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eIF4B phosphorylation at Ser504 links synaptic activity with protein translation in physiology and pathology. Sci Rep 2017; 7:10563. [PMID: 28874824 PMCID: PMC5585320 DOI: 10.1038/s41598-017-11096-1] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/10/2017] [Accepted: 08/14/2017] [Indexed: 12/20/2022] Open
Abstract
Neuronal physiology requires activity-driven protein translation, a process in which translation initiation factors are key players. We focus on eukaryotic initiation factor 4B (eIF4B), a regulator of protein translation, whose function in neurons is undetermined. We show that neuronal activity affects eIF4B phosphorylation and identify Ser504 as a phosphorylation site regulated by casein kinases and sensitive to the activation of metabotropic glutamate receptors. Ser504 phosphorylation increases eIF4B recruitment to the pre-initiation complex and influences eIF4B localization at synapses. Moreover, Ser504 phosphorylation modulates the translation of protein kinase Mζ. Therefore, by sensing synaptic activity, eIF4B could adjust translation to neuronal needs, promoting adaptive changes in synaptic plasticity. We also show that Ser504 phosphorylation is increased in vivo in a rat model of epilepsy during epileptogenesis i.e. when translation drives maladaptive synaptic changes. We propose eIF4B as a mediator between neuronal activity and translation, with relevance in the control of synaptic plasticity.
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2
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Gallie DR. The role of the poly(A) binding protein in the assembly of the Cap-binding complex during translation initiation in plants. ACTA ACUST UNITED AC 2014; 2:e959378. [PMID: 26779409 DOI: 10.4161/2169074x.2014.959378] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/15/2014] [Revised: 05/19/2014] [Accepted: 06/17/2014] [Indexed: 12/30/2022]
Abstract
Translation initiation in eukaryotes requires the involvement of multiple initiation factors (eIFs) that facilitate the binding of the 40 S ribosomal subunit to an mRNA and assemble the 80 S ribosome at the correct initiation codon. eIF4F, composed of eIF4E, eIF4A, and eIF4G, binds to the 5'-cap structure of an mRNA and prepares an mRNA for recruitment of a 40 S subunit. eIF4B promotes the ATP-dependent RNA helicase activity of eIF4A and eIF4F needed to unwind secondary structure present in a 5'-leader that would otherwise impede scanning of the 40 S subunit during initiation. The poly(A) binding protein (PABP), which binds the poly(A) tail, interacts with eIF4G and eIF4B to promote circularization of an mRNA and stimulates translation by promoting 40 S subunit recruitment. Thus, these factors serve essential functions in the early steps of protein synthesis. Their assembly and function requires multiple interactions that are competitive in nature and determine the nature of interactions between the termini of an mRNA. In this review, the domain organization and partner protein interactions are presented for the factors in plants which share similarities with those in animals and yeast but differ in several important respects. The functional consequences of their interactions on factor activity are also discussed.
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Affiliation(s)
- Daniel R Gallie
- Department of Biochemistry; University of California ; Riverside, CA USA
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3
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Insights from a Paradigm Shift: How the Poly(A)-Binding Protein Brings Translating mRNAs Full Circle. ACTA ACUST UNITED AC 2014. [DOI: 10.1155/2014/873084] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
In recent years, our thinking of how the initiation of protein synthesis occurs has changed dramatically. Initiation was thought to involve only events occurring at or near the 5′-cap structure, which serves as the binding site for the cap-binding complex, a group of translation initiation factors (eIFs) that facilitate the binding of the 40 S ribosomal subunit to an mRNA. Because the poly(A)-binding protein (PABP) binds the poly(A) tail present at the 3′-terminus of an mRNA, it was long thought to play no role in translation initiation. In this review, I present evidence from my laboratory that has contributed to the paradigm shift in how we think of mRNAs during translation. The depiction of mRNAs as straight molecules in which the poly(A) tail is far from events occurring at the 5′-end has now been replaced by the concept of a circular mRNA where the interaction between PABP and the cap-binding complex bridges the termini of an mRNA and promotes translation initiation. The research from my laboratory supports the new paradigm that translation of most mRNAs requires a functional and physical interaction between the termini of an mRNA.
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4
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Shagam LI, Terenin IM, Andreev DE, Dunaevsky JE, Dmitriev SE. In vitro activity of human translation initiation factor eIF4B is not affected by phosphomimetic amino acid substitutions S422D and S422E. Biochimie 2012; 94:2484-90. [PMID: 22750809 DOI: 10.1016/j.biochi.2012.06.021] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/01/2012] [Accepted: 06/20/2012] [Indexed: 10/28/2022]
Abstract
Eukaryotic translation initiation factor eIF4B is necessary for ribosomal scanning through structured mRNA leaders. In higher eukaryotes, eIF4B serves as a downstream effector of several signaling pathways. In response to mitogenic stimuli, eIF4B undergoes multiple phosphorylations which are thought to regulate its activity. Recently, Ser422 was identified as a predominant site for human eIF4B phosphorylation via several signaling pathways, and phosphomimetic amino acid substitutions S422D or S422E were shown to activate eIF4B in living cells. However, stimulatory role of these modifications has never been analyzed directly. Here, using both mammalian reconstituted translation initiation assay and complete cell-free translation system, we perform a comparison of recombinant eIF4B derivatives with the wild type recombinant protein, and do not find any difference in their activities. On the contrary, native eIF4B purified from HeLa cells reveals significantly higher activity in both assays. Thus, the effects of S422D and S422E substitutions on eIF4B activity in living cells observed previously either require some other protein modification(s), or may only be manifested in an intact cell. Our study raises the question on whether the phosphorylation of Ser422 is sufficient for eIF4B activation observed upon mitogenic stimulation.
