Evidence for a second phosphorylation site on eIF-2 alpha from rabbit reticulocytes

Signaling Pathways Involved in the Regulation of mRNA Translation

Translation is a key step in the regulation of gene expression and one of the most energy-consuming processes in the cell. In response to various stimuli, multiple signaling pathways converge on the translational machinery to regulate its function. To date, the roles of phosphoinositide 3-kinase (PI3K)/AKT and the mitogen-activated protein kinase (MAPK) pathways in the regulation of translation are among the best understood. Both pathways engage the mechanistic target of rapamycin (mTOR) to regulate a variety of components of the translational machinery. While these pathways regulate protein synthesis in homeostasis, their dysregulation results in aberrant translation leading to human diseases, including diabetes, neurological disorders, and cancer. Here we review the roles of the PI3K/AKT and MAPK pathways in the regulation of mRNA translation. We also highlight additional signaling mechanisms that have recently emerged as regulators of the translational apparatus.

Phosphorylation stoichiometries of human eukaryotic initiation factors

Eukaryotic translation initiation factors are the principal molecular effectors regulating the process converting nucleic acid to functional protein. Commonly referred to as eIFs (eukaryotic initiation factors), this suite of proteins is comprised of at least 25 individual subunits that function in a coordinated, regulated, manner during mRNA translation. Multiple facets of eIF regulation have yet to be elucidated; however, many of the necessary protein factors are phosphorylated. Herein, we have isolated, identified and quantified phosphosites from eIF2, eIF3, and eIF4G generated from log phase grown HeLa cell lysates. Our investigation is the first study to globally quantify eIF phosphosites and illustrates differences in abundance of phosphorylation between the residues of each factor. Thus, identification of those phosphosites that exhibit either high or low levels of phosphorylation under log phase growing conditions may aid researchers to concentrate their investigative efforts to specific phosphosites that potentially harbor important regulatory mechanisms germane to mRNA translation.

Regulation of mRNA translation by signaling pathways

mRNA translation is the most energy consuming process in the cell. In addition, it plays a pivotal role in the control of gene expression and is therefore tightly regulated. In response to various extracellular stimuli and intracellular cues, signaling pathways induce quantitative and qualitative changes in mRNA translation by modulating the phosphorylation status and thus the activity of components of the translational machinery. In this work we focus on the phosphoinositide 3-kinase (PI3K)/AKT and the mitogen-activated protein kinase (MAPK) pathways, as they are strongly implicated in the regulation of translation in homeostasis, whereas their malfunction has been linked to aberrant translation in human diseases, including cancer.

Phosphorylation of human eukaryotic initiation factor 2γ: novel site identification and targeted PKC involvement

Eukaryotic translation requires a suite of proteins known as eukaryotic initiation factors (eIFs). These molecular effectors oversee the highly regulated initiation phase of translation. Essential to eukaryotic translation initiation is the protein eIF2, a heterotrimeric protein composed of the individually distinct subunits eIF2α, eIF2β, and eIF2γ. The ternary complex, formed when eIF2 binds to GTP and Met-tRNA(i), is responsible for shuttling Met-tRNA(i) onto the awaiting 40S ribosome. As a necessary component for translation initiation, much attention has been given to the phosphorylation of eIF2α. Despite several previous investigations into eIF2 phosphorylation, most have centered on α- or β-subunit phosphorylation and little is known regarding γ-subunit phosphorylation. Herein, we report eight sites of phosphorylation on the largest eIF2 subunit with seven novel phosphosite identifications via high resolution mass spectrometry. Of the eight sites identified, three are located in either the switch regions or nucleotide binding pocket domain. In addition, we have identified a possible kinase of eIF2, protein kinase C (PKC), which is capable of phosphorylating threonine 66 (thr-66) on the intact heterotrimer. These findings may shed new light on the regulation of ternary complex formation and alternate molecular effectors involved in this process prior to 80S ribosome formation and subsequent translation elongation and termination.

Mechanism and regulation of eukaryotic protein synthesis

This review presents a description of the numerous eukaryotic protein synthesis factors and their apparent sequential utilization in the processes of initiation, elongation, and termination. Additionally, the rare use of reinitiation and internal initiation is discussed, although little is known biochemically about these processes. Subsequently, control of translation is addressed in two different settings. The first is the global control of translation, which is effected by protein phosphorylation. The second is a series of specific mRNAs for which there is a direct and unique regulation of the synthesis of the gene product under study. Other examples of translational control are cited but not discussed, because the general mechanism for the regulation is unknown. Finally, as is often seen in an active area of investigation, there are several observations that cannot be readily accommodated by the general model presented in the first part of the review. Alternate explanations and various lines of experimentation are proposed to resolve these apparent contradictions.

