SIRT4 regulates fatty acid oxidation and mitochondrial gene expression in liver and muscle cells

SIRT4, a member of the sirtuin family, has been implicated in the regulation of insulin secretion by modulation of glutamate dehydrogenase. However, the role of this enzyme in the regulation of metabolism in other tissues is unknown. In this study we investigated whether depletion of SIRT4 would enhance liver and muscle metabolic functions. To do this SIRT4 was knocked down using an adenoviral shRNA in mouse primary hepatocytes and myotubes. We observed a significant increase in gene expression of mitochondrial and fatty acid metabolism enzymes in hepatocytes with reduced SIRT4 levels. SIRT4 knockdown also increased SIRT1 mRNA and protein levels both in vitro and in vivo. In agreement with the increased fatty acid oxidation (FAO) gene expression, we showed a significant increase in FAO in SIRT4 knockdown primary hepatocytes compared with control, and this effect was dependent on SIRT1. In primary myotubes, knockdown of SIRT4 resulted in increased FAO, cellular respiration, and pAMPK levels. When SIRT4 was knocked down in vivo by tail vein injection of a shRNA adenovirus, we observed a significant increase in hepatic mitochondrial and FAO gene expression consistent with the findings in primary hepatocytes. Taken together these findings demonstrate that SIRT4 inhibition increases fat oxidative capacity in liver and mitochondrial function in muscle, which might provide therapeutic benefits for diseases associated with ectopic lipid storage such as type 2 diabetes.

JNK1 phosphorylates SIRT1 and promotes its enzymatic activity

SIRT1 is a NAD-dependent deacetylase that regulates a variety of pathways including the stress protection pathway. SIRT1 deacetylates a number of protein substrates, including histones, FOXOs, PGC-1α, and p53, leading to cellular protection. We identified a functional interaction between cJUN N-terminal kinase (JNK1) and SIRT1 by coimmunoprecipitation of endogenous proteins. The interaction between JNK1 and SIRT1 was identified under conditions of oxidative stress and required activation of JNK1 via phosphorylation. Modulation of SIRT1 activity or protein levels using nicotinamide or RNAi did not modify JNK1 activity as measured by its ability to phosphorylate cJUN. In contrast, human SIRT1 was phosphorylated by JNK1 on three sites: Ser27, Ser47, and Thr530 and this phosphorylation of SIRT1 increased its nuclear localization and enzymatic activity. Surprisingly, JNK1 phosphorylation of SIRT1 showed substrate specificity resulting in deacetylation of histone H3, but not p53. These findings identify a mechanism for regulation of SIRT1 enzymatic activity in response to oxidative stress and shed new light on its role in the stress protection pathway.

Phosphatidylinositol 3-kinase signaling inhibits DAF-16 DNA binding and function via 14-3-3-dependent and 14-3-3-independent pathways

In Caenorhabditis elegans, an insulin-like signaling pathway to phosphatidylinositol 3-kinase (PI 3-kinase) and AKT negatively regulates the activity of DAF-16, a Forkhead transcription factor. We show that in mammalian cells, C. elegans DAF-16 is a direct target of AKT and that AKT phosphorylation generates 14-3-3 binding sites and regulates the nuclear/cytoplasmic distribution of DAF-16 as previously shown for its mammalian homologs FKHR and FKHRL1. In vitro, interaction of AKT- phosphorylated DAF-16 with 14-3-3 prevents DAF-16 binding to its target site in the insulin-like growth factor binding protein-1 gene, the insulin response element. In HepG2 cells, insulin signaling to PI 3-kinase/AKT inhibits the ability of a GAL4 DNA binding domain/DAF-16 fusion protein to activate transcription via the insulin-like growth factor binding protein-1-insulin response element, but not the GAL4 DNA binding site, which suggests that insulin inhibits the interaction of DAF-16 with its cognate DNA site. Elimination of the DAF-16/1433 association by mutation of the AKT/14-3-3 sites in DAF-16, prevents 14-3-3 inhibition of DAF-16 DNA binding and insulin inhibition of DAF-16 function. Similarly, inhibition of the DAF-16/14-3-3 association by exposure of cells to the PI 3-kinase inhibitor LY294002, enhances DAF-16 DNA binding and transcription activity. Surprisingly constitutively nuclear DAF-16 mutants that lack AKT/14-3-3 binding sites also show enhanced DNA binding and transcription activity in response to LY294002, pointing to a 14-3-3-independent mode of regulation. Thus, our results demonstrate at least two mechanisms, one 14-3-3-dependent and the other 14-3-3-independent, whereby PI 3-kinase signaling regulates DAF-16 DNA binding and transcription function.