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Affiliation(s)
- Lev I Shagam
- Belozersky Institute of Physico-Chemical Biology, Lomonosov Moscow State University, Bldg. A, Leninskie Gory, Moscow 119234, Russia
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Shahbazian D, Parsyan A, Petroulakis E, Hershey J, Sonenberg N. eIF4B controls survival and proliferation and is regulated by proto-oncogenic signaling pathways. Cell Cycle 2010; 9:4106-9. [PMID: 20948310 DOI: 10.4161/cc.9.20.13630] [Citation(s) in RCA: 101] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
Abstract
Messenger RNA translation or protein synthesis, is a fundamental biological process affecting cell growth, survival and proliferation. Initiation is the rate limiting and hence the most regulated step of translation. In eukaryotes, translation initiation is facilitated by multiple protein factors collectively called eIFs (for eukaryotic translation initiation factors). The complex consisting of the eIF4 group factors including the mRNA cap-binding eIF4E protein, large scaffolding protein eIF4G and RNA helicase eIF4A is assisted by the eIF4B co-factor to unwind local secondary structures and create a ribosome landing pad on mRNA. Recruitment of the ribosome and augmentation in the mRNA scanning process culminates in the positioning of the ribosome over the start codon. Deregulated translational control is believed to play an important role in oncogenic transformation. Indeed, many eIFs are bona fide proto-oncogenes. In many types of human cancers, eIFs are either overexpressed or ectopically activated by Ras-MAPK and PI3K-mTOR signaling cascades, resulting in increased survival and accelerated proliferation. In this review we will analyze the bulk of data describing eIF4B and its role in cell survival and proliferation. Recent studies have shown that eIF4B is phosphorylated and activated by Ras-MAPK and PI3K-mTOR signaling cascades. In addition, eIF4B regulates translation of proliferative and pro-survival mRNAs. Moreover, eIF4B depletion in cancer cells attenuates proliferation, sensitizes them to genotoxic stress-driven apoptosis. Taken together, these findings identify eIF4B as a potential target for development of anti-cancer therapies.
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Affiliation(s)
- David Shahbazian
- Department of Pharmacology, School of Medicine, Yale University, New Haven, CT, USA
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6
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Dempsey JM, Mahoney SJ, Blenis J. mTORC1-Mediated Control of Protein Translation. ACTA ACUST UNITED AC 2010. [DOI: 10.1016/s1874-6047(10)28001-0] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/17/2023]
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7
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Mahoney SJ, Dempsey JM, Blenis J. Cell signaling in protein synthesis ribosome biogenesis and translation initiation and elongation. PROGRESS IN MOLECULAR BIOLOGY AND TRANSLATIONAL SCIENCE 2009; 90:53-107. [PMID: 20374739 DOI: 10.1016/s1877-1173(09)90002-3] [Citation(s) in RCA: 55] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
Protein synthesis is a highly energy-consuming process that must be tightly regulated. Signal transduction cascades respond to extracellular and intracellular cues to phosphorylate proteins involved in ribosomal biogenesis and translation initiation and elongation. These phosphorylation events regulate the timing and rate of translation of both specific and total mRNAs. Alterations in this regulation can result in dysfunction and disease. While many signaling pathways intersect to control protein synthesis, the mTOR and MAPK pathways appear to be key players. This chapter briefly reviews the mTOR and MAPK pathways and then focuses on individual phosphorylation events that directly control ribosome biogenesis and translation.
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Affiliation(s)
- Sarah J Mahoney
- Department of Cell Biology, Harvard Medical School, Boston, Massachusetts 02115, USA
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8
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Van Der Kelen K, Beyaert R, Inzé D, De Veylder L. Translational control of eukaryotic gene expression. Crit Rev Biochem Mol Biol 2009; 44:143-68. [PMID: 19604130 DOI: 10.1080/10409230902882090] [Citation(s) in RCA: 97] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/20/2022]
Abstract
Translational control mechanisms are, besides transcriptional control and mRNA stability, the most determining for final protein levels. A large number of accessory factors that assist the ribosome during initiation, elongation, and termination of translation are required for protein synthesis. Cap-dependent translational control occurs mainly during the initiation step, involving eukaryotic initiation factors (eIFs) and accessory proteins. Initiation is affected by various stimuli that influence the phosphorylation status of both eIF4E and eIF2 and through binding of 4E-binding proteins to eIF4E, which finally inhibits cap- dependent translation. Under conditions where cap-dependent translation is hampered, translation of transcripts containing an internal ribosome entry site can still be supported in a cap-independent manner. An interesting example of translational control is the switch between cap-independent and cap-dependent translation during the eukaryotic cell cycle. At the G1-to-S transition, translation occurs predominantly in a cap-dependent manner, while during the G2-to-M transition, cap-dependent translation is inhibited and transcripts are predominantly translated through a cap-independent mechanism.
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9
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Ma XM, Blenis J. Molecular mechanisms of mTOR-mediated translational control. Nat Rev Mol Cell Biol 2009; 10:307-18. [DOI: 10.1038/nrm2672] [Citation(s) in RCA: 1895] [Impact Index Per Article: 126.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/08/2023]
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10
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AGC kinases regulate phosphorylation and activation of eukaryotic translation initiation factor 4B. Oncogene 2008; 28:95-106. [PMID: 18836482 DOI: 10.1038/onc.2008.367] [Citation(s) in RCA: 65] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Abstract
Eukaryotic translation initiation factor 4B (eIF4B) plays a critical role during the initiation of protein synthesis and its activity can be regulated by multiple phosphorylation events. In a search for novel protein kinase B (PKB/c-akt) substrates, we identified eIF4B as a potential target. Using an in vitro kinase assay, we found that PKB can directly phosphorylate eIF4B on serine 422 (ser422). Activation of a conditional PKB mutant, interleukin-3 (IL-3) or insulin stimulation resulted in PKB-dependent phosphorylation of this residue in vivo. This was prevented by pretreatment of cells with the phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 or pharmacological inhibition of PKB. Pretreatment of cells with rapamycin, inhibiting mTOR or U0126 to inhibit MEK, had little effect on eIF4B ser422 phosphorylation. In contrast, following amino-acid refeeding, eIF4B ser422 phosphorylation was found to be mammalian target of rapamycin (mTOR)-dependent. We further identified eIF4B ser406 as a novel mitogen-regulated phosphorylation site. Insulin-induced phosphorylation of eIF4B ser406 was dependent on both MEK and mTOR activity. Utilizing a novel translational control luciferase assay, we could further demonstrate that phosphorylation of ser406 or ser422 is essential for optimal translational activity of eIF4B. These data provide novel insights into complex multikinase regulation of eIF4B phosphorylation and reveal an important mechanism by which PKB can regulate translation, potentially critical for the transforming capacity of this AGC kinase family member.