Partial purification of a novel N-ethylmaleimide-activated translational inhibitor from adult rat brain

A translational inhibitor that is activated by N-ethylmaleimide treatment can be found in the postmicrosomal fraction prepared from the brain of adult rats, but it is almost undetectable in the same fraction prepared from suckling animals. The inhibitor is thermolabile and remains in the supernatant fraction after precipitation at pH 5. During the purification procedure, the inhibitor in its unactivated state binds to the anion exchanger (diethylaminoethyl-cellulose) but not to the cation exchanger (phosphocellulose). Treatment with N-ethylmaleimide increases inhibitor affinity for the cation exchanger, and this chromatographic step purifies the inhibitor by 143-fold. Both the thermolabile nature and the behavior of the inhibitory activity during the different steps of the purification procedure suggest that this activity is most probably due to a protein. Although the addition of initiation factor 2 reverses the inhibition of protein synthesis in the presence of ATP and Mg2+, the inhibitor does not phosphorylate any of the initiation factor subunits "in vitro," which indicates that it does not contain any intrinsic protein kinase activity. However, its presence in both a crude and a purified preparation of a kinase of the alpha subunit of a brain eukaryotic initiation factor increases the phosphorylation of the alpha subunit of the initiation factor. The mechanism of action of this inhibitor is discussed.

A synthetic peptide substrate for initiation factor-2 kinases

A peptide P(45-56) corresponding to residues 45-56 (sequence: ILLSELSRRRIR) of eIF-2 alpha was synthesised. It was phosphorylated by both of the well characterised eIF-2 alpha kinases viz.; the heme-controlled repressor (HCR) and the double stranded RNA-dependent inhibitor (dsI). Of four other protein kinases tested only protein kinase C (PKC) phosphorylated P(45-56), with complete dependence on phosphatidylserine. Only the residue corresponding to serine-51 in eIF-2 alpha was phosphorylated by HCR, dsI or PKC. The phosphorylation of the peptide by dsI and the phosphorylation of eIF-2 alpha by dsI or HCR showed sigmoidal kinetics with respect to substrate concentration.

Phosphorylation of only serine-51 in protein synthesis initiation factor-2 is associated with inhibition of peptide-chain initiation in reticulocyte lysates

We have examined the phosphorylation of the alpha-subunit of initiation factor-2 (eIF-2 alpha) in reticulocyte lysates in which translational shut-off was induced by haem-deficiency or by double-stranded RNA. To maximise the phosphorylation of eIF-2 alpha, lysates were supplemented with the broad spectrum phosphatase inhibitor microcystin. Under all conditions tested, serine-51 was the only residue to become labelled. This is consistent with the observation of only two species of eIF-2 alpha in isoelectric focusing/immunoblotting analyses of lysates treated as described above.

Eukaryotic protein synthesis initiation factor 2. A target for inactivation by proanthocyanidin

Polyproanthocyanidin (PPA), a phenolic polymer isolated from the plant Alhagi kirgisorum S. was found to interact strongly with eukaryotic initiation factor 2 (eIF-2), thereby inhibiting reactions involving this protein. When added to a rabbit reticulocyte lysate system, PPA blocks in vitro translation and it appears to selectively bind and precipitate a relatively small number of proteins including eIF-2 and regulin. The phosphorylation of purified regulin and eIF-2 by casein kinase II (CK II) and the heme-sensitive eIF-2 alpha kinase, respectively, was also inhibited by the polyphenolic compound. The natural fluorescence of PPA was utilized to compare its interaction with eIF-2 and regulin to that with other natural and synthetic polypeptides.