DAF-16 recruits the CREB-binding protein coactivator complex to the insulin-like growth factor binding protein 1 promoter in HepG2 cells

Insulin negatively regulates expression of the insulin-like growth factor binding protein 1 (IGFBP-1) gene by means of an insulin-responsive element (IRE) that also contributes to glucocorticoid stimulation of this gene. We find that the Caenorhabditis elegans protein DAF-16 binds the IGFBP-1 small middle dotIRE with specificity similar to that of the forkhead (FKH) factor(s) that act both to enhance glucocorticoid responsiveness and to mediate the negative effect of insulin at this site. In HepG2 cells, DAF-16 and its mammalian homologs, FKHR, FKHRL1, and AFX, activate transcription through the IGFBP-1.IRE; this effect is inhibited by the viral oncoprotein E1A, but not by mutants of E1A that fail to interact with the coactivator p300/CREB-binding protein (CBP). We show that DAF-16 and FKHR can interact with both the KIX and E1A/SRC interaction domains of p300/CBP, as well as the steroid receptor coactivator (SRC). A C-terminal deletion mutant of DAF-16 that is nonfunctional in C. elegans fails to bind the KIX domain of CBP, fails to activate transcription through the IGFBP-1.IRE, and inhibits activation of the IGFBP-1 promoter by glucocorticoids. Thus, the interaction of DAF-16 homologs with the KIX domain of CBP is essential to basal and glucocorticoid-stimulated transactivation. Although AFX interacts with the KIX domain of CBP, it does not interact with SRC and does not respond to glucocorticoids or insulin. Thus, we conclude that DAF-16 and FKHR act as accessory factors to the glucocorticoid response, by recruiting the p300/CBP/SRC coactivator complex to an FKH factor site in the IGFBP-1 promoter, which allows the cell to integrate the effects of glucocorticoids and insulin on genes that carry this site.

IRE-ABP (insulin response element-A binding protein), an SRY-like protein, inhibits C/EBPalpha (CCAAT/enhancer-binding protein alpha)-stimulated expression of the sex-specific cytochrome P450 2C12 gene

In primary hepatocytes, overexpression of an insulin response element-A binding protein (IRE-ABP), a member of the SRY family of high-mobility group (HMG) proteins, inhibits CCAAT/enhancer-binding protein alpha (C/EBPalpha)-mediated activation of the female-specific cytochrome P450 2C12 (CYP2C12) gene, but not the male-specific cytochrome P450 2C11 (CYP2C11) gene. IRE-ABP and C/EBPalpha have overlapping specificity for the C/EBPalpha target site in the CYP2C12 promoter and compete for binding to CYP2C12 DNA in vitro. In contrast, IRE-ABP and C/EBPalpha bind distinct sequences in the CYP2C11 promoter. A single amino acid substitution in the HMG domain of IRE-ABP impairs its ability to bind DNA and to inhibit the effect of C/EBPalpha on CYP2C12 gene expression. Therefore, the ability of IRE-ABP to inhibit C/EBPalpha-stimulated CYP2C12 gene expression requires a functional DNA-binding domain. Taken together, our findings suggest that SRY-like proteins can bind to a subset of sequences recognized by the C/EBP family of DNA-binding proteins and modulate gene transcription in a context-specific manner.

Cellular radiosensitivity, radioresistant DNA synthesis, and defect in radioinduction of p53 in fibroblasts from atherosclerosis patients

Earlier studies have suggested that both cancer and atherosclerosis may follow a common pathway in the early stage of development and share certain risk factors. One report indicated that the gene responsible for the radiosensitive, cancer-prone, multisystem disorder ataxia telangiectasia (AT) may increase the risk of developing ischemic heart disease. The present studies were carried out to find similarities, if any, between atherosclerosis patients and AT homozygotes or heterozygotes (ATHs) in their cellular/molecular response to ionizing radiation, which acts as a carcinogen as well as an atherogen. Fibroblast cell strains developed from healthy subjects and from AT homozygotes, ATHs, and atherosclerosis patients were compared for (1) survival, by the colony-forming assay and (2) DNA synthesis inhibition after irradiation, determined by [3H]thymidine incorporation, cell cycle distribution, and the expression of p53 and p21 proteins, analyzed by flow cytometry. Fibroblasts from the atherosclerosis patients as a group, compared with the healthy subjects, showed enhanced sensitivity to chronic (low-dose-rate) irradiation. A majority of the cell strains representing atherosclerosis patients exhibited varying degrees of radioresistant DNA synthesis (RDS), with roughly 33% showing an AT-like and the rest an ATH-like response. All cell strains with an AT-like and one quarter with an ATH-like RDS were found to be defective in the radioinduction of both p53 and p21 proteins, which are concerned with cell cycle regulation. An absence of G1 arrest after irradiation was observed in cell strains lacking a radioinduced expression of p53 and p21. Cellular/molecular defects leading to increased radiosensitivity, reduced induction of p53/p21, and cell cycle deregulation found to be associated with cancer-prone disorders such as AT may constitute important risk factors for atherosclerosis as well.

