Nucleoside diphosphate regulation of overall rates of protein biosynthesis acting at the level of initiation

Phytomedicines as potential inhibitors of β amyloid aggregation: significance to Alzheimer's disease

Throughout the history of drug development, plants have been an important source for the discovery of novel therapeutically active compounds for many diseases. The ethnopharmacological approach has provided several leads to identify potential new drugs from plant sources, including those for memory disorders. For the treatment of Alzheimer's disease the drug discovery focus shifted from cholinesterase inhibitors, to other targets primarily based on two key neuropathological hallmarks, namely the hyperphosphorylation of the tau protein resulting in the formation of neurofibrillary tangles (NFTs), and the increased formation and aggregation of amyloid-beta peptide (Aβ) derived from amyloid precursor protein (APP). The present article aims to provide a comprehensive literature survey of plants and their constituents that have been tested for Aβ aggregation, thus possibly relieving several features of Alzheimer's disease (AD).

Exogenous nucleosides accelerate differentiation of rat intestinal epithelial cells

Several studies have reported differing data on the effect of exogenous nucleosides and nucleotides on the proliferation and differentiation in various intestinal cell lines and explants. To study whether exogenous nucleosides modulate intestinal cell differentiation, IEC-6 cells were differentiated in the presence or absence of a nucleoside mixture (cytidine, uridine, guanosine and inosine, 30 microM each), and the concentrations of nucleoside derivatives were determined by HPLC. Cell differentiation was assessed by electron microscopy, alkaline phosphatase activity and Rnd3 gene expression. The present results showed that uridine, guanosine and inosine were cleared from culture media (up to 32, 63 and 100 % in proliferating cells, and 31, 80 and 94 % in differentiated cells, respectively) whereas cytidine concentrations increased. Differentiation of IEC-6 cells was associated with a significant increase in intracellular nucleotide concentrations. Clearance of nucleosides correlated with a significant increase in the intracellular nucleotide pool in proliferating and differentiated IEC-6 cells. Intracellular guanosine nucleotides increased 2.5- and 5-fold in nucleoside-supplemented proliferating and differentiated cells, respectively. At 24 h, nucleoside-supplemented differentiated IEC-6 cells had significantly higher energy charge and GTP levels than non-supplemented ones. These modifications paralleled changes in cell differentiation as indicated by increased alkaline phosphatase activity, prolonged microvilli formation and accelerated down-regulation of Rnd3 gene expression. The present findings suggest that exogenous nucleosides were selectively taken up by IEC-6 cells, increased the intracellular nucleotide pool, GTP and energy charge, and favoured cell morphological and functional changes during differentiation.

An economical method for cell-free protein synthesis using glucose and nucleoside monophosphates

Cell-free protein synthesis reactions have not been seriously considered as a viable method for commercial protein production mainly because of high reagent costs and a lack of scalable technologies. Here we address the first issue by presenting a cell-free protein synthesis system with comparable protein yields that removes the most expensive substrates and lowers the cell-free reagent cost by over 75% (excluding extract, polymerase, and plasmid) while maintaining high energy levels. This system uses glucose as the energy source and nucleoside monophosphates (NMPs) in place of nucleoside triphosphates (NTPs) as the nucleotide source. High levels of nucleoside triphosphates are generated from the monophosphates within 20 min, and the subsequent energy charge is similar in reactions beginning with either NTPs or NMPs. Furthermore, significant levels (>0.2 mM) of all NTPs are still available at the end of a 3-h incubation, and the total nucleotide pool is stable throughout the reaction. The glucose/NMP reaction was scaled up to milliliter scale using a thin film approach. Significant yields of active protein were observed for two proteins of vastly different size: chloramphenicol acetyl transferase (CAT, 25 kDa) and beta-galactosidase (472 kDa). The glucose/NMP cell-free reaction system dramatically reduces reagent costs while supplying high protein yields.

