Fatty acyl-CoA-acyl-CoA-binding protein complexes activate the Ca2+ release channel of skeletal muscle sarcoplasmic reticulum

The Ryanodine Receptor as a Sensor for Intracellular Environments in Muscles

The ryanodine receptor (RyR) is a Ca²⁺ release channel in the sarcoplasmic reticulum of skeletal and cardiac muscles and plays a key role in excitation-contraction coupling. The activity of the RyR is regulated by the changes in the level of many intracellular factors, such as divalent cations (Ca²⁺ and Mg²⁺), nucleotides, associated proteins, and reactive oxygen species. Since these intracellular factors change depending on the condition of the muscle, e.g., exercise, fatigue, or disease states, the RyR channel activity will be altered accordingly. In this review, we describe how the RyR channel is regulated under various conditions and discuss the possibility that the RyR acts as a sensor for changes in the intracellular environments in muscles.

Comprehensive Characterization of Toxoplasma Acyl Coenzyme A-Binding Protein TgACBP2 and Its Critical Role in Parasite Cardiolipin Metabolism

Toxoplasma gondii is one of the most successful human parasites, infecting nearly one-third of the total world population. T. gondii tachyzoites residing within parasitophorous vacuoles (PVs) can acquire fatty acids both via salvage from host cells and via de novo synthesis pathways for membrane biogenesis. However, although fatty acid fluxes are known to exist in this parasite, how fatty acids flow through Toxoplasma lipid metabolic organelles, especially mitochondria, remains unknown. In this study, we demonstrated that Toxoplasma expresses an active ankyrin repeat containing protein TgACBP2 to coordinate cardiolipin metabolism. Specifically, HMA acquisition resulting from heterologous functional expression of MAF1 rescued growth and lipid metabolism defects in ACBP2-deficient type II parasites, manifesting the complementary role of host mitochondria in parasite cardiolipin metabolism. This work highlights the importance of TgACBP2 in parasite cardiolipin metabolism and provides evidence for metabolic association of host mitochondria with T. gondii.

Acyl coenzyme A (CoA)-binding protein (ACBP) can bind acyl-CoAs with high specificity and affinity, thus playing multiple roles in cellular functions. Mitochondria of the apicomplexan parasite Toxoplasma gondii have emerged as key organelles for lipid metabolism and signaling transduction. However, the rationale for how this parasite utilizes acyl-CoA-binding protein to regulate mitochondrial lipid metabolism remains unclear. Here, we show that an ankyrin repeat-containing protein, TgACBP2, is localized to mitochondria and displays active acyl-CoA-binding activities. Dephosphorylation of TgACBP2 is associated with relocation from the plasma membrane to the mitochondria under conditions of regulation of environmental [K⁺]. Under high [K⁺] conditions, loss of ACBP2 induced mitochondrial dysfunction and apoptosis-like cell death. Disruption of ACBP2 caused growth and virulence defects in the type II strain but not in type I parasites. Interestingly, mitochondrial association factor-1 (MAF1)-mediated host mitochondrial association (HMA) restored the growth ability of ACBP2-deficient type II parasites. Lipidomics analysis indicated that ACBP2 plays key roles in the cardiolipin metabolism of type II parasites and that MAF1 expression complemented the lipid metabolism defects of ACBP2-deficient type II parasites. In addition, disruption of ACBP2 caused attenuated virulence of Prugniuad (Pru) parasites for mice. Taking the results collectively, these data indicate that ACBP2 is critical for the growth and virulence of type II parasites and for the growth of type I parasites under high [K⁺] conditions.

Regulation of very-long acyl chain ceramide synthesis by acyl-CoA-binding protein

Ceramide and more complex sphingolipids constitute a diverse group of lipids that serve important roles as structural entities of biological membranes and as regulators of cellular growth, differentiation, and development. Thus, ceramides are vital players in numerous diseases including metabolic and cardiovascular diseases, as well as neurological disorders. Here we show that acyl-coenzyme A-binding protein (ACBP) potently facilitates very-long acyl chain ceramide synthesis. ACBP increases the activity of ceramide synthase 2 (CerS2) by more than 2-fold and CerS3 activity by 7-fold. ACBP binds very-long-chain acyl-CoA esters, which is required for its ability to stimulate CerS activity. We also show that high-speed liver cytosol from wild-type mice activates CerS3 activity, whereas cytosol from ACBP knock-out mice does not. Consistently, CerS2 and CerS3 activities are significantly reduced in the testes of ACBP^-/- mice, concomitant with a significant reduction in long- and very-long-chain ceramide levels. Importantly, we show that ACBP interacts with CerS2 and CerS3. Our data uncover a novel mode of regulation of very-long acyl chain ceramide synthesis by ACBP, which we anticipate is of crucial importance in understanding the regulation of ceramide metabolism in pathogenesis.