Studies of the p53 gene mutation in Saudi non-Hodgkin's lymphoma

Tumor biopsies (paraffin embedded tissue) obtained from 45 Saudi patients with non-Hodgkin's lymphoma (NHL) were examined for the incidence of p53 mutations screened by polymerase chain reaction (PCR) and single strand conformation polymorphism (SSCP) analysis. DNA sequencing was carried out to confirm the occurrence of p53 mutation in PCR products showing abnormal migration by SSCP analysis. Only 1/45 samples showed the incidence of a homozygous mutation at codon 179 (exon 5) of the p53 gene that replaces histidine with tyrosine. The data showed that the frequency of p53 mutations was very low in Saudi NHL. Our results are consistent with the general observation that the p53 mutation is rather infrequent in hematopoietic malignancies like NHLs.

Reduced induction of P53 protein by gamma-irradiation in ataxia telangiectasia cells without constitutional mutations in exons 5, 6, 7, and 8 of the p53 gene

Ataxia telangiectasia (AT) is an autosomal recessive disease of childhood with several phenotypic characteristics. One of the hallmarks of this syndrome is its hypersensitivity to ionizing radiation, which is believed to be due to defects in DNA repair/processing. In addition to radio-resistant DNA synthesis, both fibroblasts and lymphoblastoid cell lines derived from these patients have been shown to have an impaired G1 arrest and prolonged G2 accumulation of cells indicating a defect in the regulation of cell cycle after irradiation. Since the (tumor suppressor) p53 protein has been reported to participate in the regulation of G1 arrest after irradiation, the possibility of p53 gene mutation and deregulating cell cycle in AT needed to be examined. We used the PCR amplification and DNA sequencing methods to detect mutations in the hypermutable exons (5-8) of germline p53 in fibroblast cells from 3 AT homozygotes. No mutation was found in any of these exons. In order to determine the role of the p53 protein in G1 arrest, its levels were measured before and after gamma-irradiation by flow cytometry in both AT and normal cells. Radiation-induced p53 protein levels in the AT cells varied from 6 to 60% compared to the normal cells, indicating a reduced induction of the protein in AT. These results suggest that mutation in the AT gene affects the p53 induction by irradiation and may, thus, alter the cell cycle regulation in the AT patients.

A PCR analysis of bcl-2 gene rearrangement in nodal and extranodal non-hodgkins-lymphoma (nhl) - some unusual observations in saudi patients

Non-Hodgkin's lymphoma (NHL) is the most common malignancy referred to our institute which is the largest tertiary referral cancer centre in Saudi Arabia, The proportion of follicular low grade NHL appears to be extremely small in this population (<5% of all NHL). To date, there is no data available regarding any correlation between bcl-2 gene rearrangement and different cell types of nodal and extranodal NHLs in Saudi patients. We used a sequential polymerase chain reaction (PCR) technique to determine the frequency of bcl-2/J(H) recombination occurring via the major breakpoint region (mbr) in 16 GI tract NHLs including 4 MALT lymphomas and 13 follicular (nodal) NHLs. The results showed only 2/13 (15%) nodal follicular NHLs with bcl-2/J(H) fusion DNA whereas 9/16 (56%) of the extranodal NHLs with at least 2 of them exhibiting MALT characteristics were positive for the bcl-2 gene rearrangement. A breakdown of the proportion of extranodal NHLs of different cell types showing bcl-2 rearrangement via mbr was as follows: 5/8 diffuse large non-cleaved cell (DLNCC), 1/3 diffuse small cleaved cell (DSCC), 1/1 follicular small cleaved cell (FSCC) and 2/4 MALTs. The PCR amplified bcl-2/J(H) fusion DNA from 5 randomly selected tumors (2 MALTs, 1 DLNCC, 1 DSCC and 1 nodal follicular lymphoma) were cloned and sequenced. All 5 of them showed different bcl-2/J(H) N-regions confirming the clonality of each tumor sample. The data indicating a very low incidence of bcl-2 translocation in nodal follicular NHLs and a surprisingly high incidence of it in extranodal NHLs are intriguing, and quite contrary to the findings in Western patients. These unusual observations warrant further studies and may suggest that different genetic events are involved in the development of extranodal NHLs including MALT and follicular center-cell NHLs in Saudi patients.