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11
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Llorens F, Duarri A, Sarró E, Roher N, Plana M, Itarte E. The N-terminal domain of the human eIF2beta subunit and the CK2 phosphorylation sites are required for its function. Biochem J 2006; 394:227-36. [PMID: 16225457 PMCID: PMC1386020 DOI: 10.1042/bj20050605] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
CK2 (protein kinase CK2) is known to phosphorylate eIF2 (eukaryotic translation initiation factor 2) in vitro; however, its implication in this process in living cells has remained to be confirmed. The combined use of chemical inhibitors (emodin and apigenin) of CK2 together with transfection experiments with the wild-type of the K68A kinase-dead mutant form of CK2alpha evidenced the direct involvement of this protein kinase in eIF2beta phosphorylation in cultured HeLa cells. Transfection of HeLa cells with human wild-type eIF2beta or its phosphorylation site mutants showed Ser2 as the main site for constitutive eIF2beta phosphorylation, whereas phosphorylation at Ser67 seems more restricted. In vitro phosphorylation of eIF2beta also pointed to Ser2 as a preferred site for CK2 phosphorylation. Overexpression of the eIF2beta S2/67A mutant slowed down the rate of protein synthesis stimulated by serum, although less markedly than the overexpression of the Delta2-138 N-terminal-truncated form of eIF2beta (eIF2beta-CT). Mutation at Ser2 and Ser67 did not affect eIF2beta integrating into the eIF2 trimer or being able to complex with eIF5 and CK2alpha. The eIF2beta-CT form was also incorporated into the eIF2 trimer but did not bind to eIF5. Overexpression of eIF2beta slightly decreased HeLa cell viability, an effect that was more evident when overexpressing the eIF2beta S2/67A mutant. Cell death was particularly marked when overexpressing the eIF2beta-CT form, being detectable at doses where eIF2beta and eIF2beta S2/67A were ineffective. These results suggest that Ser2 and Ser67 contribute to the important role of the N-terminal region of eIF2beta for its function in mammals.
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Affiliation(s)
- Franc Llorens
- Departament de Bioquímica i Biologia Molecular, Unitat de Bioquímica de Ciències, Universitat Autònoma de Barcelona, Edifici Cs, Campus de Bellaterra, 08193 Bellaterra, Barcelona, Spain.
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12
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Crozier SJ, Vary TC, Kimball SR, Jefferson LS. Cellular energy status modulates translational control mechanisms in ischemic-reperfused rat hearts. Am J Physiol Heart Circ Physiol 2005; 289:H1242-50. [PMID: 15894572 DOI: 10.1152/ajpheart.00859.2004] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Mechanisms regulating ischemia and reperfusion (I/R)-induced changes in mRNA translation in the heart are poorly defined, as are the factors that initiate these changes. Because cellular energy status affects mRNA translation under physiological conditions, it is plausible that I/R-induced changes in translation may in part be a result of altered cellular energy status. Therefore, the purpose of the studies described herein was to compare the effects of I/R with those of altered energy substrate availability on biomarkers of mRNA translation in the heart. Isolated adult rat hearts were perfused with glucose or a combination of glucose plus palmitate, and effects of I/R on various biomarkers of translation were subsequently analyzed. When compared with hearts perfused with glucose plus palmitate, hearts perfused with glucose alone exhibited increased phosphorylation of eukaryotic elongation factor (eEF)2, the alpha-subunit of eukaryotic initiation factor (eIF)2, and AMP-activated protein kinase (AMPK), and these hearts also exhibited enhanced association of eIF4E with eIF4E binding protein (4E-BP)1. Regardless of the energy substrate composition of the buffer, phosphorylation of eEF2 and AMPK was greater than control values after ischemia. Phosphorylation of eIF2alpha and eIF4E and the association of eIF4E with 4E-BP1 were also greater than control values after ischemia but only in hearts perfused with glucose plus palmitate. Reperfusion reversed the ischemia-induced increase in eEF2 phosphorylation in hearts perfused with glucose and reversed ischemia-induced changes in eIF4E, eEF2, and AMPK phosphorylation in hearts perfused with glucose plus palmitate. Because many ischemia-induced changes in mRNA translation are mimicked by the removal of a metabolic substrate under normal perfusion conditions, the results suggest that cellular energy status represents an important modulator of I/R-induced changes in mRNA translation.
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Affiliation(s)
- Stephen J Crozier
- Department of Cellular and Molecular Physiology, The Pennsylvania State University College of Medicine, PO Box 850, Hershey, PA 17033, USA
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13
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Abstract
The evolutionarily conserved checkpoint protein kinase, TOR (target of rapamycin), has emerged as a major effector of cell growth and proliferation via the regulation of protein synthesis. Work in the last decade clearly demonstrates that TOR controls protein synthesis through a stunning number of downstream targets. Some of the targets are phosphorylated directly by TOR, but many are phosphorylated indirectly. In this review, we summarize some recent developments in this fast-evolving field. We describe both the upstream components of the signaling pathway(s) that activates mammalian TOR (mTOR) and the downstream targets that affect protein synthesis. We also summarize the roles of mTOR in the control of cell growth and proliferation, as well as its relevance to cancer and synaptic plasticity.
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Affiliation(s)
- Nissim Hay
- Department of Biochemistry and Molecular Genetics, University of Illinois at Chicago, 60607, USA.
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14
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Orton KC, Ling J, Waskiewicz AJ, Cooper JA, Merrick WC, Korneeva NL, Rhoads RE, Sonenberg N, Traugh JA. Phosphorylation of Mnk1 by caspase-activated Pak2/gamma-PAK inhibits phosphorylation and interaction of eIF4G with Mnk. J Biol Chem 2004; 279:38649-57. [PMID: 15234964 DOI: 10.1074/jbc.m407337200] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The mitogen-activated protein kinase-interacting kinase 1 (Mnk1) is phosphorylated by caspase-cleaved protein kinase Pak2/gamma-PAK but not by Cdc42-activated Pak2. Phosphorylation of Mnk1 is rapid, reaching 1 mol/mol within 15 min of incubation with Pak2. A kinetic analysis of the phosphorylation of Mnk1 by Pak2 yields a K(m) of 0.6 microm and a V(max) of 14.9 pmol of (32)P/min/microg of Pak2. Two-dimensional tryptic phosphopeptide mapping of Mnk1 phosphorylated by Pak2 yields two distinct phosphopeptides. Analysis of the phosphopeptides by automated microsequencing and manual Edman degradation identified the sites in Mnk1 as Thr(22) and Ser(27). Mnk1, activated by phosphorylation with Erk2, phosphorylates the eukaryotic initiation factor (eIF) 4E and the eIF4G components of eIF4F. Phosphorylation of Mnk1 by Pak2 does not activate Mnk1, as measured with either eIF4E or eIF4F as substrate. Phosphorylation of Erk2-activated Mnk1 by Pak2 has no effect on phosphorylation of eIF4E but reduces phosphorylation of eIF4G by Mnk1 by up to 50%. Phosphorylation of Mnk1 by Pak2 inhibits binding of eIF4G peptides containing the Mnk1 binding site by up to 80%. When 293T cells are subjected to apoptotic induction by hydrogen peroxide, Mnk1 is phosphorylated at both Thr(22) and Ser(27). These results indicate a role for Pak2 in the down-regulation of translation initiation in apoptosis by phosphorylation of Mnk1.