Long-chain acyl-CoA esters in metabolism and signaling: Role of acyl-CoA binding proteins

Long-chain fatty acyl-CoA esters are key intermediates in numerous lipid metabolic pathways, and recognized as important cellular signaling molecules. The intracellular flux and regulatory properties of acyl-CoA esters have been proposed to be coordinated by acyl-CoA-binding domain containing proteins (ACBDs). The ACBDs, which comprise a highly conserved multigene family of intracellular lipid-binding proteins, are found in all eukaryotes and ubiquitously expressed in all metazoan tissues, with distinct expression patterns for individual ACBDs. The ACBDs are involved in numerous intracellular processes including fatty acid-, glycerolipid- and glycerophospholipid biosynthesis, β-oxidation, cellular differentiation and proliferation as well as in the regulation of numerous enzyme activities. Little is known about the specific roles of the ACBDs in the regulation of these processes, however, recent studies have gained further insights into their in vivo functions and provided further evidence for ACBD-specific functions in cellular signaling and lipid metabolic pathways. This review summarizes the structural and functional properties of the various ACBDs, with special emphasis on the function of ACBD1, commonly known as ACBP.

Heat shock and structural proteins associated with meat tenderness in Nellore beef cattle, a Bos indicus breed

Nellore beef cattle, a Bos indicus (Zebu) breed, is well adapted to tropical conditions and has allowed Brazil to become one of the largest producers of red meat. Nevertheless, B. indicus breeds are reported to have less tender meat than Bos taurus. This study was designed to identify genes associated with meat tenderness and thus provides important information for breeding programs. A group of 138 animals was evaluated for longissimus thoracis muscle shear force (SF). Animals with the highest and lowest SF values (six animals each) were then selected for protein abundance studies. Samples were subjected to two-dimensional gel electrophoresis (2-DE) followed by peptide sequencing through mass spectrometry (MS) to identify differentially expressed proteins associated with SF values. Seventeen differentially expressed spots were observed (p<0.05) between the two groups. The 13 proteins identified included structural proteins (alpha actin-1, MLC1, MLC3, MLC2F and tropomyosin), related to cell organization (HSPB1 and HSP70), metabolism (beta-LG, ACBD6 and Complex III subunit I) and some uncharacterized proteins. Results confirm the existence of differentially expressed proteins associated with SF, which can lead to a better understanding of mechanisms involved in meat tenderness.

Malignant hyperthermia-like syndrome and carnitine palmitoyltransferase II deficiency with heterozygous R503C mutation

We describe a child who developed a malignant hyperthermia-like syndrome after exposure to succinylcholine and halothane. Many features of a typical malignant hyperthermia episode were present, including tachydysrhythmia, tachypnea, and fever in association with metabolic acidosis, hyperCKemia, myglobinemia, and rapid recovery without residual effects upon administration of dantrolene, sodium bicarbonate, and active cooling. Muscle rigidity, hypercarbia, and hyperkalemia were not observed. The patient was found to be heterozygous for a mutation in the carnitine palmitoyltransferase II gene (CPT2) encoding an arginine to cysteine substitution at amino acid 503 (R503C) with reduced activity of the enzyme.