A molecular study of EBV DNA and p53 mutations in nasopharyngeal carcinoma of Saudi Arab patients

Tumor biopsies obtained from 25 Saudi Arab patients with nasopharyngeal carcinoma (NPC) were examined for the presence of Epstein-Barr virus (EBV) DNA detected by the polymerase chain reaction (PCR) and for the incidence of p53 mutations screened by a combination of PCR, single strand conformation polymorphism (SSCP) and PCR-restriction fragment length polymorphism (PCR-RFLP). DNA sequencing was carried out to confirm the occurrence of p53 mutation. While 92% of the tumor specimens were found to carry EBV DNA, only 1/25 showed the incidence of a homozygous mutation at codon 248 of the p53 gene. The data showed that despite a high association of EBV infection with Saudi NPC, the frequency of p53 mutations was very low. Our results are consistent with the worldwide observation of infrequent p53 mutations in NPC.

Apparent absence of BCL-2 rearrangement in ten Saudi follicular lymphomas: A preliminary report

Specific chromosomal translocations are often found to be associated with distinct types of human neoplasms. The t(14;18) is considered to be a cytogenetic hallmark of the follicular lymphomas found mostly in American patients. This chromosomal translocation occurs through a 3' untranslated region (either major breakpoint region [mbr] or minor cluster region [mcr]) of the bcl-2 proto-oncogene in chromosome 18 and the JH region of the immunoglobulin (Ig) gene in chromosome 14. In the present study, the polymerase chain reaction (PCR) was used to detect a fusion DNA fragment generated from bcl-2/JH rearrangement either through mbr or mcr in 10 follicular lymphomas in Saudi Arab patients. Surprisingly, none of these cases showed any evidence of bcl-2 gene rearrangement through mbr or mcr. The absence of usual bcl-2/JH recombination in Saudi follicular lymphomas is intriguing and may suggest that the occurrence and mode of bcl-2 gene rearrangement in this lymphoma varies in different patient populations.

Models of insulin action on metabolic and growth response genes

In ongoing studies aimed at elucidating the mechanism of insulin action on the expression of genes that modulate glucose utilization and cell growth, we have focused on the inductive effect of insulin on transcription of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and the early growth response gene, Egr-1. Insulin acutely stimulates the expression of both genes in 3T3 adipocytes; however, in primary adipocytes, chronic insulin exposure has opposing effects on the expression of these genes. GAPDH mRNA is decreased in the epididymal fat cells of diabetic animals and is increased over control levels when insulin is replaced, while Egr-1 mRNA levels are increased in diabetic animals. These observations, coupled with the finding that insulin-stimulated Egr-1 gene transcription is impaired in a Chinese hamster ovarian (CHO) cell line that displays normal metabolic responses but impaired ability to regulate DNA synthesis, support the conclusion that insulin regulation of Egr-1, a growth response gene, and GAPDH, a metabolic response gene, are mediated by distinct pathways. We present evidence that supports the role of protein phosphorylation in mediating the effect of insulin on activation of Egr-1 and GAPDH gene transcription.

Identification of a core motif that is recognized by three members of the HMG class of transcriptional regulators: IRE-ABP, SRY, and TCF-1 alpha

Insulin induces glyceraldehyde-3-phosphate dehydrogenase (GADPH) gene transcription in part by regulating one or more proteins that bind a cis-acting element, IRE-A. We have recently cloned a protein, IRE-ABP, that binds the IRE-A element. IRE-ABP is a member of the HMG class of transcriptional regulators and is 67% identical within its HMG box domain to the candidate gene for the testis-determining factor, SRY. IRE-ABP and SRY share binding specificity for the IRE-A motif. This sequence is also highly conserved with a core motif, 5'-Py-ctttg(a/t)-3', contained in T-cell specific genes that have high affinity for TCF-1 alpha, another member of the HMG class of transcriptional regulators. Thus, diverse members of the HMG family interact with similar nucleotide sequences to regulate expression of genes that initiate and maintain the differentiated phenotype. We have found this core motif in the upstream region of many genes that are positively and negatively regulated by insulin. These observations suggest that IRE-ABP or a related family member may coordinate the expression of these genes. The HMG family of proteins has diverse functions ranging from the regulation of differentiation and mating type in yeast to the regulation of tissue- and species-specific gene expression in mammals. Insulin regulates GAPDH gene transcription in a tissue-specific manner. We propose that members of the IRE-ABP family play an important role in controlling tissue specificity of the insulin response.