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Affiliation(s)
- Kevin C Orton
- Department of Biochemistry, University of California, Riverside, Riverside, California 92521, USA
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15
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Raught B, Peiretti F, Gingras AC, Livingstone M, Shahbazian D, Mayeur GL, Polakiewicz RD, Sonenberg N, Hershey JWB. Phosphorylation of eucaryotic translation initiation factor 4B Ser422 is modulated by S6 kinases. EMBO J 2004; 23:1761-9. [PMID: 15071500 PMCID: PMC394247 DOI: 10.1038/sj.emboj.7600193] [Citation(s) in RCA: 343] [Impact Index Per Article: 17.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/10/2003] [Accepted: 03/08/2004] [Indexed: 11/08/2022] Open
Abstract
The eucaryotic translation initiation factor 4B (eIF4B) stimulates the helicase activity of the DEAD box protein eIF4A to unwind inhibitory secondary structure in the 5' untranslated region of eucaryotic mRNAs. Here, using phosphopeptide mapping and a phosphospecific antiserum, we identify a serum-responsive eIF4B phosphorylation site, Ser422, located in an RNA-binding region required for eIF4A helicase-promoting activity. Ser422 phosphorylation appears to be regulated by the S6Ks: (a) Ser422 phosphorylation is sensitive to pharmacological inhibitors of phosphoinositide-3 kinase and the mammalian target of rapamycin; (b) S6K1/S6K2 specifically phosphorylate Ser422 in vitro; and (c) rapamycin-resistant S6Ks confer rapamycin resistance upon Ser422 phosphorylation in vivo. Substitution of Ser422 with Ala results in a loss of activity in an in vivo translation assay, indicating that phosphorylation of this site plays an important role in eIF4B function. We therefore propose that eIF4B may mediate some of the effects of the S6Ks on translation.
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Affiliation(s)
- Brian Raught
- Department of Biochemistry and McGill Cancer Centre, McGill University, Montréal, Québec, Canada
| | - Franck Peiretti
- Department of Biological Chemistry, School of Medicine, University of California, Davis, CA, USA
| | - Anne-Claude Gingras
- Department of Biochemistry and McGill Cancer Centre, McGill University, Montréal, Québec, Canada
| | - Mark Livingstone
- Cell Signaling Technology, Inc., 166B Cummings Center, Beverly, MA, USA
| | - David Shahbazian
- Department of Biochemistry and McGill Cancer Centre, McGill University, Montréal, Québec, Canada
| | - Greg L Mayeur
- Department of Biological Chemistry, School of Medicine, University of California, Davis, CA, USA
| | | | - Nahum Sonenberg
- Department of Biochemistry and McGill Cancer Centre, McGill University, Montréal, Québec, Canada
| | - John WB Hershey
- Department of Biological Chemistry, School of Medicine, University of California, Davis, CA, USA
- Department of Biological Chemistry, School of Medicine, University of California at Davis, Health Science Drive, Davis, CA 95616, USA. Tel.: +1 530 752 3235; Fax: +1 530 752 3516; E-mail:
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16
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Abstract
Rapamycin potently inhibits downstream signaling from the target of rapamycin (TOR) proteins. These evolutionarily conserved protein kinases coordinate the balance between protein synthesis and protein degradation in response to nutrient quality and quantity. The TOR proteins regulate (i) the initiation and elongation phases of translation, (ii) ribosome biosynthesis, (iii) amino acid import, (iv) the transcription of numerous enzymes involved in multiple metabolic pathways, and (v) autophagy. Intriguingly, recent studies have also suggested that TOR signaling plays a critical role in brain development, learning, and memory formation.
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Affiliation(s)
- B Raught
- Department of Biochemistry and McGill Cancer Centre, McGill University, 3655 Promenade Sir-William-Osler, Montréal, QC H3G 1Y6 Canada
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Uma S, Yun BG, Matts RL. The heme-regulated eukaryotic initiation factor 2alpha kinase. A potential regulatory target for control of protein synthesis by diffusible gases. J Biol Chem 2001; 276:14875-83. [PMID: 11278914 DOI: 10.1074/jbc.m011476200] [Citation(s) in RCA: 41] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022] Open
Abstract
Nitric oxide (NO) has been reported to inhibit protein synthesis in eukaryotic cells by increasing the phosphorylation of the alpha-subunit of eukaryotic initiation factor (eIF) 2. However, the mechanism through which this increase occurs has not been characterized. In this report, we examined the effect of the diffusible gases nitric oxide (NO) and carbon monoxide (CO) on the activation of the heme-regulated eIF2alpha kinase (HRI) in rabbit reticulocyte lysate. Spectral analysis indicated that both NO and CO bind to the N-terminal heme-binding domain of HRI. Although NO was a very potent activator of HRI, CO markedly suppressed NO-induced HRI activation. The NO-induced activation of HRI was transduced through the interaction of NO with the N-terminal heme-binding domain of HRI and not through S-nitrosylation of HRI. We postulate that the regulation of HRI activity by diffusible gases may be of wider physiological significance, as we further demonstrate that NO generators increase eIF2alpha phosphorylation levels in NT2 neuroepithelial and C2C12 myoblast cells and activate HRI immunoadsorbed from extracts of these non-erythroid cell lines.
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Affiliation(s)
- S Uma
- Department of Biochemistry and Molecular Biology, Oklahoma State University, Stillwater, Oklahoma 74078, USA
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Affiliation(s)
- A C Gingras
- Department of Biochemistry, McGill University, Montréal, Québec H3G 1Y6, Canada
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19
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Young TE, Gallie DR. Programmed cell death during endosperm development. PLANT MOLECULAR BIOLOGY 2000; 44:283-301. [PMID: 11199389 DOI: 10.1007/978-94-010-0934-8_4] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/29/2023]
Abstract
The endosperm of cereals functions as a storage tissue in which the majority of starch and seed storage proteins are synthesized. During its development, cereal endosperm initiates a cell death program that eventually affects the entire tissue with the exception of the outermost cells, which differentiate into the aleurone layer and remain living in the mature seed. To date, the cell death program has been described for maize and wheat endosperm, which exhibits common and unique elements for each species. The progression of endosperm programmed cell death (PCD) in both species is accompanied by an increase in nuclease activity and the internucleosomal degradation of nuclear DNA, hallmarks of apoptosis in animals. Moreover, ethylene and abscisic acid are key to mediating PCD in cereal endosperm. The progression of the cell death program in developing maize endosperm follows a highly organized pattern whereas in wheat endosperm, PCD initiates stochastically. Although the essential characteristics of cereal endosperm PCD are now known, the molecular mechanisms responsible for its execution remain to be identified.