Acyl-CoA-binding protein (ACBP) localizes to the endoplasmic reticulum and Golgi in a ligand-dependent manner in mammalian cells

In the present study, we microinjected fluorescently labelled liver bovine ACBP (acyl-CoA-binding protein) [FACI-50 (fluorescent acyl-CoA indicator-50)] into HeLa and BMGE (bovine mammary gland epithelial) cell lines to characterize the localization and dynamics of ACBP in living cells. Results showed that ACBP targeted to the ER (endoplasmic reticulum) and Golgi in a ligand-binding-dependent manner. A variant Y28F/K32A-FACI-50, which is unable to bind acyl-CoA, did no longer show association with the ER and became segregated from the Golgi, as analysed by intensity correlation calculations. Depletion of fatty acids from cells by addition of FAFBSA (fatty-acid-free BSA) significantly decreased FACI-50 association with the Golgi, whereas fatty acid overloading increased Golgi association, strongly supporting that ACBP associates with the Golgi in a ligand-dependent manner. FRAP (fluorescence recovery after photobleaching) showed that the fatty-acid-induced targeting of FACI-50 to the Golgi resulted in a 5-fold reduction in FACI-50 mobility. We suggest that ACBP is targeted to the ER and Golgi in a ligand-binding-dependent manner in living cells and propose that ACBP may be involved in vesicular trafficking.

Modulation of hSlo BK current inactivation by fatty acid esters of CoA

Lipid metabolism influences membrane proteins, including ion channels, in health and disease. Fatty acid esters of CoA are important intermediates in fatty acid metabolism and lipid biosynthesis. In the present study, we examined the effect of acyl-CoAs on hSlo BK currents. Arachidonoyl-CoA (C(20)-CoA) induced beta2-dependent inhibition of hSlo-alpha current when applied intracellularly but not extracellularly. This action was also mimicked by other long-chain acyl-CoAs such as oleoyl-CoA (C(18)-CoA) and palmitoyl-CoA (C(16)-CoA), but not acetyl-CoA (C(2)-CoA, shorter chain), suggesting that the length of acyl chains, rather than CoA headgroups, is critical. When hSlo-alpha inactivation was induced by a free synthetic cationic beta2 NH2-terminus inactivation ball peptide, long-chain acyl-CoAs inhibited hSlo-alpha current and facilitated inactivation. The precursor fatty acids also facilitated the ball peptide-induced inactivation in a chain length-dependent manner, whereas sphingosine (positively charged) slowed this inactivation. When the beta2-induced inactivation was compared with that of the ball peptide, there was a negative shift in the steady state inactivation, slower recovery, and a reduced voltage-dependence of inactivation onset. These data suggest that electrostatic interactions with the cytosolic inactivation domain of beta2 mediate acyl-CoA modulation of BK currents. BK channel inactivation may be a specific target for lipid modulation in physiological and pathophysiological conditions.

Acyl-CoA binding proteins; structural and functional conservation over 2000 MYA

Besides serving as essential substrates for beta-oxidation and synthesis of triacylglycerols and more complex lipids like sphingolipids and sterol esters, long-chain fatty acyl-CoA esters are increasingly being recognized as important regulators of enzyme activities and gene transcription. Acyl-CoA binding protein, ACBP, has been proposed to play a pivotal role in the intracellular trafficking and utilization of long-chain fatty acyl-CoA esters. Depletion of acyl-CoA binding protein in yeast results in aberrant organelle morphology incl. fragmented vacuoles, multi-layered plasma membranes and accumulation of vesicles of variable sizes. In contrast to synthesis and turn-over of glycerolipids, the levels of very-long-chain fatty acids, long-chain bases and ceramide are severely affected by Acb1p depletion, suggesting that Acb1p, rather than playing a general role, serves specific roles in cellular lipid metabolism.

Acyl coenzyme A-binding protein augments bid-induced mitochondrial damage and cell death by activating mu-calpain

Activation of calpain has been shown to occur in some contexts of cell injury and to be essential for loss of cell viability. Part of this may be mediated at the mitochondrial level. It has been demonstrated that calpain activity is necessary for the complete discharge of apoptosis-inducing factor from the mitochondrial intermembrane space and can cause the cleavage of full-length Bid to a more potent truncated form (Polster, B. M., Basanez, G., Etxebarria, A., Hardwick, J. M., and Nicholls, D. G. (2005) J. Biol. Chem. 280, 6447-6454). In this study, we identify acyl-CoA-binding protein (ACBP) as playing a critical role in the activation of calpain upon exposure of mitochondria to both full-length Bid and truncated Bid (t-Bid). Suppression of ACBP levels by small interfering RNA inhibited the t-Bid-induced activation of mitochondrial mu-calpain and release of apoptosis-inducing factor from the mitochondrial intermembrane space and the cleavage of full-length Bid to t-Bid. Moreover, ACBP required the presence of the peripheral benzodiazepine receptor (for which ACBP is a ligand) to be retained at the mitochondria, to activate mu-calpain, and to amplify Bid-induced mitochondrial damage.