Multiple insulin-responsive elements regulate transcription of the GAPDH gene

Multiple elements in the upstream region of the GAPDH gene play a role in mediating the acute and chronic effect of insulin on GAPDH gene expression. The complexity of this regulation provides many layers of control. In differentiated tissues, the transcriptional response to insulin results from the additive effects of g/TRE, IRE-A and IRE-B. The gTRE may interact with newly synthesized c-fos/c-jun heterodimer to activate GAPDH gene transcription. Studies are underway to determine whether protein synthesis inhibitors affect the regulation of GAPDH. Because there are several elements that mediate the effect, it will be difficult to determine the significance of these findings until each cis-acting factor and its binding protein can be studied in isolation. IRE-A and IRE-B act together to promote a 5- to 8-fold insulin effect on HGAPDH-CAT in H35 hepatoma cells and a 3-fold effect in 3T3 adipocytes. We have succeeded in detecting an insulin-sensitive DNA-binding protein referred to as IREA-BP with an element -480 to -435. Insulin treatment of differentiated 3T3 adipocytes for 1 hr results in a 4-fold increase in the amount of this binding protein, as estimated by the amount of 32P-labelled oligonucleotide retarded on non-denaturing PAGE (11). The effect of insulin on IRP-B is comparable. Furthermore, IREA-BP is induced during the process of fasting and refeeding rats, an important in vivo correlate with our tissue culture models (11). These observations imply that the binding proteins IREA-BP and IRP-B are essential components in the signal transduction pathway of insulin action on GAPDH gene expression in metabolically active tissues such as fat and liver. Differentiation-dependence and tissue-specificity are achieved through multiple regulatory elements and involve pre- and post-translational regulation of multiple transcription factors. IREA-BP is present in preadipocytes but activity in highly induced upon differentiation of preadipocytes to adipocytes. The IRE-B (-408 to -269) DNA binding protein is not detected in 3T3 preadipocytes. A gC/EBP like-protein takes part in the formation of this complex which may explain the inductive effect of differentiation on binding. Finally, footprint and cotransfection studies indicate that the differentiation-dependent protein C/EBP also regulates GAPDH gene transcription through a motif located within one hundred nucleotides of the promoter. We have begun to clone the IRE-A and IRE-B DNA binding proteins. An IRE-A binding protein that footprints the 3' domain of the IRE-A has been cloned.(ABSTRACT TRUNCATED AT 400 WORDS)

DNA-binding properties of the product of the testis-determining gene and a related protein

THE upstream region of the human glyceraldehyde-3-phosphate dehydrogenase gene contains an insulin-response element (IRE-A) responsible for insulin-dependent transcription of the gene. The open reading frame of a rat complementary DNA encoding a protein (IRE-ABP) that binds to this sequence contains an HMG box motif that is 67% identical to the mouse candidate gene for the testis-determining factor SRY, and 98% identical to the mouse SRY-like gene, a4. Here we report that IRE-ABP and SRY bind to IRE-A DNA with comparable specificity in a DNase-I footprinting assay. Two females with sex reversal were found to have a single amino-acid substitution in the HMG box domain of SRY at position 3 and 7, respectively. SRY derivatives containing corresponding mutations do not make contact with IRE-A DNA. These results are direct evidence that mouse SRY-like proteins are sequence-specific DNA-binding proteins and identify two amino acids critical to this interaction. Moreover, IRE-A is a candidate SRY-response element.

An insulin response element in the glyceraldehyde-3-phosphate dehydrogenase gene binds a nuclear protein induced by insulin in cultured cells and by nutritional manipulations in vivo

Two independent cis-acting insulin response elements (IREs) in the gene encoding glyceraldehyde-3-phosphate dehydrogenase [D-glyceraldehyde-3-phosphate: NAD+ oxidoreductase (phosphorylating), EC 1.2.1.12], designated IRE-A and IRE-B, are sufficient to direct insulin-inducible gene expression. Using the electrophoretic mobility shift assay, a 4-fold increase in the amount of IRE-A DNA bound to nuclear proteins was detected when extracts isolated from insulin-stimulated differentiated 3T3-L1 cells or from the liver of rats refed a high-carbohydrate/low-fat diet after a 72-hr fast were compared to control nuclear extracts. The points of contact between protein and IRE-A DNA may represent a sequence recognized by at least one class of insulin-sensitive transcription factor(s).