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Affiliation(s)
- T E Young
- Department of Biochemistry, University of California, Riverside 92521-0129, USA.
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20
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Caraglia M, Budillon A, Vitale G, Lupoli G, Tagliaferri P, Abbruzzese A. Modulation of molecular mechanisms involved in protein synthesis machinery as a new tool for the control of cell proliferation. EUROPEAN JOURNAL OF BIOCHEMISTRY 2000; 267:3919-36. [PMID: 10866791 DOI: 10.1046/j.1432-1327.2000.01465.x] [Citation(s) in RCA: 53] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
In the past years, the attention of scientists has focused mainly on the study of the genetic information and alterations that regulate eukaryotic cell proliferation and that lead to neoplastic transformation. All therapeutic strategies against cancer are, to date, directed at DNA either with cytotoxic drugs or gene therapy. Little or no interest has been aroused by protein synthesis mechanisms. However, an increasing body of data is emerging about the involvement of translational processes and factors in control of cell proliferation, indicating that protein synthesis can be an additional target for anticancer strategies. In this paper we review the novel insights on the biochemical and molecular events leading to protein biosynthesis and we describe their involvement in cell proliferation and tumorigenesis. A possible mechanistic explanation is given by the interactions that occur between protein synthesis machinery and the proliferative signal transduction pathways and that are therefore suitable targets for indirect modulation of protein synthesis. We briefly describe the molecular tools used to block protein synthesis and the attempts made at increasing their efficacy. Finally, we propose a new multimodal strategy against cancer based on the simultaneous intervention on protein synthesis and signal transduction.
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Affiliation(s)
- M Caraglia
- Dipartimento di Biochimica e Biofisica, Seconda Università di Napoli, Italy
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21
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Metz AM, Wong KC, Malmström SA, Browning KS. Eukaryotic initiation factor 4B from wheat and Arabidopsis thaliana is a member of a multigene family. Biochem Biophys Res Commun 1999; 266:314-21. [PMID: 10600500 DOI: 10.1006/bbrc.1999.1814] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Clones of eukaryotic initiation factor (eIF) 4B from wheat and Arabidopsis thaliana were obtained from cDNA and genomic libraries. The exon/intron organization of the genes from wheat and A. thaliana is very similar. The deduced amino acid sequences for the wheat and Arabidopsis eIF4B proteins showed overall similarity to each other, but very little similarity to eIF4B from other eukaryotes. The recombinant form of eIF4B supports polypeptide synthesis in an in vitro translation system and reacts with antibodies to native wheat eIF4B. In contrast to mammalian eIF4B and eIF4A, the combination of wheat eIF4B and eIF4A does not stimulate RNA-dependent ATP hydrolysis activity; however, wheat eIF4B does stimulate eIF4F and eIF4A RNA-dependent ATP hydrolysis activity. Interestingly, eIF4B does not stimulate eIF(iso)4F and eIF4A hydrolysis activity. Gel filtration experiments indicate wheat eIF4B, like its mammalian counterpart, self-associates to form a homodimer.
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Affiliation(s)
- A M Metz
- Department of Chemistry and Biochemistry and the Institute for Cellular and Molecular Biology, University of Texas at Austin, Austin, Texas 78712, USA
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22
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Affiliation(s)
- R E Rhoads
- Department of Biochemistry, Louisiana State University Medical Center, Shreveport, Louisiana 71130-3932, USA.
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23
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Berlanga JJ, Santoyo J, De Haro C. Characterization of a mammalian homolog of the GCN2 eukaryotic initiation factor 2alpha kinase. EUROPEAN JOURNAL OF BIOCHEMISTRY 1999; 265:754-62. [PMID: 10504407 DOI: 10.1046/j.1432-1327.1999.00780.x] [Citation(s) in RCA: 209] [Impact Index Per Article: 8.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
In eukaryotic cells, protein synthesis is regulated in response to various environmental stresses by phosphorylating the alpha subunit of the eukaryotic initiation factor 2 (eIF2alpha). Three different eIF2alpha kinases have been identified in mammalian cells, the heme-regulated inhibitor (HRI), the interferon-inducible RNA-dependent kinase (PKR) and the endoplasmic reticulum-resident kinase (PERK). A fourth eIF2alpha kinase, termed GCN2, was previously characterized from Saccharomyces cerevisiae, Drosophila melanogaster and Neurospora crassa. Here we describe the cloning of a mouse GCN2 cDNA (MGCN2), which represents the first mammalian GCN2 homolog. MGCN2 has a conserved motif, N-terminal to the kinase subdomain V, and a large insert of 139 amino acids located between subdomains IV and V that are characteristic of the known eIF2alpha kinases. Furthermore, MGCN2 contains a class II aminoacyl-tRNA synthetase domain and a degenerate kinase segment, downstream and upstream of the eIF2alpha kinase domain, respectively, and both are singular features of GCN2 protein kinases. MGCN2 mRNA is expressed as a single message of approximately 5.5 kb in a wide range of different tissues, with the highest levels in the liver and the brain. Specific polyclonal anti-(MGCN2) immunoprecipitated an eIF2alpha kinase activity and recognized a 190 kDa phosphoprotein in Western blots from either mouse liver or MGCN2-transfected 293 cell extracts. Interestingly, serum starvation increased eIF2alpha phosphorylation in MGCN2-transfected human 293T cells. This finding provides evidence that GCN2 is the unique eIF2alpha kinase present in all eukaryotes from yeast to mammals and underscores the role of MGCN2 kinase in translational control and its potential physiological significance.