Micro method for determination of nonesterified fatty acid in whole blood obtained by fingertip puncture

Diagnostic tools for early identification of subjects at high risk for type 2 diabetes and other obesity-related disorders are important in prevention of these diseases. Nonesterified fatty acids (NEFAs) have been suggested to serve as a prediagnostic marker of diabetes and obesity-related disorders. In the current study, we developed a sensitive and reproducible micro method for quantification of NEFA in less than 10 microl whole blood. The method involves only two steps: (i) conversion of NEFA to fatty acid acyl-coenzyme A (acyl-CoA) esters using an acyl-CoA synthetase and (ii) quantification of the formed acyl-CoA esters with a fluorescent biosensor based on bovine acyl-CoA binding protein (ACBP). Lys50 of ACBP was mutagenized to a cysteine residue that was covalently modified with 6-bromoacetyl-2-dimethylaminonaphthalene to make a fluorescent acyl-CoA indicator (FACI-50). FACI-50 exhibits high fluorescence emission yield with maximum at 490 nm in the presence of CoA when excited at 387 nm. The addition of palmitoyl-CoA to a CoA-saturated FACI-50 lowered fluorescence emission by eightfold. Ethanol extract from 1 microl whole blood was incubated with ATP, CoA, and FACI-50. Following background fluorescence reading, NEFAs were converted to acyl-CoA by the acyl-CoA synthetase and the NEFA content was calculated from fluorescence emission changes using palmitic acid as external standard. The FACI-50 NEFA method was compared with two commercially available methods for quantification of NEFA.

Acyl-coenzyme A binding protein expression alters liver fatty acyl-coenzyme A metabolism

Although studies in vitro and in yeast suggest that acyl-CoA binding protein ACBP may modulate long-chain fatty acyl-CoA (LCFA-CoA) distribution, its physiological function in mammals is unresolved. To address this issue, the effect of ACBP on liver LCFA-CoA pool size, acyl chain composition, distribution, and transacylation into more complex lipids was examined in transgenic mice expressing a higher level of ACBP. While ACBP transgenic mice did not exhibit altered body or liver weight, liver LCFA-CoA pool size increased by 69%, preferentially in saturated and polyunsaturated, but not monounsaturated, LCFA-CoAs. Intracellular LCFA-CoA distribution was also altered such that the ratio of LCFA-CoA content in (membranes, organelles)/cytosol increased 2.7-fold, especially in microsomes but not mitochondria. The increased distribution of specific LCFA-CoAs to the membrane/organelle and microsomal fractions followed the same order as the relative LCFA-CoA binding affinity exhibited by murine recombinant ACBP: saturated > monounsaturated > polyunsaturated C14-C22 LCFA-CoAs. Consistent with the altered microsomal LCFA-CoA level and distribution, enzymatic activity of liver microsomal glycerol-3-phosphate acyltransferase (GPAT) increased 4-fold, liver mass of phospholipid and triacylglyceride increased nearly 2-fold, and relative content of monounsaturated C18:1 fatty acid increased 44% in liver phospholipids. These effects were not due to the ACBP transgene altering the protein levels of liver microsomal acyltransferase enzymes such as GPAT, lysophosphatidic acid acyltransferase (LAT), or acyl-CoA cholesterol acyltransferase 2 (ACAT-2). Thus, these data show for the first time in a physiological context that ACBP expression may play a role in LCFA-CoA metabolism.

