The N-terminal domain of rat liver carnitine palmitoyltransferase 1 contains an internal mitochondrial import signal and residues essential for folding of its C-terminal catalytic domain

Knockdown of carnitine palmitoyltransferase I (CPT1) reduces fat body lipid mobilization and resistance to starvation in the insect vector Rhodnius prolixus

The energy stored in fatty acids is essential for several critical activities of insects, such as embryogenesis, oviposition, and flight. Rhodnius prolixus is an obligatory hematophagous hemipteran and vector of Chagas disease, and it feeds infrequently on very large blood meals. As digestion slowly occurs, lipids are synthesized and accumulate in the fat body, mainly as triacylglycerol, in lipid droplets. Between feeding bouts, proper mobilization and oxidation of stored lipids are crucial for survival, and released fatty acids are oxidized by mitochondrial β-oxidation. Carnitine palmitoyl transferase I (CPT1) is the enzyme that catalyzes the first reaction of the carnitine shuttle, where the activated fatty acid, acyl-CoA, is converted to acyl-carnitine to be transported into the mitochondria. Here, we investigated the role of CPT1 in lipid metabolism and in resistance to starvation in Rhodnius prolixus. The expression of the CPT1 gene (RhoprCpt1) was determined in the organs of adult females on the fourth day after a blood meal, and the flight muscle showed higher expression levels than the ovary, fat body, and anterior and posterior midgut. RhoprCpt1 expression in the fat body dramatically decreased after feeding, and started to increase again 10 days later, but no changes were observed in the flight muscle. β-oxidation rates were determined in flight muscle and fat body homogenates with the use of ³H-palmitate, and in unfed females, they were higher in the flight muscle. In the fat body, lipid oxidation activity did not show any variation before or at different days after feeding, and was not affected by the presence of etomoxir or malonyl-CoA. We used RNAi and generated RhoprCPT1-deficient insects, which surprisingly did not show a decrease in measured ³H-palmitate oxidation rates. However, the RNAi-knockdown females presented increased amounts of triacylglycerol and larger lipid droplets in the fat body, but not in the flight muscle. When subjected to starvation, these insects had a shorter lifespan. These results indicated that the inhibition of RhoprCpt1 expression compromised lipid mobilization and affected resistance to starvation.

Mechanisms of CPT1C-Dependent AMPAR Trafficking Enhancement

In neurons, AMPA receptor (AMPAR) function depends essentially on their constituent components:the ion channel forming subunits and ion channel associated proteins. On the other hand, AMPAR trafficking is tightly regulated by a vast number of intracellular neuronal proteins that bind to AMPAR subunits. It has been recently shown that the interaction between the GluA1 subunit of AMPARs and carnitine palmitoyltransferase 1C (CPT1C), a novel protein partner of AMPARs, is important in modulating surface expression of these ionotropic glutamate receptors. Indeed, synaptic transmission in CPT1C knockout (KO) mice is diminished supporting a positive trafficking role for that protein. However, the molecular mechanisms of such modulation remain unknown although a putative role of CPT1C in depalmitoylating GluA1 has been hypothesized. Here, we explore that possibility and show that CPT1C effect on AMPARs is likely due to changes in the palmitoylation state of GluA1. Based on in silico analysis, Ser 252, His 470 and Asp 474 are predicted to be the catalytic triad responsible for CPT1C palmitoyl thioesterase (PTE) activity. When these residues are mutated or when PTE activity is inhibited, the CPT1C effect on AMPAR trafficking is abolished, validating the CPT1C catalytic triad as being responsible for PTE activity on AMPAR. Moreover, the histidine residue (His 470) of CPT1C is crucial for the increase in GluA1 surface expression in neurons and the H470A mutation impairs the depalmitoylating catalytic activity of CPT1C. Finally, we show that CPT1C effect seems to be specific for this CPT1 isoform and it takes place solely at endoplasmic reticulum (ER). This work adds another facet to the impressive degree of molecular mechanisms regulating AMPAR physiology.

