Mammalian myristoyl CoA: protein N-myristoyltransferase

The role of N-myristoyltransferase 1 in tumour development

N-myristoyltransferase 1 (NMT1) is an indispensable eukaryotic enzyme that catalyses the transfer of myristoyl groups to the amino acid terminal residues of numerous proteins. This catalytic process is required for the growth and development of many eukaryotes and viruses. Elevated expression and activity of NMT1 is observed to varying degrees in a variety of tumour types (e.g. colon, lung and breast tumours). Furthermore, an elevated level of NMT1 in tumours is associated with poor survival. Therefore, a relationship exists between NMT1 and tumours. In this review, we discuss the underlying mechanisms by which NMT1 is associated with tumour development from the perspective of oncogene signalling, involvement in cellular metabolism, and endoplasmic reticulum stress. Several NMT inhibitors used in cancer treatment are introduced. The review will provide some directions for future research.Key MessagesElevated expression and activity of NMT1 is observed to varying degrees in a variety of tumour types which creates the possibility of targeting NMT1 in tumours.NMT1-mediated myristoylation plays a pivotal role in cancer cell metabolism and may be particularly relevant to cancer metastasis and drug resistance. These insights can be used to direct potential therapeutic avenues for NMT1 inhibitors.

Protein N-myristoylation: functions and mechanisms in control of innate immunity

Protein N-myristoylation is an important fatty acylation catalyzed by N-myristoyltransferases (NMTs), which are ubiquitous enzymes in eukaryotes. Specifically, attachment of a myristoyl group is vital for proteins participating in various biological functions, including signal transduction, cellular localization, and oncogenesis. Recent studies have revealed unexpected mechanisms indicating that protein N-myristoylation is involved in host defense against microbial and viral infections. In this review, we describe the current understanding of protein N-myristoylation (mainly focusing on myristoyl switches) and summarize its crucial roles in regulating innate immune responses, including TLR4-dependent inflammatory responses and demyristoylation-induced innate immunosuppression during Shigella flexneri infection. Furthermore, we examine the role of myristoylation in viral assembly, intracellular host interactions, and viral spread during human immunodeficiency virus-1 (HIV-1) infection. Deeper insight into the relationship between protein N-myristoylation and innate immunity might enable us to clarify the pathogenesis of certain infectious diseases and better harness protein N-myristoylation for new therapeutics.

Myristoylation of EV71 VP4 is Essential for Infectivity and Interaction with Membrane Structure

The Enterovirus 71 (EV71) VP4 is co-translationally linked to myristic acid at its amino-terminal glycine residue. However, the role of this myristoylation in the EV71 life cycle remains largely unknown. To investigate this issue, we developed a myristoylation-deficient virus and reporter (luciferase) pseudovirus with a Gly-to-Ala mutation (G2A) on EV71 VP4. When transfecting the EV71-G2A genome encoding plasmid in cells, the loss of myristoylation on VP4 did not affect the expression of viral proteins and the virus morphology, however, it did significantly influence viral infectivity. Further, in myristoylation-deficient reporter pseudovirus-infected cells, the luciferase activity and viral genome RNA decreased significantly as compared to that of wild type virus; however, cytopathic effect and viral capsid proteins were not detected in myristoylation-deficient virus-infected cells. Also, although myristoylation-deficient viral RNA and proteins were detected in the second blind passage of infection, they were much fewer in number compared to that of the wild type virus. The replication of genomic RNA and negative-strand viral RNA were both blocked in myristoylation-deficient viruses, suggesting that myristoylation affects viral genome RNA release from capsid to cytoplasm. Besides, loss of myristoylation on VP4 altered the distribution of VP4-green fluorescent protein protein, which disappeared from the membrane structure fraction. Finally, a liposome leakage assay showed that EV71 myristoylation mediates the permeability of the model membrane. Hence, the amino-terminal myristoylation of VP4 is pivotal to EV71 infection and capsid-membrane structure interaction. This study provides novel molecular mechanisms regarding EV71 infection and potential molecular targets for antiviral drug design.

Protein lipid modifications--More than just a greasy ballast

Covalent lipid modifications of proteins are crucial for regulation of cellular plasticity, since they affect the chemical and physical properties and therefore protein activity, localization, and stability. Most recently, lipid modifications on proteins are increasingly attracting important regulatory entities in diverse signaling events and diseases. In all cases, the lipid moiety of modified proteins is essential to allow water-soluble proteins to strongly interact with membranes or to induce structural changes in proteins that are critical for elemental processes such as respiration, transport, signal transduction, and motility. Until now, roughly about ten lipid modifications on different amino acid residues are described at the UniProtKB database and even well-known modifications are underrepresented. Thus, it is of fundamental importance to develop a better understanding of this emerging and so far under-investigated type of protein modification. Therefore, this review aims to give a comprehensive and detailed overview about enzymatic and nonenzymatic lipidation events, will report their role in cellular biology, discuss their relevancy for diseases, and describe so far available bioanalytical strategies to analyze this highly challenging type of modification.