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Affiliation(s)
- J J Berlanga
- Centro de Biología Molecular 'Severo Ochoa', Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, Spain
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24
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Chatterjee M, Chatterjee N, Datta R, Datta B, Gupta NK. Expression and activity of p67 are induced during heat shock. Biochem Biophys Res Commun 1998; 249:113-7. [PMID: 9705841 DOI: 10.1006/bbrc.1998.9056] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
p67, a cellular glycoprotein, protects eIF2 alpha from phosphorylation by inhibitory kinases such as PKR and HCR. p67 promoter contains heat shock element (HSE). To investigate whether this HSE of p67 has any role during heat-shock, rat tumor hepatoma cells were transiently transfected with CAT reporters linked to p67 promoter with HSE and without HSE. Heat shock induced CAT activity when p67 promoter contained HSE and this induction was not observed when HSE was deleted from the p67 promoter. In response to heat-shock, the endogenous p67 mRNA was also induced to more than 36-fold, and much of it translated into protein which was modified by GlcNAc moieties. The time of induced glycosyl modification at the later stages of the heat-shock correlates with the reduced level of eIF2 alpha phosphorylation. During later stages of the heat shock of animal cells, there is a preferential translation of a small class of messages encoding heat shock proteins. Our results suggest that the expression and activity of p67 are induced at the later stages of the heat-shock, and may be involved in the preferential translation of the heat-shock messages.
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Affiliation(s)
- M Chatterjee
- Department of Chemistry, University of Nebraska at Lincoln 68588-0304, USA
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25
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Le H, Browning KS, Gallie DR. The phosphorylation state of the wheat translation initiation factors eIF4B, eIF4A, and eIF2 is differentially regulated during seed development and germination. J Biol Chem 1998; 273:20084-9. [PMID: 9685349 DOI: 10.1074/jbc.273.32.20084] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The translation initiation factors (eIF) 4B and eIF2 are phosphoproteins whose phosphorylation state differs between mature seed and leaves. We examined the isoforms of eIF4B and the alpha and beta subunits of eIF2 during the development and germination of wheat seed to determine whether the differences in their phosphorylation state are because of tissue-specific regulation or occur concomitant with changes in protein synthetic activity during development. eIF2alpha underwent phosphorylation through several intermediate isoforms that correlated with the increase and subsequent reduction in protein synthetic activity characteristic of seed development. eIF2beta and eIF4B, present as highly phosphorylated isoforms during early seed development, underwent dephosphorylation during late development. eIF4B was rapidly phosphorylated within 20 h of germination, whereas eIF2alpha did not undergo dephosphorylation until 48-60 h of growth. A third factor, eIF4A, was predominantly nonphosphorylated throughout most of seed development and germination. These observations suggest that the phosphorylation state of eIF2alpha, eIF2beta, and eIF4B is developmentally regulated in a way that correlates with the changes in protein synthetic activity but that some differences were also observed.
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Affiliation(s)
- H Le
- Department of Biochemistry, University of California, Riverside, California 92521-0129, USA
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26
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Gupta S, Bose A, Chatterjee N, Saha D, Wu S, Gupta NK. p67 transcription regulates translation in serum-starved and mitogen-activated KRC-7 cells. J Biol Chem 1997; 272:12699-704. [PMID: 9139727 DOI: 10.1074/jbc.272.19.12699] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
The regulation of protein synthesis was studied in KRC-7 cells (rat hepatoma) grown in complete medium, during serum starvation, and mitogen activation. Upon serum starvation, the cells lost almost completely p67 mRNA, p67 protein, and protein synthesis activity. After phorbol 12-myristate 13-acetate addition, the same serum-starved cells regained p67 mRNA, p67 protein, and protein synthesis activity. Also, the extracts from the serum-starved cells phosphorylated the eukaryotic initiation factor-2 (eIF-2) alpha-subunit. This eIF-2 alpha-subunit phosphorylation was not observed when the extracts from either the cells grown in complete medium or mitogen-activated cells were used (Gupta, S., Wu, S., Chatterjee, N., Ilan, J., Ilan, J., Osterman, J. C., and Gupta, N. K. (1995) Gene Expr. 5, 113-122). We now report the following. 1) The eIF-2 kinase activity was the same in the cells grown in complete medium, after serum starvation, and subsequent mitogen stimulation. However, the eIF-2 kinase in the cells grown in complete medium and also after mitogen activation of the serum-starved cells cannot phosphorylate eIF-2 alpha-subunit as these cells contain p67. After removal of endogenous p67 by p67 antibodies, the extracts from all these cells similarly phosphorylated exogenously added eIF-2. 2) None of the cell extracts showed p67 deglycosylase activity. 3) The p67 mRNA was synthesized in serum-starved cells by expression of a p67 cDNA. The appearance of p67 mRNA in the serum-starved cells was accompanied by the appearance of p67 protein. Also, the rates of protein synthesis in the serum-starved cells were restored nearly to the level observed in the confluent cells. The expression of p67 cDNA also significantly increased protein synthesis rates in the cells grown in complete medium and in mitogen-activated cells. These results show that the loss of protein synthesis activity in serum-starved cells was due to loss of p67 mRNA. The expressed p67 mRNA was stable in serum-starved cells. These results, therefore, suggest that the loss of p67 mRNA in serum-starved cells is due to loss of p67 transcription. The p67 transcription regulates translation.
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Affiliation(s)
- S Gupta
- Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68588-0304, USA
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27
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Gallie DR, Le H, Caldwell C, Tanguay RL, Hoang NX, Browning KS. The phosphorylation state of translation initiation factors is regulated developmentally and following heat shock in wheat. J Biol Chem 1997; 272:1046-53. [PMID: 8995401 DOI: 10.1074/jbc.272.2.1046] [Citation(s) in RCA: 87] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Abstract
Several translation initiation factors in mammals and yeast are regulated by phosphorylation. The phosphorylation state of these factors is subject to alteration during development, environmental stress (heat shock, starvation, or heme deprivation), or viral infection. The phosphorylation state and the effect of changes in phosphorylation of the translation initiation factors of higher plants have not been previously investigated. We have determined the isoelectric states for the wheat translation initiation factors eIF-4A, eIF-4B, eIF-4F, eIF-iso4F, and eIF-2 and the poly(A)-binding protein in the seed, during germination, and following heat shock of wheat seedlings using two-dimensional gel electrophoresis and Western analysis. We found that the developmentally induced changes in isoelectric state observed during germination or the stress-induced changes were consistent with changes in phosphorylation. Treatment of the phosphorylated forms of the factors with phosphatases confirmed that the nature of the modification was due to phosphorylation. The isoelectric states of eIF-4B, eIF-4F (eIF-4E, p26), eIF-iso4F (eIF-iso4E, p28), and eIF-2alpha (p42) were altered during germination, suggesting that phosphorylation of these factors is developmentally regulated and correlates with the resumption of protein synthesis that occurs during germination. The phosphorylation of eIF-2beta (p38) or poly(A)-binding protein did not change either during germination or following a thermal stress. Only the phosphorylation state of two factors, eIF-4A and eIF-4B, changed following a heat shock, suggesting that plants may differ significantly from animals in the way in which their translational machinery is modified in response to a thermal stress.