Insulin stimulates glyceraldehyde-3-phosphate dehydrogenase gene expression through cis-acting DNA sequences

Glyceraldehyde-3-phosphate dehydrogenase [GAPDH; D-glyceraldehyde-3-phosphate:NAD+ oxidoreductase (phosphorylating), EC 1.2.1.12] mRNA levels are induced by physiologic concentrations of insulin in cultured 3T3-F442A adipocyte and H35 hepatoma cell lines. To examine the mechanism by which insulin regulates GAPDH mRNA levels in these two insulin-sensitive tissues, we have isolated a functional human GAPDH gene. When stably transfected and expressed in 3T3-F442A preadipocytes and H35 hepatoma cells, the intact human GAPDH gene is induced 10-fold by insulin in 3T3-F442A adipocytes and 3-fold by insulin in H35 hepatoma lines, which is similar to the induction obtained with the endogenous gene. A human GAPDH-chloramphenicol acetyltransferase construct, containing sequences -487 to +20 of the human gene fused to the chloramphenicol acetyltransferase gene, is regulated by insulin in stably transfected 3T3 adipocytes and stably or transiently transfected H35 hepatoma cell lines, whereas the Rous sarcoma virus-chloramphenicol acetyltransferase fusion protein is not. Thus, the inductive effect of insulin on human GAPDH gene expression is mediated through cis-acting sequences located between -487 and +20 of the human GAPDH gene.

Protein synthesis in yeast Saccharomyces cerevisiae. Purification of Co-eIF-2A and 'mRNA-binding factor(s)' and studies of their roles in Met-tRNAf.40S.mRNA complex formation

Antibodies prepared against a homogeneous preparation of Co-eIF-2A20 [Ahmad et al. (1985) J. Biol. Chem. 260, 6955-6959] reacted with several polypeptides including an 80-kDa polypeptide present in a crude yeast ribosomal salt wash. This 80-kDa polypeptide, containing Co-eIF-2A (Co-eIF-2A80) activity, has been extensively purified using a two-step purification procedure involving an immunoaffinity column chromatograph prepared using antibodies against Co-eIF-2A20 (fraction II) and hydroxyapatite chromatography (fraction III). The factors, eIF-2 + homogeneous Co-eIF-2A80 (fraction III) promoted Met-tRNAf.40S complex formation with an AUG codon but not with a physiological mRNA or a polyribonucleotide messenger poly(U,G) whereas eIF-2 + a partially purified Co-eIF-2A80 preparation (fraction II) promoted Met-tRNAf.40S complex formation with an AUG codon as well as with globin mRNA and poly(U,G) messenger. This factor-promoted Met-tRNAf binding to 40S ribosomes depends absolutely on the presence of a polyribonucleotide messenger containing an initiation codon (such as AUG or GUG). Other polyribonucleotide messengers tested, such as poly(U), poly(A) and poly(A,C) were completely ineffective in this binding reaction. This result indicates that the Met-tRNAf.40S.mRNA complex is formed by a direct interaction between Met-tRNAf, 40S ribosomes and the initiation site in mRNA. A mechanism has been proposed for Met-tRNAf.40S.mRNA complex formation in yeast.

A comparative study of the characteristics of eIF-2 and eIF-2-ancillary factor activities from yeast Saccharomyces cerevisiae and rabbit reticulocytes

The characteristics of yeast eukaryotic initiation factor 2 (eIF-2) and Co-eIF-2A have been studied and compared with those of the corresponding factors from rabbit reticulocytes. 1) Unlike eIF-2r, purified eIF-2y did not contain bound GDP. 2) Purified eIF-2y preparation contained GTPase activity and dephosphorylated GTP to GDP. 3) An anti-eIF-2r preparation which predominantly precipitated the gamma-subunit (Mr 54,000) of eIF-2r also precipitated the larger subunit (Mr 54,000) of eIF-2y. 4) Unlike eIF-2r, ternary complex formation by eIF-2y was not inhibited by Mg2+. 5) Both Co-eIF-2A20y and Co-eIF-2r significantly enhanced Met-tRNAf binding to eIF-2y and, again, Mg2+ did not have any effect on this stimulated Met-tRNAf binding to eIF-2y. 6) Both Co-eIF-2A20y and Co-eIF-2r were similarly effective in stimulating Met-tRNAf binding to eIF-2r in the absence of Mg2+. However, in the presence of Mg2+, Co-eIF-2A20y was significantly less effective than Co-eIF-2r as Co-eIF-2A20y did not promote displacement of GDP from eIF-2r X GDP. 7) eIF-2y bound [3H]GDP and this binding was significantly enhanced in the presence of Mg2+. Also, [3H]GDP in the preformed eIF-2y X [3H]GDP complex was rapidly exchanged with exogenously added unlabeled GDP in the presence of Mg2+. Co-eIF-2A20y had no effect on GDP binding to eIF-2y nor on GDP exchange reactions. 8) Reticulocyte heme-regulated protein synthesis inhibitor, which phosphorylated almost completely (in excess of 80%) the alpha-subunit (Mr 38,000) of eIF-2r, also phosphorylated similarly the smaller subunit (Mr 36,000) of eIF-2y. However, such phosphorylation had no significant effect on ternary complex formation, GDP binding, and GDP exchange reactions.