The myristoylated amino-terminus of an Arabidopsis calcium-dependent protein kinase mediates plasma membrane localization

Calcium-dependent protein kinases (CDPK) are a major group of calcium-stimulated kinases found in plants and some protists. Many CDPKs are membrane-associated, presumably because of lipid modifications at their amino termini. We investigated the subcellular location and myristoylation of AtCPK5, a member of the Arabidopsis CDPK family. Most AtCPK5 was associated with the plasma membrane as demonstrated by two-phase fractionation of plant microsomes and by in vivo detection of AtCPK5-GFP fusion proteins. AtCPK5 was a substrate for plant N-myristoyltransferase and myristoylation was prevented by converting the glycine at the proposed site of myristate attachment to alanine (G2A). In transgenic plants, a G2A mutation completely abolished AtCPK5 membrane association, indicating that myristoylation was essential for membrane binding. The first sixteen amino acids of AtCPK5 were sufficient to direct plasma membrane localization. In addition, differentially phosphorylated forms of AtCPK5 were detected both in planta and after expression of AtCPK5 in a cell-free plant extract. Our results demonstrate that AtCPK5 is myristoylated at its amino terminus and that myristoylation is required for membrane binding.

Protein myristoylation in health and disease

N-myristoylation is the attachment of a 14-carbon fatty acid, myristate, onto the N-terminal glycine residue of target proteins, catalysed by N-myristoyltransferase (NMT), a ubiquitous and essential enzyme in eukaryotes. Many of the target proteins of NMT are crucial components of signalling pathways, and myristoylation typically promotes membrane binding that is essential for proper protein localisation or biological function. NMT is a validated therapeutic target in opportunistic infections of humans by fungi or parasitic protozoa. Additionally, NMT is implicated in carcinogenesis, particularly colon cancer, where there is evidence for its upregulation in the early stages of tumour formation. However, the study of myristoylation in all organisms has until recently been hindered by a lack of techniques for detection and identification of myristoylated proteins. Here we introduce the chemistry and biology of N-myristoylation and NMT, and discuss new developments in chemical proteomic technologies that are meeting the challenge of studying this important co-translational modification in living systems.

Expression and activities of N-myristoyltransferase and calcineurin in normal and inflamed lungs

The role of N-myristoyltransferase and calcineurin is well established in signaling pathways. However, there are no data on their expression and activities in normal and inflamed lungs. The mechanisms of lung inflammation induced following administration of lipopolysaccharides (LPS) or exposure to swine barn air remain unclear. Therefore, we examined expression and activities of N-myristoyltransferase and calcineurin in normal and inflamed lungs of rats. Histopathology showed acute inflammation in the lungs of rats exposed to barn air or LPS but not of control rats. There was no difference in the activities of N-myristoyltransferase and calcineurin among the control, barn-exposed, and LPS-treated rat lungs. Although N-myristoyltransferase and calcineurin were localized in airway epithelium, blood vessel walls, alveolar macrophages, and septa in the lungs of rats from all the groups, the staining intensity was increased in the lungs from rats exposed to intravenous LPS or barn air. Densitometric analyses of Western blots of 55- and 60-kDa polypeptide bands corresponding to N-myristoyltransferase and calcineurin, respectively, in the lung homogenates revealed no differences among the groups. These results show that expression of myristoyltransferase and calcineurin in lung epithelium and endothelium and a cell-specific increase in immunohistochemical expression.

Biochemical characterization of bovine brain myristoyl-CoA:protein N-myristoyltransferase type 2

Protein N-myristoylation is a lipidic modification which refers to the covalent attachment of myristate, a 14-carbon saturated fatty acid, to the N-terminal glycine residue of a number of mammalian, viral, and fungal proteins. In this paper, we have cloned the gene coding for myristoyl-CoA:protein N-myristoyltransferase (NMT) from Bos tarus brain. The open reading frame codes for a 410-amino-acid protein and overexpressed in Escherichia coli. Kinetic studies suggested that bovine brain NMT2 and human NMT1 show significant differences in their peptide substrate specificities. The metal ion Ca(2+) had stimulatory effects on NMT2 activity while Mn(2+) and Zn(2+) inhibited the enzyme activity. In addition, NMT2 activity was inhibited by various organic solvents and other detergents while NMT1 had a stimulatory effect. Biochemical characterization suggested that both forms of NMT have unique characteristics. Further analysis towards functional role NMT2 will lead the development of therapeutic target for the progression of various diseases such as cancer, cardiovascular diseases, and neurodegenerative diseases.