The substrate specificity of protein kinases which phosphorylate the alpha subunit of eukaryotic initiation factor 2

The alpha subunit of eukaryotic protein synthesis initiation factor (eIF-2 alpha) is phosphorylated at a single serine residue (Ser51) by two distinct and well-characterized protein kinase, the haem-controlled repressor (HCR) and the double-stranded RNA-activated inhibitor (dsI). The sequence adjacent to Ser51 is rich in basic residues (Ser51-Arg-Arg-Arg-Ile-Arg) suggesting that they may be important in the substrate specificity of the two kinases, as is the case for several other protein kinases. A number of proteins and synthetic peptides containing clusters of basic residues were tested as substrates for HCR and dsI. Both kinases were able to phosphorylate histones and protamines ar multiple sites as judged by two-dimensional mapping of the tryptic phosphopeptides. These data also showed that the specificities of the two kinases were different from one another and from the specificities of two other protein kinases which recognise basic residues, cAMP-dependent protein kinase and protein kinase C. In histones, HCR phosphorylated only serine residues while dsI phosphorylated serine and threonine. Based on phosphoamino acid analyses and gel filtration of tryptic fragments, dsI was capable of phosphorylating both 'sites' in clupeine Y1 and salmine A1, whereas HCR acted only on the N-terminal cluster of serines in these protamines. The specificities of HCR and dsI were further studied using synthetic peptides with differing configurations of basic residues. Both kinases phosphorylated peptides containing C-terminal clusters of arginines on the 'target' serine residue, provided that they were present at positions +3 and/or +4 relative to Ser51. However, peptides containing only N-terminal basic residues were poor and very poor substrates for dsI and HCR, respectively. These findings are consistent with the disposition of basic residues near the phosphorylation site in eIF-2 alpha and show that the specificities of HCR and dsI differ from other protein kinases whose specificities have been studied.

Intracellular messengers and the control of protein synthesis

The molecular events responsible for controlling cell growth and development, as well as their coordinate interaction is only beginning to be revealed. At the basis of these controlling events are hormones, growth factors and mitogens which, through transmembrane signalling trigger an array of cellular responses, initiated by receptor-associated tyrosine kinases, which in turn either directly or indirectly mediate their effects through serine/threonine protein kinases. Utilizing the obligatory response of activation of protein synthesis in cell growth and development, we describe efforts to work backwards along the regulatory pathway to the receptor, identifying those molecular components involved in modulating the rate of translation. We begin by describing the components and steps of protein synthesis and then discuss in detail the regulatory pathways involved in the mitogenic response of eukaryotic cells and during meiotic maturation of oocytes. Finally we discuss possible future work which will further our understanding of these systems.

Protein kinase recognition sequence motifs

Protein kinases play a crucial role in the regulation of many cellular processes. They alter the functions of their target proteins by phosphorylating specific serine, threonine and tyrosine residues. Identification of phosphorylation site sequences and studies with corresponding model peptides have provided clues to how these important enzymes recognize their substrate proteins. This knowledge has made it possible to identify potential sites of phosphorylation in newly sequenced proteins as well as to construct specific model substrates and inhibitors.

Phosphorylation of protein synthesis initiation factor-2. Identification of the site in the alpha-subunit phosphorylated in reticulocyte lysates

The data presented here show that serine-51 of the alpha-subunit of eukaryotic initiation factor eIF-2 is the only residue phosphorylated by the eIF-2 alpha-specific kinases HCR (haem-controlled repressor) and dsI (double-stranded RNA-activated inhibitor) in vitro. This confirms our earlier finding that serine-48 is not labelled by either kinase. Methodology appropriate for the examination of phosphorylation sites in eIF-2 alpha in whole cells and their extracts has been developed, and used to study the site(s) in eIF-2 alpha labelled in reticulocyte lysates. Only serine-51 became phosphorylated under conditions of haem-deficiency or in the presence of double-stranded RNA. No evidence for a second phosphorylation site on the alpha-subunit was obtained with the lysates and conditions used here.

Two phosphorylation sites on eIF-2 alpha

Protein synthesis in mammalian cells can be regulated through phosphorylation/dephosphorylation of the alpha subunit of initiation factor 2, eIF-2. Two specific kinases have been identified that apparently phosphorylate the same site(s). Controversy exists as to whether serine-48 is a phosphorylation site in addition to serine-51. A recent publication is discussed that, in this author's view, answers the question of the phosphorylation sites. It is suggested that phosphorylation proceeds sequentially with serine-51 being the first and serine-48 the second phosphorylation site. Phosphorylation of both sites is required for inhibition of protein synthesis.