Mammalian stress granules represent sites of accumulation of stalled translation initiation complexes

In eukaryotic cells subjected to environmental stress, untranslated mRNA accumulates in discrete cytoplasmic foci that have been termed stress granules. Recent studies have shown that in addition to mRNA, stress granules also contain 40S ribosomal subunits and various translation initiation factors, including the mRNA binding proteins eIF4E and eIF4G. However, eIF2, the protein that transfers initiator methionyl-tRNA(i) (Met-tRNA(i)) to the 40S ribosomal subunit, has not been detected in stress granules. This result is surprising because the eIF2. GTP. Met-tRNA(i) complex is thought to bind to the 40S ribosomal subunit before the eIF4G. eIF4E. mRNA complex. In the present study, we show in both NIH-3T3 cells and mouse embryo fibroblasts that stress granules contain not only eIF2 but also the guanine nucleotide exchange factor for eIF2, eIF2B. Moreover, we show that phosphorylation of the alpha-subunit of eIF2 is necessary and sufficient for stress granule formation during the unfolded protein response. Finally, we also show that stress granules contain many, if not all, of the components of the 48S preinitiation complex, but not 60S ribosomal subunits, suggesting that they represent stalled translation initiation complexes.

Evidence that ternary complex (eIF2-GTP-tRNA(i)(Met))-deficient preinitiation complexes are core constituents of mammalian stress granules

Environmental stress-induced phosphorylation of eIF2alpha inhibits protein translation by reducing the availability of eIF2-GTP-tRNA(i)Met, the ternary complex that joins initiator tRNA(Met) to the 43S preinitiation complex. The resulting untranslated mRNA is dynamically routed to discrete cytoplasmic foci known as stress granules (SGs), a process requiring the related RNA-binding proteins TIA-1 and TIAR. SGs appear to be in equilibrium with polysomes, but the nature of this relationship is obscure. We now show that most components of the 48S preinitiation complex (i.e., small, but not large, ribosomal subunits, eIF3, eIF4E, eIF4G) are coordinately recruited to SGs in arsenite-stressed cells. In contrast, eIF2 is not a component of newly assembled SGs. Cells expressing a phosphomimetic mutant (S51D) of eIF2alpha assemble SGs of similar composition, confirming that the recruitment of these factors is a direct consequence of blocked translational initiation and not due to other effects of arsenite. Surprisingly, phospho-eIF2alpha is recruited to SGs that are disassembling in cells recovering from arsenite-induced stress. We discuss these results in the context of a translational checkpoint model wherein TIA and eIF2 are functional antagonists of translational initiation, and in which lack of ternary complex drives SG assembly.

A new method of ATP regeneration rate measurement using a pH meter

A new method was developed for measuring the ATP regeneration rate by creatine kinase from creatine phosphate and ADP. The ATP regeneration rate was calculated to be 0.20 mM x s(-1) at room temperature from the pH change of the regeneration mixture.

Adaptation to hypoxia alters energy metabolism in rat heart

The present study characterized metabolic changes in the heart associated with long-term exposure to hypoxia, a potent stimulus for pulmonary hypertension and right ventricular hypertrophy. When anesthetized rats adapted to chronic hypoxia spontaneously respired room air, their mean right intraventricular peak systolic pressure (RVSP) was twice that in normal control animals with the same arterial PO2. RVSP was linearly related to right ventricular mass (r = 0.78). Oxidative capacity (O2 consumption) of homogenates of right and left ventricles from both groups of rats was measured with one of the following substrates: pyruvate, glutamate, acetate, and palmitoyl-L-carnitine. Oxidation of all substrates was significantly greater in the left than in the right ventricle in normal rats but not in hypoxia-adapted animals, where it was the same, within the experimental error. O2 consumption by the left ventricle was greater in control than in experimental rats, but right ventricular O2 consumption was similar in the two groups. Maximal reaction velocity of cytochrome-c oxidase was about the same in the two ventricles, and there were no significant differences between control and hypoxia-adapted animals. HPLC analyses showed significantly higher aspartate levels and aspartate-to glutamate concentration ratios in both ventricles of hypoxic rats than in corresponding tissues from controls, indicative of a decreased flux through the malate-aspartate shuttle under conditions of O2 limitation. Myocardial glutamine levels were lower in hypoxic rats, and glutamine-to-glutamate concentration ratios decreased, although primarily in the pressure-overloaded right ventricle. These findings indicate that normal energy metabolism in the left ventricle differs from that in the right and that the differences, particularly those of amino acid metabolism, are markedly influenced by chronic exposure to hypoxia.