Potential role of N-myristoyltransferase in cancer

Colorectal cancer is the second leading cause of malignant death, and better preventive strategies are needed. The treatment of colonic cancer remains difficult because of the lack of effective chemotherapeutic agents; therefore it is important to continue to search for cellular functions that can be disrupted by chemotherapeutic drugs resulting in the inhibition of the development and progression of cancer. The current knowledge of the modification of proteins by myristoylation involving myristoyl-CoA: protein N-myristoyltransferase (NMT) is in its infancy. This process is involved in the pathogenesis of cancer. We have reported for the first time that NMT activity and protein expression were higher in human colorectal cancer, gallbladder carcinoma and brain tumors. In addition, an increase in NMT activity appeared at an early stage in colonic carcinogenesis. It is conceivable therefore that NMT can be used as a potential marker for the early detection of cancer. These observations lead to the possibility of developing NMT specific inhibitors, which may be therapeutically useful. We proposed that HSC70 and/or enolase could be used as an anticancer therapeutic target. This review summarized the status of NMT in cancer which has been carried in our laboratory.

Myristoyl-CoA:protein N-myristoyltransferase, an essential enzyme and potential drug target in kinetoplastid parasites

Co-translational modification of eukaryotic proteins by N-myristoylation aids subcellular targeting and protein-protein interactions. The enzyme that catalyzes this process, N-myristoyltransferase (NMT), has been characterized in the kinetoplastid protozoan parasites, Leishmania and Trypanosoma brucei. In Leishmania major, the single copy NMT gene is constitutively expressed in all parasite stages as a 48.5-kDa protein that localizes to both membrane and cytoplasmic fractions. Leishmania NMT myristoylates the target acylated Leishmania protein, HASPA, when both are co-expressed in Escherichia coli. Gene targeting experiments have shown that NMT activity is essential for viability in Leishmania. In addition, overexpression of NMT causes gross changes in parasite morphology, including the subcellular accumulation of lipids, leading to cell death. This phenotype is more extreme than that observed in Saccharomyces cerevisiae, in which overexpression of NMT activity has no obvious effects on growth kinetics or cell morphology. RNA interference assays in T. brucei have confirmed that NMT is also an essential protein in both life cycle stages of this second kinetoplastid species, suggesting that this enzyme may be an appropriate target for the development of anti-parasitic agents.

A fluorescence-based high performance liquid chromatographic method for the characterization of palmitoyl acyl transferase activity

Although protein palmitoylation is essential for targeting many important signaling proteins to the plasma membrane, the mechanism by which palmitoylation occurs is uncharacterized, since the enzyme(s) responsible for this modification remain unidentified. To study palmitoyl acyl transferase (PAT) activity, we developed an in vitro palmitoylation (IVP) assay using a fluorescently labeled substrate peptide, mimicking the N-terminal palmitoylation motif of proteins such as non-receptor Src-related tyrosine kinases. The palmitoylated and non-palmitoylated forms of the peptide were resolved by reverse-phase HPLC and detected by fluorescence. The method was optimized for PAT activity using lysates from the MCF-7 and Hep-G2 human tumor cell lines. The PAT activity was inhibited by boiling, reducing the incubation temperature, or adding 10 microM 2-bromopalmitate, a known palmitoylation inhibitor. This IVP assay provides the first method that is suitable to study all facets of the palmitoylation reaction, including peptide palmitoylation by PAT(s), depalmitoylation by thioesterases, and evaluation of potential palmitoylation inhibitors.

Exogenous myristic acid acylates proteins in cultured rat hepatocytes

Fatty acid acylation is a functionally important modification of proteins. In the liver, however, acylated proteins remain largely unknown. This work was aimed at investigating fatty acid acylation of proteins in cultured rat hepatocytes. Incubation of these cells with [9,10-3H] myristic acid followed by two-dimensional electrophoresis separation of the delipidated cellular proteins and autoradiography evidenced the reproducible and selective incorporation of radioactivity from the precursor into 18 well-resolved proteins in the 10--120 kDa range and the 4--7 pH range. Radiolabeling of these proteins resulted from covalent linkage to the precursor [9,10-3H] myristic acid or to its elongation product, palmitic acid. The majority of the covalent linkages between the proteins and the fatty acids were broken by base hydrolysis, which indicated that the linkage was of thioester or ester-type. Only one of the studied proteins was attached to myristic acid via an amide linkage which resisted the basic treatment but was broken by acid hydrolysis. After incubation with [9,10-3H] palmitic acid, only two proteins previously detected with myristic acid were radiolabeled. Finally, the identified acylated proteins may be grouped into two classes: proteins involved in signal transduction (the alpha subunit of a heterotrimeric G protein and several small G proteins) and cytoskeletal proteins (cytokeratins, actin).