Purification and properties of eukaryotic initiation factor 2 and its ancillary protein factor (Co-eIF-2A) from yeast Saccharomyces cerevisiae

Two peptide chain initiation factor activities, eIF-2y and Co-eIF-2A20y, were purified from the high speed supernatant fraction of the yeast Saccharomyces cerevisiae and their properties were studied. 1) In sodium dodecyl sulfate-polyacrylamide gels, purified eIF-2y showed two major polypeptide bands corresponding to molecular weights of 54,000 and 36,000. The Mr 54,000 band was significantly more intense than the Mr 36,000 band, indicating the possible presence of two polypeptides of equal molecular weight in this band. The molecular weight of eIF-2y, determined using a density gradient centrifugation method, was approximately 140,000. 2) In sodium dodecyl sulfate-polyacrylamide gel, purified Co-eIF-2A20y showed a single polypeptide band corresponding to a molecular weight of 20,000. A similar molecular weight for Co-eIF-2A20y was also found using a density gradient centrifugation method. 3) In partial reactions, eIF-2y bound Met-tRNAf in the presence of Mg2+. The reaction required GTP. Co-eIF-2A20y stimulated Met-tRNAf binding to eIF-2y (2-3-fold) and also rendered the complex stable to 3 X 10(-5) M aurintricarboxylic acid. 4) This Co-eIF-2A20y activity was heat-labile and N-ethylmaleimide-insensitive. 5) Antibodies were prepared by injecting rabbits with homogeneous Co-eIF-2A20y. Such anti-Co-eIF-2A20y inhibited (60%) protein synthesis in a yeast cell-free protein synthesizing system and completely blocked Co-eIF-2A20y stimulation of Met-tRNAf. 40 S initiation complex formation. Protein synthesis inhibition by anti-Co-eIF-2A20y was almost completely reversed by preincubation of the antibodies specifically with homogeneous Co-eIF-2A20y.

Protein synthesis in rabbit reticulocytes. A study of the roles of Co-eIF-2, Co-eIF-2A80, and GDP in peptide chain initiation

The roles of Co-eIF-2, Co-eIF-2A80, and GDP in ternary complex and Met-tRNAf X 40 S initiation complex formation were studied. 1) Partially purified eukaryotic initiation factor 2 (eIF-2) (50% pure) preparations contained 0.4-0.6 pmol of bound GDP/pmol of eIF-2. eIF-2 purity was calculated from ternary complex formation in the absence of Mg2+ and in the presence of excess Co-eIF-2. 2) In the absence of Mg2+, approximately 30% of the potentially active eIF-2 molecules formed ternary complexes, and both Co-eIF-2 and Co-eIF-2A80 were equally effective in full activation of the eIF-2 molecules for ternary complex formation. 3) In the presence of Mg2+, approximately 10% of the potentially active eIF-2 molecules formed ternary complexes in the absence of ancillary factors, and the ancillary factors Co-eIF-2A80 and Co-eIF-2 raised the incorporation to 20 and 50% of the eIF-2 molecules, respectively. 4) In the absence of Mg2+, [3H]GDP in preformed eIF-2 X [3H]GDP was readily displaced by GTP during ternary complex formation. 5) In the presence of Mg2+, [3H]GDP remained tightly bound to eIF-2 and ternary complex formation was inhibited. Co-eIF-2, but not Co-eIF-2A80, was effective in promoting [3H]GDP displacement and the former was more effective in promoting ternary complex formation than the latter. 6) eIF-2 X [3H]GDP was converted to eIF-2 X [3H] GTP by incubation in the presence of nucleoside-5'-diphosphate kinase and ATP, but the eIF-2 X [3H]GTP thus formed did not bind Met-tRNAf in the presence of Mg2+ and required exogeneous addition of Co-eIF-2 and GTP for ternary complex formation and GTP displacement. 7) In the absence of Mg2+, the increased ternary complex formed in the presence of eIF-2 X [3H] GDP and Co-eIF-2A80 (with accompanying loss of [3H] GDP) was inactive in a subsequent reaction, which involves Met-tRNAf transfer to 40 S ribosomes (in the presence of Mg2+), and required trace amounts of Co-eIF-2 for such activity. Based on the above observations, we have suggested a two-step activation of eIF-2 molecules by the Co-eIF-2 protein complex for functional ternary complex formation. One of these steps involves the Co-eIF-2A component of Co-eIF-2. This activation results in stimulated Met-tRNAf binding to eIF-2 and is most apparent in the absence of Mg2+ and with aged eIF-2 molecules.(ABSTRACT TRUNCATED AT 400 WORDS)