Evolutionary analysis of the carnitine- and choline acyltransferases suggests distinct evolution of CPT2 versus CPT1 and related variants

Carnitine/choline acyltransferases play diverse roles in energy metabolism and neuronal signalling. Our knowledge of their evolutionary relationships, important for functional understanding, is incomplete. Therefore, we aimed to determine the evolutionary relationships of these eukaryotic transferases. We performed extensive phylogenetic and intron position analyses. We found that mammalian intramitochondrial CPT2 is most closely related to cytosolic yeast carnitine transferases (Sc-YAT1 and 2), whereas the other members of the family are related to intraorganellar yeast Sc-CAT2. Therefore, the cytosolically active CPT1 more closely resembles intramitochondrial ancestors than CPT2. The choline acetyltransferase is closely related to carnitine acetyltransferase and shows lower evolutionary rates than long chain acyltransferases. In the CPT1 family several duplications occurred during animal radiation, leading to the isoforms CPT1A, CPT1B and CPT1C. In addition, we found five CPT1-like genes in Caenorhabditis elegans that strongly group to the CPT1 family. The long branch leading to mammalian brain isoform CPT1C suggests that either strong positive or relaxed evolution has taken place on this node. The presented evolutionary delineation of carnitine/choline acyltransferases adds to current knowledge on their functions and provides tangible leads for further experimental research.

The Origin and Diversity of Cpt1 Genes in Vertebrate Species

The Carnitine palmitoyltransferase I (Cpt1) gene family plays a crucial role in energy homeostasis since it is required for the occurrence of fatty acid β-oxidation in the mitochondria. The exact gene repertoire in different vertebrate lineages is variable. Presently, four genes are documented: Cpt1a, also known as Cpt1a1, Cpt1a2; Cpt1b and Cpt1c. The later is considered a mammalian innovation resulting from a gene duplication event in the ancestor of mammals, after the divergence of sauropsids. In contrast, Cpt1a2 has been found exclusively in teleosts. Here, we reassess the overall evolutionary relationships of Cpt1 genes using a combination of approaches, including the survey of the gene repertoire in basal gnathostome lineages. Through molecular phylogenetics and synteny studies, we find that Cpt1c is most likely a rapidly evolving orthologue of Cpt1a2. Thus, Cpt1c is present in other lineages such as cartilaginous fish, reptiles, amphibians and the coelacanth. We show that genome duplications (2R) and variable rates of sequence evolution contribute to the history of Cpt1 genes in vertebrates. Finally, we propose that loss of Cpt1b is the likely cause for the unusual energy metabolism of elasmobranch.

A panoramic overview of mitochondria and mitochondrial redox biology

Mitochondria dysfunction was first described in the 1960s. However, the extent and mechanisms of mitochondria dysfunction’s role in cellular physiology and pathology has only recently begun to be appreciated. To adequately evaluate mitochondria-mediated toxicity, it is not only necessary to understand mitochondria biology, but discerning mitochondrial redox biology is also essential. The latter is intricately tied to mitochondrial bioenergetics. Mitochondrial free radicals, antioxidants, and antioxidant enzymes are players in mitochondrial redox biology. This review will provide an across-the-board, albeit not in-depth, overview of mitochondria biology and mitochondrial redox biology. With accumulating knowledge on mitochondria biology and mitochondrial redox biology, we may devise experimental methods with adequate sensitivity and specificity to evaluate mitochondrial toxicity, especially in vivo in living organisms, in the near future.

Molecular characterization, tissue distribution and kinetic analysis of carnitine palmitoyltransferase I in juvenile yellow catfish Pelteobagrus fulvidraco

Up to date, only limited information is available on genetically and functionally different isoforms of CPT I enzyme in fish. In the study, molecular characterization and their tissue expression profile of three CPT Iα isoforms (CPT Iα1a, CPT Iα1b and CPT Iα2a) and a CPT Iβ isoform from yellow catfish Pelteobagrus fulvidraco is determined. The activities and kinetic features of CPT I from several tissues have also been analyzed. The four CPT I isoforms in yellow catfish present distinct differences in amino acid sequences and structure. They are widely expressed in liver, heart, white muscle, spleen, intestine and mesenteric adipose tissue of yellow catfish at the mRNA level, but with the varying levels. CPT I activity and kinetics show tissue-specific differences stemming from co-expression of different isoforms, indicating more complex pathways of lipid utilization in fish than in mammals, allowing for precise control of lipid oxidation in individual tissue.