Myristic acid, unlike palmitic acid, is rapidly metabolized in cultured rat hepatocytes

This study was designed to examine and compare the metabolism of myristic and palmitic acids in cultured rat hepatocytes. [1-(14)C]-Labeled fatty acids were solubilized with albumin at 0.1 mmol/L in culture medium. Incubation with 24-hr cultured hepatocytes was carried out for 12 hr. Myristic acid was more rapidly (P < 0.05) taken up by the cells than was palmitic acid (86.9 +/- 0.9% and 68.3 +/- 5.7%, respectively, of the initial radioactivity was cleared from the medium after 4 hr incubation). Incorporation into cellular lipids, however, was similar after the same time (33.4 +/- 2.8% and 34.9 +/- 9.3%, respectively, of initial radioactivity). In the early phase of the incubation (30 min), myristic acid was more rapidly incorporated into cellular triglycerides than was palmitic acid (7.4 +/- 0.9% and 3.6 +/- 1.9%, respectively, of initial radioactivity). However, after 12 hr incubation, the radioactivity of cellular triglycerides, cellular phospholipids, and secreted triglycerides was significantly higher with palmitic acid as precursor. Myristic acid oxidation was significantly higher than that of palmitic acid (14.9 +/- 2.2% and 2.3 +/- 0.6%, respectively, of the initial radioactivity was incorporated into the beta-oxidation products after 4 hr). Myristic acid was also more strongly elongated to radiolabeled palmitic acid (12.2 +/- 0.8% of initial radioactivity after 12 hr) than palmitic acid was to stearic acid (5.1 +/- 1.3% of initial radioactivity after 12 hr). The combination of elongation and beta-oxidation results in the rapid disappearance of C14:0 in hepatocytes whereas C16:0 is esterified to form glycerolipids. This study provides evidence that myristic acid is more rapidly metabolized in cultured hepatocytes than is palmitic acid.

Molecular cloning, genomic organization, and biochemical characterization of myristoyl-CoA:protein N-myristoyltransferase from Arabidopsis thaliana

Myristoyl-CoA:protein N-myristoyltransferase (NMT, EC 2.3.1.97) catalyzes the co-translational addition of myristic acid to the amino-terminal glycine residue of a number of important proteins of diverse functions. We have isolated a full-length Arabidopsis thaliana cDNA encoding NMT (AtNMT1), the first described from a higher plant. This AtNMT1 cDNA clone has an open reading frame of 434 amino acids and a predicted molecular mass of 48,706 Da. The primary structure is 50% identical to the mammalian NMTs. Analyses of Southern blots, genomic clones, and database sequences suggested that the A. thaliana genome contains two copies of NMT gene, which are present on different chromosomes and have distinct genomic organizations. The recombinant AtNMT1 expressed in Escherichia coli exhibited a high catalytic efficiency for the peptides derived from putative plant myristoylated proteins AtCDPK6 and Fen kinase. The AtNMT was similar to the mammalian NMTs with respect to a relative specificity for myristoyl CoA among the acyl CoA donors and also inhibition by the bovine brain NMT inhibitor NIP(71). The AtNMT1 expression profile indicated ubiquity in roots, stem, leaves, flowers, and siliques (approximately 1.7 kb transcript and approximately 50 kDa immunoreactive polypeptide) but a greater level in the younger tissue, which are developmentally very active. NMT activity was also evident in all these tissues. Subcellular distribution studies indicated that, in leaf extracts, approximately 60% of AtNMT activity was associated with the ribosomal fractions, whereas approximately 30% of the activity was observed in the cytosolic fractions. The NMT is biologically important to plants, as noted from the stunted development when the AtNMT1 was down-regulated in transgenic Arabidopsis under the control of an enhanced CaMV 35S promoter. The results presented in this study provide the first direct molecular evidence for plant protein N-myristoylation and a mechanistic basis for understanding the role of this protein modification in plants.

N-myristoyltransferase

Myristoylation refers to the co-translational addition of a myristoyl group to an amino-terminal glycine residue of a protein by an ubiquitously distributed enzyme myristoyl-CoA:protein N-myristoyltransferase (NMT, EC 2.3.1.97). This review describes the basic enzymology, molecular cloning and regulation of NMT activity in various pathophysiological processes such as colon cancer and diabetes.