Regulation of Ca2+ movements by cyclovirobuxine D in ECV304 endothelial cells

In Fura-2/loaded ECV304 endothelial cells cyclovirobuxine D promoted a transient increase in cytosolic free Ca2+ originating from both an intracellular pool sensitive to the endoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin and the extracellular space. The intracellular pool was apparently different from that mobilized by other agents acting through IP3 generation. The integrity of the plasma membrane was an absolute requirement. In cells treated with digitonin, cyclovirobuxine D did not promote any Ca2+ release from the intracellular stores even at high concentrations and in the absence or presence of thapsigargin or sodium azide, the inhibitors of the endoplasmic reticular or mitochondrial Ca2+ uptake. Furthermore, cyclovirobuxine D was effective in halting the persistent increase in cytosolic Ca2+ caused by thapsigargin, inhibiting the operation of the "capacitative" Ca2+ membrane channels as demonstrated by the decrease in the extent of both Ca2+-overshoot and Mn2+ influx. Additional effects of cyclovirobuxine D included a depolarization of plasma membrane apparently related to an enhanced influx of Na+ from the extracellular space. The results obtained indicate that cyclovirobuxine D markedly affects intracellular Ca2+ homeostasis in ECV304 endothelial cells by both promoting a discharge of intracellular pools and by interfering with the operation of store-dependent channels via plasma membrane depolarization.

Role of palmitoylation/depalmitoylation reactions in G-protein-coupled receptor function

G-protein-coupled receptors (GPCRs) constitute one of the largest protein families in the human genome. They are subject to numerous post-translational modifications, including palmitoylation. This review highlights the dynamic nature of palmitoylation and its role in GPCR expression and function. The palmitoylation of other proteins involved in GPCR signaling, such as G-proteins, regulators of G-protein signaling, and G-protein-coupled receptor kinases, is also discussed.

Glutathione transport in the endo/sarcoplasmic reticulum

Glutathione transport through the endo/sarcoplasmic reticulum (ER/SR) membrane might play a role in the maintenance of the thiol redox potential difference between the lumen and the cytosol. The transport of glutathione (both GSH and glutathione disulfide, GSSG) is entirely different in the ER and SR membranes. The transport measurements based on either rapid filtration or light scattering techniques revealed that the SR membrane transports glutathione much faster than the hepatic ER membrane or microsomal membranes prepared from heart or brain. The fastest transport has been measured in the membrane of muscle terminal cisternae, which is enriched in ryanodine receptor type 1 (RyR1). All the studied membranes have been found to be equally impermeable to various hydrophilic substances of similar size to glutathione, thus the glutathione transport in muscle microsomes and terminal cysternae as well as the correlation between the rate of glutathione transport and the abundance of RyR1 are specific. In both muscle microsomes and terminal cysternae, glutathione influx can be either inhibited or activated by antagonists and agonists of the ryanodine receptor, respectively, while these agents do not influence the transport of other small permeant molecules. These findings strongly suggest that the ryanodine receptor channel activity is directly associated with glutathione transport activity in the skeletal muscle sarcoplasmic reticulum membrane.

Fluorescently labelled bovine acyl-CoA-binding protein acting as an acyl-CoA sensor: interaction with CoA and acyl-CoA esters and its use in measuring free acyl-CoA esters and non-esterified fatty acids

Long-chain acyl-CoA esters are key metabolites in lipid synthesis and beta-oxidation but, at the same time, are important regulators of intermediate metabolism, insulin secretion, vesicular trafficking and gene expression. Key tools in studying the regulatory functions of acyl-CoA esters are reliable methods for the determination of free acyl-CoA concentrations. No such method is presently available. In the present study, we describe the synthesis of two acyl-CoA sensors for measuring free acyl-CoA concentrations using acyl-CoA-binding protein as a scaffold. Met24 and Ala53 of bovine acyl-CoA-binding protein were replaced by cysteine residues, which were covalently modified with 6-bromoacetyl-2-dimethylaminonaphthalene to make the two fluorescent acyl-CoA indicators (FACIs) FACI-24 and FACI-53. FACI-24 and FACI-53 showed fluorescence emission maximum at 510 and 525 nm respectively, in the absence of ligand (excitation 387 nm). Titration of FACI-24 and FACI-53 with hexadecanoyl-CoA and dodecanoyl-CoA increased the fluorescence yield 5.5-and 4.7-fold at 460 and 495 nm respectively. FACI-24 exhibited a high, and similar increase in, fluorescence yield at 460 nm upon binding of C14-C20 saturated and unsaturated acyl-CoA esters. Both indicators bind long-chain (>C14) acyl-CoA esters with high specificity and affinity (K(d)=0.6-1.7 nM). FACI-53 showed a high fluorescence yield for C8-C12 acyl chains. It is shown that FACI-24 acts as a sensitive acyl-CoA sensor for measuring the concentration of free acyl-CoA, acyl-CoA synthetase activity and the concentrations of free fatty acids after conversion of the fatty acid into their respective acyl-CoA esters.