Packing of transmembrane domain 2 of carnitine palmitoyltransferase-1A affects oligomerization and malonyl-CoA sensitivity of the mitochondrial outer membrane protein

The purpose of this study was to investigate the sequence-dependence of oligomerization of transmembrane domain 2 (TM2) of rat carnitine palmitoyltransferase 1A (rCPT1A), to elucidate the role of this domain in the function of the full-length enzyme. Oligomerization of TM2 was studied qualitatively using complementary genetic assays that facilitate measurement of helix-helix interactions in the Escherichia coli inner membrane, and multiple quantitative biophysical methods. The effects of TM2-mutations on oligomerization and malonyl-CoA inhibition of the full-length enzyme (expressed in the yeast Pichia pastoris) were quantified. Changes designed to disrupt close-packing of the GXXXG(A) motifs reduced the oligomeric state of the corresponding TM2 peptides from hexamer to trimer (or lower), a reduction also observed on mutation of the TM2 sequence in the full-length enzyme. Disruption of these GXXXG(A) motifs had a parallel effect on the malonyl-CoA sensitivity of rCPT1A, reducing the IC(50) from 30.3 ± 5.0 to 3.0 ± 0.6 μM. For all measurements, wild-type rCPT1A was used as a control alongside various appropriate (e.g., molecular mass) standards. Our results suggest that sequence-determined, TM2-mediated oligomerization is likely to be involved in the modulation of malonyl-CoA inhibition of CPT1A in response to short- and long-term changes in protein-protein and protein-lipid interactions that occur in vivo.

Structural insight into function and regulation of carnitine palmitoyltransferase

The control of fatty acid translocation across the mitochondrial membrane is mediated by the carnitine palmitoyltransferase (CPT) system. Modulation of its functionality has simultaneous effects on fatty acid and glucose metabolism. This encourages use of the CPT system as drug target for reduction of gluconeogenesis and restoration of lipid homeostasis, which are beneficial in the treatment of type 2 diabetes mellitus and obesity. Recently, crystal structures of CPT-2 were determined in uninhibited forms and in complexes with inhibitory substrate-analogs with anti-diabetic properties in animal models and in clinical studies. The CPT-2 crystal structures have advanced understanding of CPT structure-function relationships and will facilitate discovery of novel inhibitors by structure-based drug design. However, a number of unresolved questions regarding the biochemistry and pharmacology of CPT enzymes remain and are addressed in this review.

Colicin N binds to the periphery of its receptor and translocator, outer membrane protein F

Colicins kill Escherichia coli after translocation across the outer membrane. Colicin N displays an unusually simple translocation pathway, using the outer membrane protein F (OmpF) as both receptor and translocator. Studies of this binary complex may therefore reveal a significant component of the translocation pathway. Here we show that, in 2D crystals, colicin is found outside the porin trimer, suggesting that translocation may occur at the protein-lipid interface. The major lipid of the outer leaflet interface is lipopolysaccharide (LPS). It is further shown that colicin N binding displaces OmpF-bound LPS. The N-terminal helix of the pore-forming domain, which is not required for pore formation, rearranges and binds to OmpF. Colicin N also binds artificial OmpF dimers, indicating that trimeric symmetry plays no part in the interaction. The data indicate that colicin is closely associated with the OmpF-lipid interface, providing evidence that this peripheral pathway may play a role in colicin transmembrane transport.