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Affiliation(s)
- D R Gallie
- Department of Biochemistry, University of California, Riverside 92521-0129, USA.
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28
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Alcázar A, Rivera J, Gómez-Calcerrada M, Muñoz F, Salinas M, Fando JL. Changes in the phosphorylation of eukaryotic initiation factor 2 alpha, initiation factor 2B activity and translational rates in primary neuronal cultures under different physiological growing conditions. BRAIN RESEARCH. MOLECULAR BRAIN RESEARCH 1996; 38:101-8. [PMID: 8737673 DOI: 10.1016/0169-328x(95)00335-p] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
Phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 (eIF-2) is one of the best known mechanisms regulating protein synthesis in a wide range of eukaryotic cells, from yeast to human. To determine whether this mechanism operates in primary neuronal cells, we have cultured primary neuronal cells for 7 days under two optimal growing conditions, complete medium (containing 15% serum) and serum-free medium, and determined the protein synthesis rate, eukaryotic initiation 2 and 2B (eIF-2B) activities, as well as the level of phosphorylation of eIF-2. Cells cultured in serum-free medium exhibited a lower rate of protein synthesis (75%), concomitant to a decreased eIF-2 activity (71%), and slightly higher eIF-2(alpha P) levels (from 10 to 16% of total eIF-2) with respect to cells cultured in complete media. eIF-2B activity, as measured at saturating eIF-2. GDP concentrations (assay independent on the presence of eIF-2(alpha P)) was similar under the two culture conditions. When neurons cultured in serum-free medium are exposed to complete medium for only 24 h, there is a clear decrease in the phosphorylation of eIF-2 alpha (16-3%). This decrease correlates in time with an increase in the protein synthesis rate (154%), as well as eIF-2 activity (236%). The increased levels of eIF-2(alpha P), a competitive inhibitor of eIF-2B in the guanine-exchange reaction, are responsible for the decreased eIF-2B activity found in the neurons cultured in serum-free medium. Additionally, eIF-2(alpha P) is accountable for the lower effect of exogenous eIF-2B in ternary complex formation from preformed eIF-2. GDP in the serum-free media. These changes in phosphorylation of eIF-2 alpha in normal mammalian cells in response to changes in the extracellular medium are reported here for the first time.
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Affiliation(s)
- A Alcázar
- Departamento de Investigación, Hospital Ramón y Cajal, Madrid, Spain
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29
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Kleijn M, Voorma HO, Thomas AA. Phosphorylation of eIF-4E and initiation of protein synthesis in P19 embryonal carcinoma cells. J Cell Biochem 1995; 59:443-52. [PMID: 8749714 DOI: 10.1002/jcb.240590405] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
Mitogenic stimulation of protein synthesis is accompanied by an increase in eIF-4E phosphorylation. The effect on protein synthesis by induction of differentiation is less well known. We treated P19 embryonal carcinoma cells with the differentiating agent retinoic acid and found that protein synthesis increased during the first hour of addition. However, the phosphorylation state, as well as the turnover of phosphate on eIF-4E, remained unchanged. Apparently, the change in protein synthesis after RA addition is regulated by another mechanism than eIF-4E phosphorylation. By using P19 cells overexpressing the EGF receptor, we show that the signal transduction pathway that leads to phosphorylation of eIF-4E is present in P19 cells; the EGF-induced change in phosphorylation of eIF-4E in these cells is likely to be regulated by a change in eIF-4E phosphatase activity. These results suggest that the onset of retinoic acid-induced differentiation is triggered by a signal transduction pathway which involves changes in protein synthesis, but not eIF-4E phosphorylation.
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Affiliation(s)
- M Kleijn
- Department of Molecular Cell Biology, University of Utrecht, Netherlands
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30
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Duncan RF. Cordycepin blocks recovery of non-heat-shock mRNA translation following heat shock in Drosophila. EUROPEAN JOURNAL OF BIOCHEMISTRY 1995; 233:784-92. [PMID: 8521843 DOI: 10.1111/j.1432-1033.1995.784_3.x] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
Treatment of cells with cordycepin (3-deoxyadenosine), an inhibitor of cytoplasmic adenylation, blocks the restoration of normal translation following heat shock. Cordycepin also reduces heat-shock protein 70 (Hsp70) protein synthesis greater than 10-fold, while having little to no effect on mRNA accumulation. Parallel analysis of the poly(A)-binding protein detects no change in its abundance during heat shock or subsequent recovery. These results suggest that normal, non-heat-shock mRNA translational repression during heat shock may be caused by deadenylation, and that readenylation is required for restoration of activity. However, three independent analyses of the adenylation status of mRNAs during heat shock and recovery indicate that no significant changes in polyadenylation occur. (a) The total poly(A) content decreases by only about 10% during heat shock; (b) the size of the poly(A) tract decreases only marginally, from an average length of 75-90 nucleotides in non-heated cells to 45-60 nucleotides during heat shock; (c) virtually all mRNAs bind to oligo d(T)-cellulose, whether extracted from normal-temperature, heat-shock or recovered cells. Our results are most consistent with a model where the process of readenylation, rather than the specific poly(A) tail length, influences translational activation during recovery, paralleling a proposed model for the activation of translation during Xenopus oocyte maturation.