Genomic organization of human myristoyl-CoA: protein N-myristoyltransferase-1

Myristoylation is a biochemical modification of proteins in which the lipid myristate becomes covalently bound to various cellular, viral, and oncoproteins catalyzed by a monomeric enzyme myristoyl-CoA:protein N-myristoyltransferase (NMT). This modification is important for the biological activity of several proteins, especially the regulation of several oncoproteins involved in various types of cancers. Complementary DNA encoding human NMT-1 (hNMT-1) has been previously reported; however, the genomic organization of hNMT-1 has not been available. Attempts to amplify genomic fragments corresponding to hNMT-1 cDNA sequence yielded only one fragment. We have searched databases using both the cDNA and sequence of one of the intron sequence and this identified a human BAC clone sequence from chromosome 17. Alignment of hNMT-1 cDNA coding information on human chromosome 17 resulted in the complete structural identity of 23,960 bp of the hNMT-1 gene. The hNMT-1 gene is composed of 11 exons and 10 introns with consensus GT/AG boundaries. Finally, we show that 140 bp from the 5' end of recently reported full-length cDNA of hNMT-1 was not part of this genomic region raising the possibility for posttranscriptional modification in generating larger transcripts likely by trans splicing. Further, the availability of this genomic sequence will assist in unraveling the molecular basis for several observed NMT isoforms.

Recombinant bovine spleen myristoyl CoA: protein N-myristoyltransferase

Myristoyl-CoA:protein N-myristoyltransferase (NMT) is an essential eukaryotic enzyme that catalyzes the co-translational transfer of myristate to the NH2-terminal glycine residue of a number of important proteins of diverse function. Recently, we have isolated full length cDNA encoding bovine spleen NMT [27] the full length cDNA was cloned and expressed in E. coli, resulting in the expression of functionally active 50 kDa NMT. Using the combination of SP-Sepharose fast flow and Mono S fast protein liquid chromatography, the enzyme was purified 20-fold with a high yield. The spleen NMT (sNMT) fusion protein exhibited an apparent molecular weight of 53 kDa on SDS-PAGE. Upon cleavage by the Enterokinase the sNMT exhibited an apparent molecular weight of 50 kDa without loss of catalytic activity. The two synthetic peptide substrates based on the N-terminal sequence of pp60src (GSSKSKMR) and cAMP dependent protein kinase (GNAAAKKRR) have different kinetic parameters of Km values of 40 and 200 microM. Recombinant sNMT was also potently inhibited by Ni2+ (histidine binder) in a concentration dependent manner with a half maximal inhibition of 280 microM. The E. coli expressed sNMT was homogenous and showed enzyme activity.

Myristoyl-coA:protein N-myristoyltransferase from bovine cardiac muscle: molecular cloning, kinetic analysis, and in vitro proteolytic cleavage by m-calpain

Myristoyl-CoA:protein N-myristoyltransferase (NMT) catalyzes the attachment of myristate onto the amino terminal glycine residue of select polypeptides. Cardiac tissue expresses high levels of cAMP-dependent protein kinase whose catalytic subunit is myristoylated; however, cardiac muscle extracts were found to contain low NMT activities. Northern blot analysis of bovine heart poly(A)+ RNA probed with bovine spleen NMT cDNA revealed a 1.7-kb mRNA. Western blot analysis of cardiac muscle extracts with human NMT antibody indicated a prominent immunoreactive band with a molecular mass of 50 kDa. The expression of mRNA and protein levels in cardiac muscle is not correlated with NMT activities, suggesting the presence of regulators of the enzyme activity. We have isolated the cDNA encoding bovine cardiac muscle NMT (cNMT) by reverse transcription polymerase chain reaction. The single long open reading frame of 1248 bp of bovine cNMT specifies a protein of 416 amino acids with a predicted mass of 46,686 Da. The cDNA clone expressed in Escherichia coli resulted in the production of functionally active 50-kDa NMT. Ultrastructural and immunolocalization of NMT utilizing the immunogold labeling technique demonstrated cytoplasmic distribution with occasional mitochondrial and myofilaments localization of the NMT antibody. Cardiac muscle NMT has a higher affinity for myristoyl-CoA than toward palmitoyl-CoA. Substrate specificity indicated that cNMT has a higher affinity toward pp60src and M2 gene segment of reovirus type 3-derived peptide substrates than toward cAMP-dependent protein kinase-derived peptide. Primary translational product of cNMT sequence contained several regions rich in proline, glutamic acid, serine, and threonine, which are known as "PEST" regions. PEST-FIND analysis of the amino acid sequences indicated eight PEST regions were present in the cNMT. These PEST regions are suggested to be recognized by specific proteases, particularly Ca(2+)-dependent neutral proteases, calpains, which are responsible for the degradation of PEST-containing proteins. We have demonstrated the abolishment of NMT activity and NMT protein degradation in vitro by m-calpain. The proteolysis of cNMT by m-calpain and the abolishment of NMT activity was prevented by the calpain inhibitor, calpastatin. These observations indicate that calpains may regulate NMT activity.