Acyl-CoA binding protein expression is fiber type- specific and elevated in muscles from the obese insulin-resistant Zucker rat

Accumulation of acyl-CoA is hypothesized to be involved in development of insulin resistance. Acyl-CoA binds to acyl-CoA binding protein (ACBP) with high affinity, and therefore knowledge about ACBP concentration is important for interpreting acyl-CoA data. In the present study, we used a sandwich enzyme-linked immunosorbent assay to quantify ACBP concentration in different muscle fiber types. Furthermore, ACBP concentration was compared in muscles from lean and obese Zucker rats. Expression of ACBP was highest in the slow-twitch oxidative soleus muscle and lowest in the fast-twitch glycolytic white gastrocnemius (0.46 +/- 0.02 and 0.16 +/- 0.005 microg/mg protein, respectively). Expression of ACBP was soleus > red gastrocnemius > extensor digitorum longus > white gastrocnemius. Similar fiber type differences were found for carnitine palmitoyl transferase (CPT)-1, and a correlation was observed between ACBP and CPT-1. Muscles from obese Zucker rats had twice the triglyceride content, had approximately twice the long-chain acyl CoA content, and were severely insulin resistant. ACBP concentration was approximately 30% higher in all muscles from obese rats. Activities of CPT-1 and 3-hydroxy-acyl-CoA dehydrogenase were increased in muscles from obese rats, whereas citrate synthase activity was similar. In conclusion, ACBP expression is fiber type-specific with the highest concentration in oxidative muscles and the lowest in glycolytic muscles. The 90% increase in the concentration of acyl-CoA in obese Zucker muscle compared with only a 30% increase in the concentration of ACBP supports the hypothesis that an increased concentration of free acyl-CoA is involved in the development of insulin resistance.

Ryanodine receptor channel-dependent glutathione transport in the sarcoplasmic reticulum of skeletal muscle

We found that glutathione transport across endo/sarcoplasmic reticulum membranes correlates with the abundance of ryanodine receptor type 1 (RyR1). The transport was the fastest in muscle terminal cisternae, fast in muscle microsomes and slow in liver, heart, and brain microsomes. Glutathione influx could be inhibited by RyR1 blockers and the inhibitory effect was counteracted by RyR1 agonists. The effect of blockers was specific to glutathione, as the transport of other small molecules was not hindered. Therefore, the glutathione transport activity seems to be associated with RyR1 in sarcoplasmic reticulum.

Depletion of acyl-coenzyme A-binding protein affects sphingolipid synthesis and causes vesicle accumulation and membrane defects in Saccharomyces cerevisiae

Deletion of the yeast gene ACB1 encoding Acb1p, the yeast homologue of the acyl-CoA-binding protein (ACBP), resulted in a slower growing phenotype that adapted into a faster growing phenotype with a frequency >1:10(5). A conditional knockout strain (Y700pGAL1-ACB1) with the ACB1 gene under control of the GAL1 promoter exhibited an altered acyl-CoA profile with a threefold increase in the relative content of C18:0-CoA, without affecting total acyl-CoA level as previously reported for an adapted acb1Delta strain. Depletion of Acb1p did not affect the general phospholipid pattern, the rate of phospholipid synthesis, or the turnover of individual phospholipid classes, indicating that Acb1p is not required for general glycerolipid synthesis. In contrast, cells depleted for Acb1p showed a dramatically reduced content of C26:0 in total fatty acids and the sphingolipid synthesis was reduced by 50-70%. The reduced incorporation of [(3)H]myo-inositol into sphingolipids was due to a reduced incorporation into inositol-phosphoceramide and mannose-inositol-phosphoceramide only, a pattern that is characteristic for cells with aberrant endoplasmic reticulum to Golgi transport. The plasma membrane of the Acb1p-depleted strain contained increased levels of inositol-phosphoceramide and mannose-inositol-phosphoceramide and lysophospholipids. Acb1p-depleted cells accumulated 50- to 60-nm vesicles and autophagocytotic like bodies and showed strongly perturbed plasma membrane structures. The present results strongly suggest that Acb1p plays an important role in fatty acid elongation and membrane assembly and organization.

