm Mn(2+), 0.075% Triton X-100, and pH 7.3. The addition of dithiothreitol produced little effect, but sulfhydryl inhibitors reduced activity in both systems. Cytidine diphosphate-diglyceride exhibited an apparent Michaelis constant for the endoplasmic reticulum enzyme of 2.8 mum and for the mitochondrial enzyme of 2.0 mum; the maximum reaction rate was achieved at about 20 mum. For the second substrate, glycerol-phosphate, the apparent Michaelis constant for both fractions was about 50 mum and maximum velocity was reached at 400 mum. The specific activity of the mitochondrial enzyme was generally twice that of the endoplasmic reticulum.

Journal Article↗
PubMed1985

Purification of phosphatidylinositol synthetase from rat brain by CDP-diacylglycerol affinity chromatography and properties of the purified enzyme.

A purification procedure for rat brain phosphatidylinositol synthetase (PI synthetase; CDP-1,2-diacyl-sn-glycerol:myo-inositol 3-phosphatidyltransferase; EC 2.7.8.11) is described. The enzyme was purified 200-250-fold from the homogenate by solubilization with Triton X-100 from microsomal membranes and affinity chromatography on CDP-diacylglycerol-Sepharose. Elution of enzyme activity required the presence of Triton X-100, CDP-diacylglycerol, and either phosphatidylcholine or asolectin. The product that was obtained in 5-10% yield from whole brain and in 70% yield from the microsomal fraction contained three protein bands as determined by sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The final preparation contained levels of CDP-diacylglycerol hydrolase and CDP-diacylglycerol: sn-glycero-3-phosphate 3-phosphatidyltransferase activities that were less than 1% of PI synthetase activity. The purified enzyme displayed a pH optimum of 8.5-9.0, required either Mg2+ or Mn2+ and exhibited a Km of 4.6 mM for myo-inositol.

Animals↗
PubMed1993

Measurement of receptor-activated phosphoinositide turnover in rat brain: nonequivalence of inositol phosphate and CDP-diacylglycerol formation.

Two methods for the measurement of receptor-activated phosphoinositide turnover were evaluated for their degree of correspondence in slices of rat brain; they involved the Li(+)-dependent accumulations of either [3H]-inositol-labeled inositol phosphates or [3H]cytidine-labeled CDP-diacylglycerol. In contrast to the expectation that the ratio of these two responses would remain approximately constant, varying degrees of correspondence were obtained. The two extremes are exemplified by carbachol, which elicited large increases in both inositol phosphate and CDP-diacylglycerol labeling, and endothelin, which gave a robust inositol phosphate response with little or no accumulation of 3H-CDP-diacylglycerol. No instance of the presence of the latter response in the absence of 3H-inositol phosphate accumulation was observed. Measurement of 3H-CDP-diacylglycerol accumulation thus may add additional insight into the regulation of phosphoinositide turnover and the complex action of Li+.

Animals↗
PubMed1993

Lithium enhances muscarinic receptor-stimulated CDP-diacylglycerol formation in inositol-depleted SK-N-SH neuroblastoma cells.

The psychotherapeutic action of Li+ in brain has been proposed to result from the depletion of cellular inositol secondary to its block of inositol monophosphatase. This action is thought to slow phosphoinositide resynthesis, thereby attenuating stimulated phosphoinositidase-mediated signal transduction in affected cells. In the present study, the effect of Li+ on muscarinic receptor-stimulated formation of the immediate precursor of phosphatidylinositol, CDP-diacylglycerol (CDP-DAG), has been examined in human SK-N-SH neuroblastoma cells that have been cultured under conditions that alter the cellular content of myo-inositol. Resting neuroblastoma cells, like brain cells in vivo, were found to concentrate inositol from the culture medium, achieving an intracellular level of 60.0 +/- 4 nmol/mg of protein. The addition of carbachol to [3H]cytidine-prelabeled cells elicited a four- to fivefold increase in the accumulation of labeled CDP-DAG. This stimulated formation of [3H]CDP-DAG was completely blocked by the addition of 10 microM atropine, was not dependent on the presence of Li+, nor was it affected by co-incubation with myo-inositol. This result was in sharp contrast to findings in rat brain slices, in which carbachol-stimulated formation of [3H]CDP-DAG was potentiated approximately 10-fold by Li+ and substantially reduced by coincubation with inositol. The formation of [3H]CDP-DAG in labeled SK-N-SH cells by carbachol was both concentration and time dependent. The order of efficacy of muscarinic ligands in stimulating [3H]-CDP-DAG accumulation paralleled that established in these cells for inositol phosphate accumulation, i.e., carbachol > or = oxotremorine-M > bethanecol > or = arecoline > oxotremorine > pilocarpine.(ABSTRACT TRUNCATED AT 250 WORDS)

Atropine↗
PubMed1993

Noradrenaline stimulation unbalances the phosphoinositide cycle in rat cerebral cortical slices.

Muscarinic cholinergic and alpha 1-adrenoceptor-mediated stimulation of phosphoinositide hydrolysis in rat cerebral cortex were compared by measuring carbachol- and noradrenaline-induced accumulation of various intermediates of the phosphoinositide cycle. Unlike carbachol, noradrenaline in the presence of guanosine 5'-O-(3-thiotriphosphate) did not stimulate phospholipase C activity in brain cortical membranes. In cortical slices, the efficacy of noradrenaline to stimulate accumulation of 3H-inositol phosphates and [32P]phosphatidic acid was 2.5 to threefold that of carbachol. However, noradrenaline was less effective than carbachol in stimulating accumulation of [3H]CDP-diacylglycerol and resynthesis of phosphatidylinositol. This was not due to calcium inhibition of CTP:phosphatidate cytidyltransferase or to different lithium requirements for carbachol- and noradrenaline-stimulated accumulation of [3H]CDP-diacylglycerol. The noradrenaline-induced unbalance of the phosphoinositide cycle, which was most apparent at relatively high concentrations of calcium (2.5 mM) in the incubation buffer, was qualitatively reproduced with ionomycin. The use of the alpha 1a-subtype-selective adrenoceptor antagonists WB4101 and 5-methylurapidil revealed a single alpha 1a-like component mediating the effects of noradrenaline. Our results suggest that the primary mechanism for phospholipase C activation by brain alpha 1 adrenoceptors involves an increase in intracellular calcium concentration.

Animals↗
PubMed2004

Phospholipid metabolism regulated by a transcription factor sensing phosphatidic acid.

Cells regulate the biophysical properties of their membranes by coordinated synthesis of different classes of lipids. Here, we identified a highly dynamic feedback mechanism by which the budding yeast Saccharomyces cerevisiae can regulate phospholipid biosynthesis. Phosphatidic acid on the endoplasmic reticulum directly bound to the soluble transcriptional repressor Opi1p to maintain it as inactive outside the nucleus. After the addition of the lipid precursor inositol, this phosphatidic acid was rapidly consumed, releasing Opi1p from the endoplasmic reticulum and allowing its nuclear translocation and repression of target genes. Thus, phosphatidic acid appears to be both an essential ubiquitous metabolic intermediate and a signaling lipid.

Active Transport, Cell Nucleus↗