Identification and quantitation of the fatty acids composing the CoA ester pool of bovine retina, heart, and liver

Several proteins found in retinal photoreceptor cells (guanylate cyclase activating protein, protein kinase A, recoverin, and transducin) are N-terminally modified with the fatty acids 12:0, 14:0, 14:1n-9, and 14:2n-6, whereas similar proteins in other tissues contain only 14:0. It has been hypothesized that the acyl-CoA pool of the retina contains amounts of 12:0, 14:1n-9, and 14:2n-6 elevated over 14:0, in comparison to other tissues, and this accounts for the specificity of N-terminal fatty acylation. To test this hypothesis, we performed fatty acid analysis on total acyl-CoAs purified from bovine retina (light-adapted), heart, and liver. We also examined the N- and S-linked fatty acid composition of the total protein pools from these tissues. Acyl-CoAs were prepared from heart, liver, and retina and separated by high performance liquid chromatography (HPLC). Identities of peaks were based on HPLC of standard 12:0, 14:0, 14:1n-9, and 14:2n-6 CoAs. Total protein was subjected to base hydrolysis followed by acidic methanolysis to release S- and N-linked fatty acids, respectively, and fatty acid phenacyl esters were prepared for HPLC analysis. Retina had levels of 12:0 (2.7 +/- 2.1%), 14:1n-9 (2.9 +/- 2.2%), and 14:2n-6 (1.6 +/- 0.7%) CoAs below that of 14:0 CoA (7.0 +/- 1.8%). Likewise, heart levels of 14:2n-6 CoA (3.7 +/- 0.1%) were near and 12:0 (2.6 +/- 0. 6%) and 14:1n-9 (0.7 +/- 0.3%) CoAs were below that of 14:0 CoA (3.8 +/- 1.0%). Liver had levels of 12:0 (16.1 +/- 5.7%) and 14:2n-6 (8.1 +/- 1.2%) CoAs above and 14:1n-9 CoA (1.2 +/- 0.6%) below that of 14:0 CoA (5.9 +/- 0.8%). Fatty acid analysis of total protein showed that all tissues contained S-linked 16:0, 18:0, and 18:1n-9. Retina proteins contained N-linked 14:0, 14:1n-9, and 14:2n-6, whereas heart and liver had only 14:0. Our findings do not support the hypothesis that the CoA ester pool of the retina is enriched with 12:0, 14:1n-9, and 14:2n-6 over 14:0, in comparison to other tissues. This suggests that alternative models must be considered for the regulation of N-terminal fatty acylation of proteins in photoreceptor cells.

Molecular cloning and biochemical characterization of bovine spleen myristoyl CoA:protein N-myristoyltransferase

Myristoyl-CoA:protein N-myristoyltransferase (NMT) is an essential eukaryotic enzyme that catalyzes the cotranslational transfer of myristate to the NH2-terminal glycine residue of a number of important proteins of diverse function. We have isolated full-length cDNA encoding bovine spleen NMT (sNMT). The single long open reading frame of 1248 bp of sNMT specifies a protein of 416 amino acids with a predicted mass of 46,686 Da. The protein coding sequence was expressed in Escherichia coli resulting in the production of functionally active 50-kDa NMT. Deletion mutagenesis showed that the C-terminus is essential for activity whereas up to 52 amino acids can be deleted from the N-terminus without affecting the function. One of the N-terminal deletions resulted in threefold higher NMT activity. Genomic Southern analysis indicated the presence of two strong hybridizing bands with three different restriction enzyme digests suggesting the possibility of two copies of the NMT gene in the bovine genome. RNA blot hybridization analysis of total cellular RNA prepared from bovine brain, heart, spleen, lung, liver, kidney, and skeletal muscle probed with bovine sNMT cDNA revealed a single 1.7-kb mRNA. Western blot analysis of various bovine tissues with human NMT peptide antibody indicated a common prominent immunoreactive band with an apparent molecular mass of 48.5-50 kDa in all tissues. Additional immunoreactive bands were observed in brain (84 and 50 kDa), lung (58 kDa), and skeletal muscle (58 kDa). Activity measurements demonstrated that brain contained the highest NMT activity followed by spleen, lung, kidney, heart, skeletal muscle, pancreas, and liver. It appears therefore that mRNA and protein expression do not correlate with NMT activity, suggesting the presence of regulators of the enzyme activity.