The mapping of interferon-induced proteins and phosphoproteins from HeLa S3 cells

Fourteen of 18 proteins induced by interferon-alpha (IFN-alpha) in HeLa S3 cells were labeled with [35S]methionine, resolved by two-dimensional electrophoresis, and assigned coordinates corresponding to HeLa proteins previously mapped by Bravo and Celis (Clin. Chem. 28, 766-781, 1982). Proteins phosphorylated with [gamma-32P]ATP in response to polyinosinic:polycytidylic acid [poly(I):poly(C)] were mapped similarly. Multiple phosphorylated species of a 72-kD protein were labeled in response to poly(I):poly(C) by extracts from IFN-treated cells but not by extracts from control cells. These are likely phosphorylated forms of the IFN-induced poly(I):poly(C)-dependent protein kinase, the enzyme responsible for the phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 (eIF-2 alpha). Two phosphorylated forms of eIF-2 alpha were labeled in extracts of IFN-treated cells. One of these is a new phosphorylated product of the double-stranded (ds) RNA-activated protein kinase.

The phosphorylation of eukaryotic initiation factor 2: a principle of translational control in mammalian cells

In eukaryotic cells, protein biosynthesis is controlled at the level of polypeptide chain initiation. During the initiation process, eukaryotic initiation factor 2 (eIF-2) catalyzes the binding of Met-tRNAf and GTP to the 40S ribosomal subunit. In a later step, eIF-2 is released from the ribosomal initiation complex, most likely as an eIF-2.GDP complex, and another initiation factor termed eIF-2B is necessary to recycle eIF-2 by displacing GDP by GTP. In rabbit reticulocytes, inhibition of protein synthesis is accompanied by the phosphorylation of the alpha-subunit of eIF-2, a process that does not render eIF-2 inactive, but prevents it from being recycled by eIF-2B. First described in rabbit reticulocytes as inhibitors of translation, two distinct eIF-2 alpha kinases are known: the haemin-controlled kinase (termed HCI) and the double-stranded RNA-activated kinase (termed DAI). eIF-2 alpha phosphorylation appears to be a reversible control mechanism since corresponding phosphatases have been described. Recent reports indicate a correlation between eIF-2 alpha phosphorylation and the inhibition of protein synthesis in several mammalian cell types under a range of physiological conditions. In this review, the physical and functional features of the known eIF-2 alpha kinases are described with respect to their role in mammalian cells and the mode of activation by cellular signals. Furthermore, the possible impact of the eIF-2/eIF-2B ratio and of the subcellular compartmentation of these factors (and the eIF-2 alpha kinases) on mammalian protein synthesis is discussed.

Generation of a mutant form of protein synthesis initiation factor eIF-2 lacking the site of phosphorylation by eIF-2 kinases

The phosphorylation of the alpha-subunit of initiation factor eIF-2 leads to an inhibition of protein synthesis in mammalian cells. We have performed site-directed mutagenesis on a cDNA encoding the alpha-subunit of human eIF-2 and have replaced the candidate sites of phosphorylation, Ser-48 and Ser-51, with alanines. The cDNAs were expressed in vitro by SP6 polymerase transcription and rabbit reticulocyte lysate translation, and the radiolabeled protein products were analyzed by high-resolution two-dimensional gel electrophoresis. The wild-type and Ser-48 mutant proteins became extensively phosphorylated by eIF-2 kinases present in the reticulocyte lysate, and when additional heme-controlled repressor or double-stranded RNA-activated kinase was present, phosphorylation of the proteins was enhanced. The Ser-51 mutant showed little covalent modification by the endogenous enzymes and showed no increase in the acidic variant with additional eIF-2 kinases, thereby suggesting that Ser-51 is the site of phosphorylation leading to repression of protein synthesis.

Generation of a mutant form of protein synthesis initiation factor eIF-2 lacking the site of phosphorylation by eIF-2 kinases

The phosphorylation of the alpha-subunit of initiation factor eIF-2 leads to an inhibition of protein synthesis in mammalian cells. We have performed site-directed mutagenesis on a cDNA encoding the alpha-subunit of human eIF-2 and have replaced the candidate sites of phosphorylation, Ser-48 and Ser-51, with alanines. The cDNAs were expressed in vitro by SP6 polymerase transcription and rabbit reticulocyte lysate translation, and the radiolabeled protein products were analyzed by high-resolution two-dimensional gel electrophoresis. The wild-type and Ser-48 mutant proteins became extensively phosphorylated by eIF-2 kinases present in the reticulocyte lysate, and when additional heme-controlled repressor or double-stranded RNA-activated kinase was present, phosphorylation of the proteins was enhanced. The Ser-51 mutant showed little covalent modification by the endogenous enzymes and showed no increase in the acidic variant with additional eIF-2 kinases, thereby suggesting that Ser-51 is the site of phosphorylation leading to repression of protein synthesis.