Recent advances in producing and selecting functional proteins by using cell-free translation

Prokaryotic and eukaryotic in vitro translation systems have recently become the focus of increasing interest for tackling fundamental problems in biochemistry. Cell-free systems can now be used to study the in vitro assembly of membrane proteins and viral particles, rapidly produce and analyze protein mutants, and enlarge the genetic code by incorporating unnatural amino acids. Using in vitro translation systems, display techniques of great potential have been developed for protein selection and evolution. Furthermore, progress has been made to efficiently produce proteins in batch or continuous cell-free translation systems and to elucidate the molecular causes of low yield and find possible solutions for this problem.

Interactions between bioenergetics and mitochondrial biogenesis

We studied the interaction between energy metabolism and mitochondrial biogenesis during myogenesis in C2C12 myoblasts. Metabolic rate was nearly constant throughout differentiation, although there was a shift in the relative importance of glycolytic and oxidative metabolism, accompanied by increases in pyruvate dehydrogenase activation state and total activity. These changes in mitochondrial bioenergetic parameters observed during differentiation occurred in the absence of a hypermetabolic stress. A chronic (3 day) energetic stress was imposed on differentiated myotubes using sodium azide to inhibit oxidative metabolism. When used at low concentrations, azide inhibited more than 70% of cytochrome oxidase (COX) activity without changes in bioenergetics (either lactate production or creatine phosphorylation) or mRNA for mitochondrial enzymes. Higher azide concentrations resulted in changes in bioenergetic parameters and increases in steady state COX II mRNA levels. Azide did not affect mtDNA copy number or mRNA levels for other mitochondrial transcripts, suggesting azide affects stability, rather than synthesis, of COX II mRNA. These results indicate that changes in bioenergetics can alter mitochondrial genetic regulation, but that mitochondrial biogenesis accompanying differentiation occurs in the absence of hypermetabolic challenge.

A beta amyloidogenesis: unique, or variation on a systemic theme?

For more than a century amyloid was considered to be an interesting, unique, but inconsequential pathologic entity that rarely caused significant clinical problems. We now recognize that amyloid is not one entity. In vivo it is a uniform organization of a disease, or process, specific protein co-deposited with a set of common structural components. Amyloid has been implicated in the pathogenesis of diseases affecting millions of patients. These range from Alzheimer's disease, adult-onset diabetes, consequences of prolonged renal dialysis, to the historically recognized systemic forms associated with inflammation and plasma cell disturbances. Strong evidence is emerging that even when deposited in local organ sites significant physiologic effects may ensue. With emphasis on A beta amyloid, we review the present definition, classification, and general in vivo pathogenetic events believed to be involved in the deposition of amyloids. This encompasses the need for an adequate amyloid precursor protein pool, whether precursor proteolysis is required prior to deposition, amyloidogenic amino acid sequences, fibrillogenic nucleating particles, and an in vivo microenvironment conducive to fibrillogenesis. The latter includes several components that seem to be part of all amyloids. The role these common components may play in amyloid accumulation, why amyloids tend to be associated with basement membranes, and how one may use these findings for anti-amyloid therapeutic strategies is also examined.

Effect of the ATP level on the overall protein biosynthesis rate in a wheat germ cell-free system

A sensitive assay which examines the effects of ATP level on the overall activity of a cell-free translation system in a protein synthesis is described. The translational activity of cell-free system was measured in terms of a rate of protein synthesis directed by the 'test' template. The test template encodes a photoluminescent protein, obelin accumulation was determined from the kinetic curves of obelin. The rate of obelin mRNA translation. Time-dependent nucleotide level measurements were conducted throughout the translation processes. It has been shown that the rate of translation decreases exponentially with the decrease of the ATP level. This fall in the overall translation rate is due in part to the mRNA becoming inactive in the translation process. This is not caused by degradation, this mRNA can be restored for translation in a fresh cell-free system by phenol treatment. The reported results provide evidence that the level of ATP unambiguously determines the translational activity of the system.