Protein synthesis in rabbit reticulocytes: a study of the mechanism of action of the protein factor RF that reverses protein synthesis inhibition in heme-deficient reticulocyte lysates

A eukaryotic initiation factor 2 (eIF-2)-ancillary protein factor Co-eIF-2 promotes displacement of GDP from eIF-2 X GDP and facilitates ternary complex (Met-tRNAf X eIF-2 X GTP) formation in the presence of Mg2+. Heme-regulated protein synthesis inhibitor, HRI, phosphorylates the alpha-subunit of eIF-2 and thus inhibits ternary complex formation as Co-eIF-2 does not displace GDP from eIF-2 alpha (P) X GDP. RF, a high molecular weight cell supernatant factor, reverses protein synthesis inhibition in heme-deficient reticulocyte lysates and also reverses HRI inhibition of ternary complex formation. RF contains Co-eIF-2 activity. In addition, an active RF preparation contains excess alpha-subunit of eIF-2 in the free and unphosphorylated form and this alpha-subunit of eIF-2 is not phosphorylated by HRI and ATP. In this paper we report (i) an active RF preparation contains excess alpha-subunit of eIF-2 and this alpha-subunit can be phosphorylated by HRI and ATP in the presence of GDP; (ii) RF promotes ternary complex formation by eIF-2 X [3H]GDP with accompanying GDP displacement; (iii) in the presence of HRI and ATP, RF promotes ternary complex formation by eIF-2 X [3H]GDP without accompanying GDP displacement; (iv) in the presence of HRI and ATP, the ternary complex formed using RF is active in Met-tRNAf X 40S initiation complex formation; (v) both the ternary complex and the Met-tRNAf X 40S complex formation in the presence of HRI and ATP are completely inhibited by prior incubation of RF with GDP; (vi) upon further fractionation of an active RF fraction, a preparation can be obtained that contains HRI-sensitive Co-eIF-2 activity. However, this preparation does not efficiently reverse protein synthesis inhibition in heme-deficient reticulocyte lysates and does not contain excess alpha-subunit of eIF-2. Based on these observations, we have suggested (a) RF provides the unphosphorylated alpha-subunit to eIF-2 alpha (P) X GDP and restores eIF-2 activity. This RF activity is inhibited as the alpha-subunit in the RF preparation becomes phosphorylated by HRI and ATP in the presence of GDP; (b) RF contains Co-eIF-2 activity, which has dual functions: (i) stimulation of ternary complex formation by eIF-2 and (ii) GDP displacement from eIF-2 X GDP during ternary complex formation. In the presence of HRI and ATP, Co-eIF-2 but does not displace GDP from eIF-2 alpha(P) X GDP.

Protein synthesis in rabbit reticulocytes: Requirements for Met-tRNAf·40S preinitiation complex formation with AUG-codon and physiological mRNAs

Under standard conditions, in the presence of GTP, highly purified eIF-2 and Co-eIF-2 factor preparations efficiently stimulated AUG-codon dependent but not physiological mRNA-dependent Met-tRNAf binding to 40S ribosomes. Replacement of GTP by a nonhydrolyzable GTP analog, GMP-PNP, in the above system, gave significant stimulation of Met-tRNAf binding to 40S ribosomes dependent on physiological mRNAs. Lower but significant stimulation of Met-tRNAf binding to 40S ribosomes was also observed when GTP was used in the presence of nucleoside 5'-diphosphate kinase (NDK) and ATP. ATP alone in the absence of NDK had no significant effect. This is the first report on the formation of a stable Met-tRNAf . 40S initiation complex dependent on physiological mRNAs and the factor requirements for such complex formation.