Rat Liver Carnitine Palmitoyltransferase 1 Forms an Oligomeric Complex within the Outer Mitochondrial Membrane

Carnitine palmitoyltransferase (CPT) 1A catalyzes the rate-limiting step in the transport of long chain acyl-CoAs from cytoplasm to the mitochondrial matrix by converting them to acylcarnitines. Located within the outer mitochondrial membrane, CPT1A activity is inhibited by malonyl-CoA, its allosteric inhibitor. In this study, we investigate for the first time the quaternary structure of rat CPT1A. Chemical cross-linking studies using intact mitochondria isolated from fed rat liver or from Saccharomyces cerevisiae expressing CPT1A show that CPT1A self-assembles into an oligomeric complex. Size exclusion chromatography experiments using solubilized mitochondrial extracts suggest that the fundamental unit of its quaternary structure is a trimer. When studied in blue native-PAGE, the CPT1A hexamer could be observed, however, suggesting that under these native conditions CPT1A trimers might be arranged as dimers. Moreover, the oligomeric state of CPT1A was found unchanged by starvation and by streptozotocin-induced diabetes, conditions characterized by changes in malonyl-CoA sensitivity of CPT1A. Finally, gel filtration analysis of several yeast-expressed chimeric CPTs demonstrates that the first 147 N-terminal residues of CPT1A, encompassing its two transmembrane segments, trigger trimerization independently of its catalytic C-terminal domain. Deletion of residues 1-82, including transmembrane 1, did not abrogate oligomerization, but the latter is limited to a trimer by the presence of the large catalytic C-terminal domain on the cytosolic face of mitochondria. Based on these findings, we proposed that the oligomeric structure of CPT1A would allow the newly formed acylcarnitines to gain direct access into the intermembrane space, hence facilitating substrate channeling.

Molecular basis for mitochondrial localization of viral particles during beet necrotic yellow vein virus infection

During infection, Beet necrotic yellow vein virus (BNYVV) particles localize transiently to the cytosolic surfaces of mitochondria. To understand the molecular basis and significance of this localization, we analyzed the targeting and membrane insertion properties of the viral proteins. ORF1 of BNYVV RNA-2 encodes the 21-kDa major coat protein, while ORF2 codes for a 75-kDa minor coat protein (P75) by readthrough of the ORF1 stop codon. Bioinformatic analysis highlighted a putative mitochondrial targeting sequence (MTS) as well as a major (TM1) and two minor (TM3 and TM4) transmembrane regions in the N-terminal part of the P75 readthrough domain. Deletion and gain-of-function analyses based on the localization of green fluorescent protein (GFP) fusions showed that the MTS was able to direct a reporter protein to mitochondria but that the protein was not persistently anchored to the organelles. GFP fused either to MTS and TM1 or to MTS and TM3-TM4 efficiently and specifically associated with mitochondria in vivo. The actual role of the individual domains in the interaction with the mitochondria seemed to be determined by the folding of P75. Anchoring assays to the outer membranes of isolated mitochondria, together with in vivo data, suggest that the TM3-TM4 domain is the membrane anchor in the context of full-length P75. All of the domains involved in mitochondrial targeting and anchoring were also indispensable for encapsidation, suggesting that the assembly of BNYVV particles occurs on mitochondria. Further data show that virions are subsequently released from mitochondria and accumulate in the cytosol.

Demonstration of N- and C-terminal domain intramolecular interactions in rat liver carnitine palmitoyltransferase 1 that determine its degree of malonyl-CoA sensitivity

We have previously proposed that changes in malonyl-CoA sensitivity of rat L-CPT1 (liver carnitine palmitoyltransferase 1) might occur through modulation of interactions between its cytosolic N- and C-terminal domains. By using a cross-linking strategy based on the trypsin-resistant folded state of L-CPT1, we have now shown the existence of such N-C (N- and C-terminal domain) intramolecular interactions both in wild-type L-CPT1 expressed in Saccharomyces cerevisiae and in the native L-CPT1 in fed rat liver mitochondria. These N-C intramolecular interactions were found to be either totally (48-h starvation) or partially abolished (streptozotocin-induced diabetes) in mitochondria isolated from animals in which the enzyme displays decreased malonyl-CoA sensitivity. Moreover, increasing the outer membrane fluidity of fed rat liver mitochondria with benzyl alcohol in vitro, which induced malonyl-CoA desensitization, attenuated the N-C interactions. This indicates that the changes in malonyl-CoA sensitivity of L-CPT1 observed in mitochondria from starved and diabetic rats, previously shown to be associated with altered membrane composition in vivo, are partly due to the disruption of N-C interactions. Finally, we show that mutations in the regulatory regions of the N-terminal domain affect the ability of the N terminus to interact physically with the C-terminal domain, irrespective of whether they increased [S24A (Ser24-->Ala)/Q30A] or abrogated (E3A) malonyl-CoA sensitivity. Moreover, we have identified the region immediately N-terminal to transmembrane domain 1 (residues 40-47) as being involved in the chemical N-C cross-linking. These observations provide the first demonstration by a physico-chemical method that L-CPT1 adopts different conformational states that differ in their degree of proximity between the cytosolic N-terminal and the C-terminal domains, and that this determines its degree of malonyl-CoA sensitivity depending on the physiological state.