The guanine nucleotide-exchange factor, eIF-2B

Eukaryotic initiation factor eIF-2B catalyses the exchange of guanine nucleotides on another translation initiation factor, eIF-2, which itself mediates the binding of the initiator Met-tRNA to the 40S ribosomal subunit during translation initiation. eIF-2B promotes the release of GDP from inactive [eIF-2.GDP] complexes, thus allowing formation of the active [eIF-2.GTP] species which subsequently binds the Met-tRNA. This guanine nucleotide-exchange step, and thus eIF-2B activity, are known to be an important control point for translation initiation. The activity of eIF-2B can be modulated in several ways. The best characterised of these involves the phosphorylation of the alpha-subunit of eIF-2 by specific protein kinases regulated by particular ligands. Phosphorylation of eIF-2 alpha leads to inhibition of eIF-2B. This mechanism is involved in the control of translation under a variety of conditions, including amino acid deprivation in yeast (Saccharomyces cerevisiae) where it causes translational upregulation of the transcription factor GCN4, and in virus-infected animal cells, where it involves a protein kinase activated by double-stranded RNA. There is now also growing evidence for direct regulation of eIF-2B. This appears likely to involve the phosphorylation of its largest subunit. Under certain circumstances eIF-2B may also be regulated by allosteric mechanisms. eIF-2B is a heteropentamer (subunits termed alpha, beta, gamma, delta and epsilon) and is thus more complex than most other guanine nucleotide-exchange factors. The genes encoding all five subunits have been cloned in yeast (exploiting the GCN4 regulatory system): all but the alpha appear to be essential for eIF-2B activity. However, this subunit may confer sensitivity to eIF-2 alpha phosphorylation. cDNAs encoding the alpha, beta, delta and epsilon subunits have been cloned from mammalian sources. There is substantial homology between the yeast and mammalian sequences. Attention is now directed towards understanding the roles of individual subunits in the function and regulation of eIF-2B.

Translational control during amino acid starvation

Amino acid starvation of mammalian cells results in a pronounced fall in the overall rate of protein synthesis. This is associated with increased phosphorylation of the alpha-subunit of the initiation factor eIF-2, which in turn impairs the activity of the guanine nucleotide exchange factor, eIF-2B. Similar mechanisms have now been found to operate in the yeast, Saccharomyces cerevisiae, where the major physiological result is to circumvent the lack of external amino acids by promoting the translation of a transcription factor, GCN4, that facilitates the expression of a number of enzymes required for amino acid biosynthesis. This article reviews current knowledge of these mechanisms in both mammalian and yeast cells and identifies questions still requiring elucidation.

Interaction of wheat germ translation initiation factor 2 with GDP and GTP

The wheat germ translation initiation factor 2 (WGeIF-2) was isolated in a homogeneous state by an efficient procedure and characterized. Its molecular mass, as determined by a gel-filtration method is approximately 150,000 Da. According to SDS-PAGE WGeIF-2 consists of four subunits with M(r) 37,000 (alpha), 40,000 (beta), 42,000 (gamma) and 52,000 (delta). The beta- and gamma-subunits (but not the alpha-subunit) of WGeIF-2 can be readily phosphorylated by the double-stranded RNA activated kinase isolated from rabbit reticulocytes. Dissociation constants for WGeIF-2 complexes with GDP and GTP were measured. In our evaluation the WGeIF-2 affinity for GDP (KdGDP = 1.5 x 10(-7) M) was only 10 times higher than for GTP (KdGTP = 1.5 x 10(-6) M), while for rabbit reticulocyte eIF-2 (RReIF-2) the difference has been estimated as as much as two orders of magnitude in accordance with the literature. Close values of dissociation constants for WGeIF-2 complexes with guanine nucleotides suggest that at a sufficiently high [GTP]/[GDP] ratio the nucleotide exchange in wheat cells may take place without the participation of specific factor (eIF-2B) which catalyzes the nucleotide exchange on eIF-2 from mammalian cells.

Inhibition of the initiation of hepatic protein synthesis during ethionine mediated ATP depletion in vivo: modification to ribosomal subunits, evidence of impaired ternary complex formation and a subcellular redistribution of eIF-2 alpha