Long-chain acylcarnitine induces Ca2+ efflux from the sarcoplasmic reticulum

Long-chain acylcarnitines increase intracellular Ca2+ (Ca2+i) and induce electrophysiologic alterations that likely contribute to the genesis of malignant ventricular arrhythmias induced during myocardial ischemia. The mechanisms by which long-chain acylcarnitines increase Ca2+i are not known, although it occurs in the presence of Ca2+ channel blockade and inhibition of Na+/Ca2+ exchange. Long-chain acylcarnitines activate Ca2+ release channels from skeletal muscle sarcoplasmic reticulum (SR), but their effect on cardiac SR is unclear. To test the hypothesis that long-chain acylcarnitines increase Ca2+i from the SR, SR-enriched membrane fractions were prepared from rabbit left ventricular myocardium using sucrose density-gradient centrifugation and characterized by marker enzyme analysis. 45Ca2+ efflux was assessed in the presence or absence of long-chain acylcarnitines. Palmitoylcarnitine and stearoylcarnitine produced concentration-dependent efflux of 45Ca2+, whereas shorter chain acylcarnitines, palmitate, and palmitoyl-coenzyme A did not. Pretreatment of cardiac SR vesicles with ryanodine did not prevent palmitoylcarnitine-induced Ca2+ release. In addition, palmitoylcarnitine did not influence specific [3H]ryanodine binding, suggesting a mechanism independent of alterations in ryanodine receptor/Ca2+ release channel binding. In summary, long-chain acylcarnitines enhance Ca2+ release from cardiac SR vesicles and may thereby mobilize Ca2+i to induce electrophysiologic derangements under conditions, such as ischemia, in which these amphiphiles accumulate.

Differential effects of acyl-CoA binding protein on enzymatic and non-enzymatic thioacylation of protein and peptide substrates

Both enzymatic and autocatalytic mechanisms have been proposed to account for protein thioacylation (commonly known as palmitoylation). Acyl-CoA binding proteins (ACBP) strongly suppress non-enzymatic thioacylation of cysteinyl-containing peptides by long-chain acyl-CoAs. At physiological concentrations of ACBP, acyl-CoAs, and membrane lipids, the rate of spontaneous acylation is expected to be too slow to contribute significantly to thioacylation of signaling proteins in mammalian cells (Leventis et al., Biochemistry 36 (1997) 5546-5553). Here we characterized the effects of ACBP on enzymatic thioacylation. A protein S-acyltransferase activity previously characterized using G-protein alpha-subunits as a substrate (Dunphy et al., J. Biol. Chem., 271 (1996) 7154-7159), was capable of thioacylating short lipid-modified cysteinyl-containing peptides. The minimum requirements for substrate recognition were a free cysteine thiol adjacent to a hydrophobic lipid anchor, either myristate or farnesyl isoprenoid. PAT activity displayed specificity for the acyl donor, efficiently utilizing long-chain acyl-CoAs, but not free fatty acid or S-palmitoyl-N-acetylcysteamine. ACBP only modestly inhibited enzymatic thioacylation of a myristoylated peptide or G-protein alpha-subunits under conditions where non-enzymatic thioacylation was reduced to background. Thus, protein S-acyltransferase remains active in the presence of physiological concentrations of ACBP and acyl-CoA in vitro and is likely to represent the predominant mechanism of thioacylation in vivo.

Role of acyl-CoA binding protein in acyl-CoA metabolism and acyl-CoA-mediated cell signaling

Long-chain acyl-CoA esters (LCA) act both as substrates and intermediates in metabolism and as regulators of various intracellular functions. Acyl-CoA binding protein (ACBP) binds LCA with high affinity and is believed to play an important role in intracellular acyl-CoA transport and pool formation and therefore also for the function of LCA as metabolites and regulators of cellular functions . The free concentration of cytosolic LCA is efficiently buffered to low nanomole concentration by ACBP and fatty acid binding protein (FABP). An additional important factor is the activity of acyl-CoA hydrolases. The estimated cellular free LCA concentration is two to four orders of magnitude lower than the concentrations reported to be necessary to regulate most LCA-affected cellular functions. Preliminary evidence indicates that the regulatory effect of LCA might be mediated by the LCA/ACBP complex.