Membrane transport of fatty acylcarnitine and free L-carnitine by rat liver microsomes

Recent studies have suggested that parts of the hepatic activities of diacylglycerol acyltransferase and acyl cholesterol acyltransferase are expressed in the lumen of the endoplasmic reticulum (ER). However the ER membrane is impermeable to the long-chain fatty acyl-CoA substrates of these enzymes. Liver microsomal vesicles that were shown to be at least 95% impermeable to palmitoyl-CoA were used to demonstrate the membrane transport of palmitoylcarnitine and free L-carnitine - processes that are necessary for an indirect route of provision of ER luminal fatty acyl-CoA through a luminal carnitine acyltransferase (CAT). Experimental conditions and precautions were established to permit measurement of the transport of [14C]palmitoylcarnitine into microsomes through the use of the luminal CAT and acyl-CoA:ethanol acyltransferase as a reporter system to detect formation of luminal [14C]palmitoyl-CoA. Rapid, unidirectional transport of free L-[3H]carnitine by microsomes was measured directly. This process, mediated either by a channel or a carrier, was inhibited by mersalyl but not by N-ethylmaleimide or sulfobetaine - properties that differentiate it from the mitochondrial inner membrane carnitine/acylcarnitine exchange carrier. These findings are relevant to the understanding of processes for the reassembly of triacylglycerols that lipidate very low density lipoprotein particles as part of a hepatic triacylglycerol lipolysis/re-esterification cycle.

Functional and structural basis of carnitine palmitoyltransferase 1A deficiency

Carnitine palmitoyltransferase 1A (CPT1A) is the key regulatory enzyme of hepatic long-chain fatty acid beta-oxidation. Human CPT1A deficiency is characterized by recurrent attacks of hypoketotic hypoglycemia. We presently analyzed at both the functional and structural levels five missense mutations identified in three CPT1A-deficient patients, namely A275T, A414V, Y498C, G709E, and G710E. Heterologous expression in Saccharomyces cerevisiae permitted to validate them as disease-causing mutations. To gain further insights into their deleterious effects, we localized these mutated residues into a three-dimensional structure model of the human CPT1A created from the crystal structure of the mouse carnitine acetyltransferase. This study demonstrated for the first time that disease-causing CPT1A mutations can be divided into two categories depending on whether they affect directly (functional determinant) or indirectly the active site of the enzyme (structural determinant). Mutations A275T, A414V, and Y498C, which exhibit decreased catalytic efficiency, clearly belong to the second class. They are located more than 20 A away from the active site and mostly affect the stability of the protein itself and/or of the enzyme-substrate complex. By contrast, mutations G709E and G710E, which abolish CPT1A activity, belong to the first category. They affect Gly residues that are essential not only for the structure of the hydrophobic core in the catalytic site, but also for the chain-length specificity of CPT isoforms. This study provides novel insights into the functionality of CPT1A that may contribute to the design of drugs for the treatment of lipid disorders.