Acute ethionine intoxication is known to induce a reversible hepatic injury in female rats by reducing the level of hepatic ATP. The injury indirectly impairs the initiation of hepatic protein synthesis, with resultant polysome disaggregation. Administration of adenine rapidly restores the ATP levels and protein synthesis. Analysis of liver polysome and ribosomal subunits reveals that polysome disaggregation occurs following 3 h of the intoxication, and reaggregation occurs following the administration of adenine. Inactive hepatic ribosomes accumulate as monomers and disomes when analysed by sucrose gradient sedimentation in low-salt buffers. High-salt buffers dissociate the inactive ribosomes into the component 40 S and 60 S subunits. The level of higher density, 1.48 g/cc, 40 S subunit increases during the inhibition of protein synthesis, while the lower density, 1.41 g/cc, 40 S subunit species does not change significantly. Hepatic microsomal and cytosolic extracts examined for their ability to support the formation of the ternary complex of eIF-2-GTP and [35S]Met-tRNAi demonstrate that during acute ethionine intoxication, ternary complex formation in the two extracts decrease 65% and 85%, respectively. These changes are coincident with polysome disaggregation. Administration of adenine to reverse the intoxication restores the ternary complex forming ability of the cytosolic extract, but does not affect the activity of the microsomal salt wash extracts. Mixing experiments indicate the accumulation of an inhibitor of ternary complex formation in the microsomal salt wash fraction. The application of quantitative western blotting demonstrates that the level of antigenic eIF-2 alpha in the microsomal salt wash extract increases 31% during the inhibition. These observations are consistent with the idea that the inhibition of the initiation of hepatic protein synthesis induced by ethionine is mediated by eIF-2 alpha phosphorylation. The latter results in an inhibition of ternary complex formation, redistribution of eIF-2 to the microsome fraction, polysomal disaggregation, and accumulation of inactive ribosomal subunits.

In vitro ischemia and protein synthesis in the rat hippocampal slice: the role of calcium and NMDA receptor activation

The rat hippocampal slice was developed as a model for investigating the effects of ischemia on protein synthesis in different cell types, as synthesis is an early functional indicator of cell damage. Five min of in vitro ischemia inhibited protein synthesis in CA1 pyramidal and subicular neurons 3 h later, despite recovery of the energy charge. Morphology of these neurons was also affected. In contrast, glia and capillary endothelial cells showed increased synthesis at this time point, and no apparent structural changes. Exposure of slices to buffer lacking calcium and containing the non-competitive NMDA receptor blocker ketamine, during the 5 min ischemia, prevented both the inhibition of protein synthesis and the morphologic changes in the neurons. However, if buffer only lacked calcium, or only contained ketamine, both forms of ischemic damage occurred. Thus, the neuronal protein synthesis inhibition and the impaired morphology appear to be mediated by either extracellular calcium or NMDA receptor activation. In contrast to the neurons, the ischemia-induced stimulation of protein synthesis in glia and capillary endothelial cells was not affected by the above treatments, indicating that neither NMDA receptor activation nor extracellular calcium is necessary for this effect.

Investigations on the function of creatine kinase in Ehrlich ascites tumor cells

1. Growth and viability of in vitro cultured Ehrlich ascites tumor cells are not significantly impaired by exogenous creatine up to 40mM. Retardation of cell growth by higher concentrations depends on cell density. 2. Ehrlich cells grown in the presence of high concentrations of creatine accumulate creatine phosphate to high levels (up to 23 nmol/10(6) cells in the presence of 40mM creatine). 3. A nearly complete interruption of glycolytic ATP production or inhibition of the oxidative ATP synthesis reduces the maximal creatine to about 40-50% of controls. 4. Studies on the intracellular distribution of creatine kinase have shown, that the enzyme is only associated with the mitochondrial fraction. Titration of isolated mitochondria with digitonin revealed that the activity is located in the inter-membrane space and partly bound to the outer site of the inner membrane. 5. By growth of Ehrlich cells in creatine-free medium it is possible to obtain "creatine phosphate-depleted" cells (creatine phosphate less than 10% of controls). The growth of creatine phosphate-depleted cells as compared to controls is significantly reduced under energetic stress situations. The protein synthesis of these cells after an energetic stress (lack of glucose and oxygen) is significantly reduced as compared to creatine phosphate containing cells. 6. It is concluded that in these cells creatine kinase/creatine phosphate is a thermodynamic buffer system and not part of an energy shuttle as is postulated for muscle cells.

Neuroendocrine and juvenile hormone effects on fat body protein synthesis during the reproductive cycle in female Blaberus discoidalis cockroaches