Human N-myristoyltransferase amino-terminal domain involved in targeting the enzyme to the ribosomal subcellular fraction

N-Myristoyltransferase (NMT) catalyzes the cotranslational acylation with myristic acid of the NH2-terminal glycines of a number of cellular and viral proteins. Most of the in vitro NMT activity (60-85%) in isoosmotic cell homogenates of human lymphoblastic leukemia (i.e. CEM and MOLT-4) and cervical carcinoma (i.e. HeLa) cells was shown to be associated with the ribosomal subcellular fractions by differential centrifugation. Also found in the ribosomal fractions was a approximately 60-kDa protein that was specifically immunoblotted with an anti-human NMT (hNMT) peptide antibody. This approximately 60-kDa protein was stable in the presence of proteolytic enzyme inhibitors but was gradually converted into a approximately 46-kDa species when stored in the absence of protease inhibitors. Sucrose density gradient centrifugation of the ribosomal fraction resulted in the hNMT activity sedimenting exactly coincident with the 260 nm absorption profile and exhibiting A260/A280 absorption ratios >1.8, indicating an association of NMT with putative ribosomal particle(s)/subunit(s). The subcellular targeting of hNMT was also examined by immunoblotting subcellular fractions from HeLa cells transfected with plasmids containing FLAG epitope-tagged hNMT inserts corresponding either to the originally assigned hNMT gene or to an alternative open reading frame initiated from an in-frame start site upstream from the assumed hNMT start site. Anti-FLAG immunoblotting of cells transfected with a plasmid containing the larger insert revealed FLAG-NMT primarily in the ribosomal fraction with an apparent molecular mass similar to the approximately 60-kDa native hNMT. In contrast, immunoblotting of cells transfected with a plasmid containing the smaller insert identified a approximately 50-kDa FLAG-NMT predominantly in the cytosolic fraction. An analysis of mixtures of CEM ribosomes and serial dilutions of purified recombinant FLAG-NMTs demonstrated that the approximately 60-kDa FLAG-NMT binds ribosomes with higher affinity than the approximately 50-kDa FLAG-NMT. These in vivo and in vitro subcellular targeting and recombinant expression experiments identify a native hNMT that is 10-12 kDa larger than the enzyme predicted by the originally assigned hNMT gene and which is apparently translated from an alternative up-stream start site. The data also indicate that although the unique NH2-terminal residues encoded by this larger open reading frame are not required for in vitro catalytic activity, they do provide signal(s) involved in targeting hNMT to the ribosomal subcellular fraction where cotranslational N-myristoylation occurs.

N-Myristoyltransferase overexpression in human colorectal adenocarcinomas

Modification of proteins by myristoylation has been proposed as a chemotherapeutic target against colon cancer because it is important in the function of various signal transduction proteins. Recently we reported that the enzyme that catalyzes this modification, N-myristoyltransferase (NMT), is elevated in colorectal adenocarcinomas [Magnuson, B. A., Raju, R. V. S., Moyana, T. N., and Sharma, R. K. (1995) J. Natl. Cancer. Inst. 87, 1630-1635]. The purpose of the present study was to investigate whether the elevated activity of NMT in colorectal adenocarcinomas is due to an increase in the production of NMT or a change in the structure of the preexisting enzyme. The expression of NMT in normal colonic mucosa and adenocarcinomas from human colorectal surgical specimens was studied by immunoblotting, and its localization was confirmed by immunohistochemistry. The molecular weight of NMT was determined by fast protein liquid chromatography. In both normal mucosa and colorectal adenocarcinomas, NMT with a molecular mass of 48.5 kDa was identified with anti-human NMT and anti-peptide antibody. However, the expression of NMT was found to be higher in the colorectal tumors. This finding was further confirmed by immunohistochemical studies which showed stronger cytoplasmic staining in the tumors. These findings represent the first description of NMT overexpression in colorectal adenocarcinomas. This has implications with regard to (i) the design of chemotherapeutic drugs and (ii) prognosis, for instance, in monitoring colorectal cancer recurrence or metastases.

Demonstration and purification of a myristoyl-CoA binding protein from bovine cardiac muscle

Protein myristoylation refers to the co-translational addition of myristoyl group to an amino-terminal glycine residue of a protein by the enzyme myristoyl-CoA:protein N-myristoyltransferase (NMT). The myristoylation reaction depends on the availability of the cellular pools of coenzyme A and myristate and their subsequent formation of myristoyl-CoA, the substrate of NMT. In the present study a myristoyl-CoA binding protein (MCBP) was purified using various column chromatographies: hydroxylapatite, DEAE Sepharose CL-6B and Sephacryl S-300 gel filtration. The purified protein exhibited an apparent molecular mass of 50 kDa on SDS-polyacrylamide gel electrophoresis. Incubation of protein with [1-(14)C]myristoyl-CoA followed by denaturing gel electrophoresis, fluorography and treatment with hydroxylamine yielded results that are highly suggestive of a covalent ester-linked acyl-protein complex. This complex formation was not observed in the crude cytosolic fractions. The addition of cytosolic fraction to a progressing acyl-protein complex, resulted in deacylation suggesting a role for thioesterase or/proteinases in the regulation of the acylation reaction in bovine cardiac muscle. The acyl-protein complex formation was not inhibited by NIP71, a 71 kDa NMT inhibitory protein from bovine brain.