Goliath, a ring-H2 mitochondrial protein, regulated by luteinizing hormone/human chorionic gonadotropin in rat leydig cells

We have cloned the rat homologue of the ring-H2 protein Goliath involved in Drosophila development. The rat Goliath mRNA (1.85 kb) was translated as a major ubiquitous protein species of 28-kDa and three larger isoforms (50, 46, and 36 kDa) expressed mainly in liver, lung, stomach, heart, and thymus and barely detectable in other tissues (kidney, skeletal muscle, brain, testis, intestine, and spleen). By immunohistochemistry on rat testis sections, we localized the protein in interstitial tissue and seminiferous tubules. In tubules, Goliath was expressed mainly in postmeiotic germ cells and to a much lesser extent in Sertoli cells. In the interstitium, Goliath was exclusively present in Leydig cells. Using a series of immunolabeling, cellular fractionation, and electron microscopy experiments, we established that Goliath is present in mitochondria of the R2C Leydig cell line. Using short-term hypophysectomized animals, we showed that Goliath is regulated by LH/hCG in Leydig cells but not in germ cells. This regulation in Leydig cells concerned only the 50-kDa isoform. This report is the first description of a differential regulation of the Goliath protein between germ cells and Leydig cells.

The liver isoform of carnitine palmitoyltransferase 1 is not targeted to the endoplasmic reticulum

Liver microsomal fractions contain a malonyl-CoA-inhibitable carnitine acyltransferase (CAT) activity. It has been proposed [Fraser, Corstorphine, Price and Zammit (1999) FEBS Lett. 446, 69-74] that this microsomal CAT activity is due to the liver form of carnitine palmitoyltransferase 1 (L-CPT1) being targeted to the endoplasmic reticulum (ER) membrane as well as to mitochondria, possibly by an N-terminal signal sequence [Cohen, Guillerault, Girard and Prip-Buus (2001) J. Biol. Chem. 276, 5403-5411]. COS-1 cells were transiently transfected to express a fusion protein in which enhanced green fluorescent protein was fused to the C-terminus of L-CPT1. Confocal microscopy showed that this fusion protein was localized to mitochondria, and possibly to peroxisomes, but not to the ER. cDNAs corresponding to truncated (amino acids 1-328) or full-length L-CPT1 were transcribed and translated in the presence of canine pancreatic microsomes. However, there was no evidence of authentic insertion of CPT1 into the ER membrane. Rat liver microsomal fractions purified by sucrose-density-gradient centrifugation contained an 88 kDa protein (p88) which was recognized by an anti-L-CPT1 antibody and by 2,4-dinitrophenol-etomoxiryl-CoA, a covalent inhibitor of L-CPT1. Abundance of p88 and malonyl-CoA-inhibitable CAT activity were increased approx. 3-fold by starvation for 24 h. Deoxycholate solubilized p88 and malonyl-CoA-inhibitable CAT activity from microsomes to approximately the same extent. The microsomal fraction contained porin, which, relative to total protein, was as abundant as in crude mitochondrial outer membranes fractions. It is concluded that L-CPT1 is not targeted to the ER membrane and that malonyl-CoA CAT in microsomal fractions is L-CPT1 that is derived from mitochondria, possibly from membrane contact sites.

The extreme C terminus of rat liver carnitine palmitoyltransferase I is not involved in malonyl-CoA sensitivity but in initial protein folding

We previously reported that the N-terminal domain (1-147 residues) of rat liver carnitine palmitoyltransferase I (L-CPTI) was essential for import into the outer mitochondrial membrane and for maintenance of a malonyl-CoA-sensitive conformation. Malonyl-CoA binding experiments using mitochondria of Saccharomyces cerevisiae strains expressing wild-type L-CPTI or previously constructed chimeric CPTs (Cohen, I., Kohl, C., McGarry, J.D., Girard, J., and Prip-Buus, C. (1998) J. Biol. Chem. 273, 29896-29904) indicated that the N-terminal domain was unable, independently of the C-terminal domain, to bind malonyl-CoA with a high affinity, suggesting that the modulation of malonyl-CoA sensitivity occurred through N/C intramolecular interactions. To assess the role of the C terminus in malonyl-CoA sensitivity, a series of C-terminal deletion mutants was generated. The kinetic properties of Delta772-773 and Delta767-773 deletion mutants were similar to those of L-CPTI, indicating that the last two highly conserved Lys residues in all known L-CPTI species were not functionally essential. By contrast, Delta743-773 deletion mutant was totally inactive and unfolded, as shown by its sensitivity to trypsin proteolysis. Because the C terminus of the native folded L-CPTI could be cleaved by trypsin without inducing protein unfolding, we concluded that the last 31 C-terminal residues constitute a secondary structural determinant essential for the initial protein folding of L-CPTI.