Acyl-CoA-binding protein is a potent m-calpain activator

Acyl-CoA-binding protein, a 20-kDa homodimer that exerts many physiological functions, promotes activation of the classic calpain forms, most markedly that of the m-isozyme. This protein factor was purified from rat skeletal muscle and was also expressed in Escherichia coli. Both native and recombinant acyl-CoA-binding proteins show the same molecular properties and an identical capacity to decrease the [Ca(2+)] required for m-calpain activity. The binding of long-chain acyl-CoAs to acyl-CoA-binding protein does not modify the activating effect on calpains. Acyl-CoA-binding protein seems to be involved in the m-calpain regulation process, whereas the previously identified UK114 activator is a specific modulator of micro-calpain. Acyl-CoA-binding protein is proposed as a new component of the Ca(2+)-dependent proteolytic system. A comparative analysis among levels of classic calpains and their activator proteins is also reported.

Acyl-coenzyme A binding protein (ACBP)

Acyl-coenzyme A binding proteins are known from a large group of eukaryote species and to bind a long chain length acyl-CoA ester with very high affinity. Detailed biochemical mapping of ligand binding properties has been obtained as well as in-depth structural studies on the bovine apo-protein and of the complex with palmitoyl-CoA using NMR spectroscopy. In the four alpha-helix bundle structure, a set of 21 highly conserved residues present in more that 90% of all known sequences of acyl-coenzyme A binding proteins constitutes three separate mini-cores. These residues are predominantly located at the helix-helix interfaces. From studies of a large set of mutant proteins the role of the conserved residues has been related to structure, function, folding and stability.

Rapamycin inhibits activation of ryanodine receptors from skeletal muscle by the fatty acyl CoA-acyl CoA binding protein complex

We previously showed (Fulceri et al., Biochem. J. 325, 423, 1997) that the fatty acyl CoA ester palmitoyl CoA (PCoA) complexed with a molar excess of its cytosolic binding protein (ACBP) causes a discrete Ca(2+) efflux or allows Ca(2+) release by suboptimal caffeine concentrations, in the Ca(2+)-preloaded terminal cisternae fraction (TC) from rabbit skeletal muscle, by activating ryanodine receptor Ca(2+) release channels (RyRC). We show here that both effects were abolished by pretreating TC with the FKBP12 ligand rapamycin (20 microM). Moreover, rapamycin reversed the Ca(2+) release induced by combined treatment with 3 mM caffeine and the PCoA-ACBP complex. Rapamycin also reduced the Ca(2+)-releasing activity by PCoA alone. Under the above experimental conditions, rapamycin removed FKBP12 from the TC membranes, as revealed by Western blot analysis. We conclude that FKBP12 associated with RyRC in the TC membrane participates in the activation of the Ca(2+) channel by fatty acyl CoA esters.

Acyl-coenzyme A causes Ca2+ release in pancreatic acinar cells

The regulation of cytosolic Ca2+ is important for a variety of cell functions. One non-inositol 1,4,5-trisphosphate (IP3) compound that may regulate Ca2+ is palmitoyl-coenzyme A (CoA), a fatty acid-CoA that is reported to cause Ca2+ release from intracellular stores of oocytes, myocytes, and hepatocytes. To study the role of palmitoyl-CoA in the pancreatic acinar cell, rat pancreatic acini were isolated by collagenase digestion, permeablized with streptolysin O, and the release of Ca2+ from internal stores was measured with fura-2. Palmitoyl-CoA released Ca2+ from internal stores (EC50 = 14 microM). The palmitoyl-CoA-sensitive pool was distinct from, and overlapping with the IP3-sensitive Ca2+ pool. The effects of submaximal doses of IP3 or cyclic ADP-ribose plus palmitoyl-CoA were additive. Fatty acid-CoA derivatives with carbon chain lengths of 16-18 were the most potent and efficacious. Ryanodine and caffeine or elevated resting [Ca2+] sensitized the Ca2+ pool to the actions of palmitoyl-CoA. Fatty acid-CoA levels in pancreatic acini were measured by extraction with 2-propanol/acetonitrile, followed by separation and quantification using reverse phase high performance liquid chromatography, and were found to be 10.17 +/- 0.93 nmol/mg protein. These data suggest the presence of an IP3-insensitive palmitoyl-CoA-sensitive Ca2+ store in pancreatic acinar cells and suggest that palmitoyl-CoA may be needed for Ca2+-induced Ca2+ release.