Myristoylation

N-myristoylation is an acylation process absolutely specific to the N-terminal amino acid glycine in proteins. This maturation process concerns about a hundred proteins in lower and higher eukaryotes involved in oncogenesis, in secondary cellular signalling, in infectivity of retroviruses and, marginally, of other virus types. Thy cytosolic enzyme responsible for this activity, N-myristoyltransferase (NMT), studied since 1987, has been purified from different sources. However, the studies of the specificities of the various NMTs have not progressed in detail except for those relating to the yeast cytosolic enzyme. Still to be explained are differences in species specificity and between various putative isoenzymes, also whether the data obtained from the yeast enzyme can be transposed to other NMTs. The present review discusses data on the various addressing processes subsequent to myristoylation, a patchwork of pathways that suggests myristoylation is only the first step of the mechanisms by which a protein associates with the membrane. Concerning the enzyme itself, there are evidences that NMT is also present in the endoplasmic reticulum and that its substrate specificity is different from that of the cytosolic enzyme(s). These differences have major implications for their differential inhibition and for their respective roles in several pathologies. For instance, the NMTs from mammalians are clearly different from those found in several microorganisms, which raises the question whether the NMT may be a new targets for fungicides. Finally, since myristoylation has a central role in virus maturation and oncogenesis, specific NMT inhibitors might lead to potent antivirus and anticancer agents.

Coenzyme A dependent myristoylation and demyristoylation in the regulation of bovine spleen N-myristoyltransferase

N-myristoyltransferase (NMT) is an essential eukaryotic enzyme that catalyzes the transfer of myristate to the NH2-terminal glycine residue of a number of important proteins of diverse function. Little is known about the control and regulation of NMT in higher eukaryotes. Bovine spleen N-myristoyltransferase has been purified and characterized [Raju, RVS, Kalra J & Sharma RK (1994) J Biol Chem 269:12080-12083]. The activation of bovine spleen NMT with thiol reducing compounds, and its inhibition by the oxidizing agent sodium iodate, suggest a role for oxidation/reduction in NMT regulation. Available knowledge concerning coenzyme A (CoA), the thiol in the cell, indicated that the agents tested on NMT could also reduce or oxidize CoA. The studies suggested that reduced CoA is the key regulator of NMT activity, while oxidized CoA did not allow NMT to promote myristoylation. Further, the process of myristoylation and demyristoylation may be governed by NMT, depending on the differential concentration of CoA. The process of demyristoylation could be blocked by excess CoA. We therefore hypothesize that the initial event in the regulation of NMT is an increase in cellular CoA concentration which could be coupled to an increase in protein myristoylation. Once the CoA concentration in the cell decreases due to oxidation, the demyristoylation process would be operative.

Distribution of myristoyl-CoA:protein N-myristoyl transferase activity in rabbit intestine

Myristoyl-CoA:protein N-myristoyl transferase (NMT) attaches the fatty acid, myristate, to the amino-terminal glycine residue of various proteins involved in cellular regulation and/or signal transduction. We report differences in the activity and properties of NMT in New Zealand rabbit small intestine, ascending colon and descending colon. The mucosa of the small intestine, ascending colon and descending colon was assayed for NMT activity using peptides of known myristoylated proteins (pp60src and catalytic subunit of cAMP dependent protein kinase). Total NMT activity per gram tissue was 5-fold higher in the small intestine and 1.5-fold higher in the ascending colon than in the descending colon. Smooth muscle from the colon also contained low levels of NMT activity. NMT activity was 2- to 3-fold higher in the particulate fraction than in the cytosolic fraction of the mucosa in the descending colon. The apparent molecular mass of NMT in the intestine mucosa was 78 kDa.

Expression of human N-myristoyltransferase in Escherichia coli. Comparison with N-myristoyltransferases expressed in different tissues

Myristoyl CoA:protein N-myristoyltransferase catalyzes the addition of myristate to the amino-terminal glycine residue of a number of eukaryotic proteins. Escherichia coli transformed with human NMT expression construct produced high levels of N-myristoyltransferase. Using the combination of ammonium sulfate precipitation, chromatography on SP-Sepharose fast flow and fast protein liquid chromatography on Mono-S, the enzyme was purified more than 100 fold with 40% yield. The hNMT fusion protein exhibited an apparent molecular weight of 53 kDa on SDS-polyacrylamide gel electrophoresis. Upon cleavage by the Enterokinase [(Asp)4-Lys], the hNMT exhibited an apparent molecular mass of 49 kDa without loss of catalytic activity. The hNMT activity could be greatly activated severalfold with the use of Tris, SDS, ethanol and acetonitrile. The catalytic activity of hNMT was potently inhibited in a concentration dependent manner by NIP71, a bovine brain NMT inhibitory protein with a half maximal inhibition of 31.0 nM. The E. coli expressed hNMT was homogeneous and showed enzyme activity.