Mitochondrial targeting and membrane anchoring of a viral replicase in plant and yeast cells

Replication of the Carnation Italian ringspot virus genomic RNA in plant cells occurs in multivesicular bodies which develop from the mitochondrial outer membrane during infection. ORF1 in the viral genome encodes a 36-kDa protein, while ORF2 codes for the 95-kDa replicase by readthrough of the ORF1 stop codon. We have shown previously that the N-terminal part of ORF1 contains the information leading to vesiculation of mitochondria and that the 36-kDa protein localizes to mitochondria. Using infection, in vivo expression of green fluorescent protein fusions in plant and yeast cells, and in vitro mitochondrial integration assays, we demonstrate here that both the 36-kDa protein and the complete replicase are targeted to mitochondria and anchor to the outer membrane with the N terminus and C terminus on the cytosolic side. Analysis of deletion mutants indicated that the anchor sequence is likely to correspond approximately to amino acids 84 to 196, containing two transmembrane domains. No evidence for a matrix-targeting presequence was found, and the data suggest that membrane insertion of the viral proteins is mediated by an import receptor-independent signal-anchor mechanism relying on the two transmembrane segments and multiple recognition signals present in the N-terminal part of ORF1.

A novel brain-expressed protein related to carnitine palmitoyltransferase I

Malonyl-CoenzymeA acts as a fuel sensor, being both an intermediate of fatty acid synthesis and an inhibitor of the two known isoforms of carnitine palmitoyltransferase I (CPT I), which control mitochondrial fatty acid oxidation. We describe here a novel CPT1 family member whose mRNA is present predominantly in brain and testis. Chromosomal locations and genome organization are reported for the mouse and human genes. The protein sequence contains all the residues known to be important for both carnitine acyltransferase activity and malonyl-CoA binding in other family members. Yeast expressed protein has no detectable catalytic activity with several different acyl-CoA esters that are good substrates for other carnitine acyltransferases, including the liver isoform of CPT I, which is also expressed in brain; however, it displays high-affinity malonyl-CoA binding. Thus this new CPT I related protein may be specialized for the metabolism of a distinct class of fatty acids involved in brain function.

Structural and functional genomics of the CPT1B gene for muscle-type carnitine palmitoyltransferase I in mammals

Muscle-type carnitine palmitoyltransferase I (M-CPT I) is a key enzyme in the control of beta-oxidation of long-chain fatty acids in the heart and skeletal muscle. Because knowledge of the mammalian genes encoding M-CPT I may aid in studies of disturbed energy metabolism, we obtained new genomic and cDNA data for M-CPT I for the human, mouse, rat, and sheep. The introns of these compact genes are 80% (mouse versus rat) and 60% (mouse versus human) identical. Sheep and goat, but not cow, pig, rodent, or human promoter sequences contain a short interspersed repeated sequence (SINE) upstream of highly conserved regulatory elements. These elements constitute two promoters in humans, sheep, and mice, and, contrary to previous reports, there is a second promoter in rats as well. Thus, the transcriptional organization of these genes is more uniform than previously supposed, with interspecies differences in the 5'-ends of the mRNAs reflecting differences in splicing; only in humans extensive splicing and splice variation is found in the 5'- and 3'-untranslated regions. In the mouse, intron retention was detected in heart, muscle, and testes and may indicate an additional mechanism of regulation of M-CPT I expression. Splice variation in the coding region was previously proposed to lead to expression of CPT I enzymes with altered malonyl-CoA sensitivity (Yu, G. S., Lu, Y. C., and Gulick, T. (1998) Biochem. J. 334, 225-231). However, when expressed in the yeast Pichia pastoris, none of three earlier described splice variants had CPT I activity. Therefore, the involvement of splice variation of M-CPT I in the modulation of malonyl-CoA inhibition of fatty acid oxidation may be less relevant than hitherto assumed.