Ovarian protein content and fat body protein synthesis were measured during the first gonotrophic cycle in virgin female Blaberus discoidalis cockroaches. Protein synthesis was measured for in vitro fat bodies from animals treated with combinations of a juvenile hormone analog (JHA = methoprene) and corpora cardiaca (CC) extracts. Ovarian protein content began to increase on Day 5 of adult life and reached its maximum at Days 20-22. Synthesis of proteins secreted by the in vitro fat body increased by 12-fold between emergence and its maximum on Day 18, then declined to nearly its original level by Day 33. Synthesis of nonsecreted, fat body proteins increased by 4-fold between emergence and a maximum on Day 21, then declined. CC extracts and JHA were administered to decapitated females to determine their regulative effects on fat body protein synthesis. The synthesis by the fat body of nonsecreted proteins was increased by both JHA and CC extracts. In contrast, synthesis of secreted proteins increased only in the presence of JHA. CC extracts, alone, had no effects on the synthesis of secreted proteins, but administration of CC extracts in combination with JHA increased the synthesis of the secreted proteins by 55% above that observed with JHA alone. SDS-PAGE of proteins secreted into the medium by the in vitro fat body demonstrated that JHA stimulated the synthesis of specific polypeptides, whereas CC extracts did not affect the synthesis of specific-secreted polypeptides. These results suggest that JH regulates specific protein synthesis by the fat body of B. discoidalis; neurohormones elevate the general capacity of the fat body for protein synthesis and amplify the specific effects of JH.

Association of glyceraldehyde-3-phosphate dehydrogenase with mono- and polyribosomes of rabbit reticulocytes

It has been shown recently that glyceraldehyde-3-phosphate dehydrogenase (GAPD) is one of the three major RNA-binding proteins of rabbit reticulocytes [Ryazanov, A. G. (1985) FEBS Lett. 192, 131-134]. It was suggested that, due to its RNA-binding capacity, GAPD can form loose dynamic complexes with polyribosomes. This communication reports that a considerable amount of GAPD activity can be found in the mono- and polyribosome fraction after sucrose gradient centrifugation of rabbit reticulocyte lysate. An increase of ionic strength, as well as the addition of exogenous RNA to the extract, result in the removal of GAPD from the complex with mono- and polyribosomes. It appears that GAPD forms the complex with polyribosomes due to the interaction with some exposed RNA regions of these structures. Although the interaction of GAPD with ribosomes is weak, it can be detected under physiological ionic conditions by the difference boundary sedimentation velocity technique. Association of GAPD with mono- and polyribosomes can be prevented by a low concentration (10 microM) of NADH, but not NAD+. A nitrocellulose filter binding assay also shows that NADH has a stronger inhibitory effect on the enzyme-RNA complex formation, as compared with NAD+. We propose that the RNA-mediated association of GAPD with mono- and polyribosomes can provide compartmentation of the energy-supplying system on these structures within the cell. This can maintain a high local concentration of ATP and GTP near the sites of protein synthesis.

5'-adenosine monophosphate inhibits ternary complex formation by rat liver eIF-2

The initiation factor 2 (eIF-2) was purified from rat liver essentially to homogeneity. The effect of physiological concentrations of AMP on ternary complex formation by eIF-2 was tested. It is shown that AMP inhibits ternary complex formation (ca. 50% at 100 microM AMP). None of the other adenine-nucleotides tested (ADP, ATP, cAMP, NADH, NAD+ and NADPH) affected ternary complex formation.

eIF-2B and the exchange of guanine nucleotides bound to eIF-2

Available data for the formation of the ternary complex eIF-2 X GTP X methionyl-tRNAi involved in eukaryotic initiation and of the inhibition of ternary complex formation by GDP have been examined with a view to determining the mechanism by which eIF-2B facilitates nucleotide exchange. Two mechanisms have been considered--first a displacement reaction in which eIF-2B displaces GDP and GTP in a manner analogous to a "ping-pong" enzyme mechanism, and secondly the possibility that binding of eIF-2B to eIF-2 nucleotide complexes enhances the rate of nucleotide exchange without itself inducing nucleotide displacement. Comparison has been made between the properties of eIF-2 and eIF-2B and of the bacterial elongation factors Tu and Ts. It seems most probable that, as previously suggested by others for Ts, eIF-2B effectively catalyses an exchange reaction through a "ping-pong" type mechanism. Possible explanations of data suggesting otherwise are put forward. Both eIF-2 and bacterial Tu are complex allosteric proteins subject to a variety of influences which in the case of eIF-2 include phosphorylation of the alpha subunit. This phosphorylation appears to change the equilibria in the reaction mechanism such that the transferred entity (eIF-2) becomes firmly bound to the catalyst (eIF-2B). Minimum rate constants for the formation of eIF-2 X eIF-2B from eIF-2 X GDP and eIF-2 X GTP and reverse reactions are derived. These values suggest that the initiation factors are likely to have to operate in a restricted environment if rates of protein synthesis seen in vivo are to be sustained.