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Affiliation(s)
- R F Duncan
- University of Southern California School of Pharmacy, Department of Molecular Pharmacology and Toxicology 90033, USA
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31
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Srivastava SP, Davies MV, Kaufman RJ. Calcium depletion from the endoplasmic reticulum activates the double-stranded RNA-dependent protein kinase (PKR) to inhibit protein synthesis. J Biol Chem 1995; 270:16619-24. [PMID: 7622470 DOI: 10.1074/jbc.270.28.16619] [Citation(s) in RCA: 112] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
Calcium depletion from the endoplasmic reticulum inhibits protein synthesis and correlates with increased phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF-2 alpha) by a mechanism that does not require ongoing protein synthesis. To elucidate whether protein synthesis inhibition requires eIF-2 alpha phosphorylation and whether eIF-2 alpha phosphorylation is mediated by the double-stranded RNA-dependent protein kinase (PKR), we studied protein synthesis in response to calcium depletion mediated by calcium ionophore A23187 in cell lines overexpressing wild-type eIF-2 alpha, a mutant eIF-2 alpha (S51A) that is resistant to phosphorylation, or a dominant negative mutant PKR (K296P in catalytic subdomain II). Expression of either mutant eIF-2 alpha or mutant PKR partially protected NIH3T3 cells from inhibition of protein synthesis upon A23187 treatment. In contrast, overexpression of wild-type PKR increased sensitivity to protein synthesis inhibition mediated by A23187 treatment. In a COS-1 monkey cell transient transfection system, increased eIF-2 alpha phosphorylation in response to A23187 treatment was inhibited by expression of the dominant negative PKR mutant. Overexpression of the PKR regulatory RNA binding domain, independent of the PKR catalytic domain, was sufficient to inhibit increased phosphorylation of eIF-2 alpha upon A23187 treatment. In addition, overexpression of the HIV TAR RNA binding protein also inhibited eIF-2 alpha phosphorylation upon A23187 treatment. Taken together, our data show that calcium depletion activates PKR to phosphorylate eIF-2 alpha, and this activation is likely mediated through the PKR RNA binding domain.
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Affiliation(s)
- S P Srivastava
- Department of Biological Chemistry, University of Michigan Medical Center, Ann Arbor 48105, USA
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32
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Li X, Chang YH. Molecular cloning of a human complementary DNA encoding an initiation factor 2-associated protein (p67). BIOCHIMICA ET BIOPHYSICA ACTA 1995; 1260:333-6. [PMID: 7873610 DOI: 10.1016/0167-4781(94)00227-t] [Citation(s) in RCA: 26] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
Rat p67 is an initiation factor-2 associated protein, which plays important roles in translational regulation. A cDNA that encodes a homologue of rat p67 was isolated from a human liver cDNA library. The encoded protein contains 478 amino acids with a calculated molecular mass of 52,891 and an isoelectric point of 5.64. The amino acid sequence is 92% identical to that of rat p67. The corresponding mRNA has a size of approximately 2.1 kb and is present in all tested human tissues.
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Affiliation(s)
- X Li
- Edward A. Doisy Department of Biochemistry and Molecular Biology, St. Louis University School of Medicine, MO 63104
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33
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Jaramillo ML, Abraham N, Bell JC. The interferon system: a review with emphasis on the role of PKR in growth control. Cancer Invest 1995; 13:327-38. [PMID: 7538039 DOI: 10.3109/07357909509094468] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Affiliation(s)
- M L Jaramillo
- Department of Medicine, University of Ottawa, Ontario, Canada
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34
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Abstract
The cap-binding complex eIF-4F plays a major role in the control of translation initiation, and overexpression of its limiting subunit, eIF-4E, leads to the deregulation of cellular growth. The recent cloning of eIF-4E binding proteins (4E-BPs) has uncovered a previously unsuspected pathway for the regulation of eIF-4E activity, through sequestration of eIF-4E as a complex with 4E-BPs.
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Affiliation(s)
- S Mader
- Department of Biochemistry, McGill University, Montreal, Quebec, Canada
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35
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Morris DR. Growth control of translation in mammalian cells. PROGRESS IN NUCLEIC ACID RESEARCH AND MOLECULAR BIOLOGY 1995; 51:339-63. [PMID: 7659778 DOI: 10.1016/s0079-6603(08)60883-1] [Citation(s) in RCA: 55] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Affiliation(s)
- D R Morris
- Department of Biochemistry, University of Washington, Seattle 98195, USA
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36
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Singh LP, Arorr AR, Wahba AJ. Phosphorylation of the guanine nucleotide exchange factor and eukaryotic initiation factor 2 by casein kinase II regulates guanine nucleotide binding and GDP/GTP exchange. Biochemistry 1994; 33:9152-7. [PMID: 8049218 DOI: 10.1021/bi00197a018] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
In mammalian cells, chain initiation factor (eIF) 2 and guanine nucleotide exchange factor (GEF) play a major role in the regulation of polypeptide chain initiation. Since guanine nucleotide exchange is the rate-limiting step in the recycling of eIF-2, we examined the effects of phosphorylation of GEF and eIF-2 on guanine nucleotide binding and the rate of GDP/GTP exchange. Phosphorylation of the 82-kDa subunit of GEF in vitro by casein kinase (CK) II results in the stimulation of guanine nucleotide exchange [Dholakia, J. N., & Wahba, A. J. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 51-54]. CK-II also phosphorylates the beta-subunit of eIF2, but the significance of this phosphorylation has not previously been investigated. In this study we demonstrate that treatment of CK-II-phosphorylated GEF or eIF-2 with alkaline phosphatase specifically removes more than 85% of the phosphate incorporated into the factors and alters guanine nucleotide binding to these proteins. In the presence of 1 mM Mg2+, the amount of GTP bound to dephosphorylated GEF is reduced 3.8-fold as compared to that of the CK-II-phosphorylated GEF. Rephosphorylation with CK-II restores GTP binding and increases 4-5-fold the activity of GEF in the exchange of eIF-2-bound GDP for free GTP. On the other hand, the extent of GDP binding to dephosphorylated eIF-2 is increased 2.3-fold as compared to that to the isolated eIF-2. The rate of GEF-catalyzed exchange of dephosphorylated eIF-2-bound GDP for GTP is approximately 2-fold slower than that with the isolated eIF-2.(ABSTRACT TRUNCATED AT 250 WORDS)
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Affiliation(s)
- L P Singh
- Department of Biochemistry, University of Mississippi Medical Center, Jackson 39216
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37
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A key transcription factor for eukaryotic initiation factor-2 alpha is strongly homologous to developmental transcription factors and may link metabolic genes to cellular growth and development. J Biol Chem 1994. [DOI: 10.1016/s0021-9258(17)32255-x] [Citation(s) in RCA: 44] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022] Open
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38
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Celis JE, Olsen E. A qualitative and quantitative protein database approach identifies individual and groups of functionally related proteins that are differentially regulated in simian virus 40 (SV40) transformed human keratinocytes: an overview of the functional changes associated with the transformed phenotype. Electrophoresis 1994; 15:309-44. [PMID: 8055864 DOI: 10.1002/elps.1150150153] [Citation(s) in RCA: 54] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
A qualitative and quantitative two-dimensional (2-D) gel database approach has been used to identify individual and groups of proteins that are differentially regulated in simian virus 40 (SV40) transformed human keratinocytes (K14). Five hundred and sixty [35S]methionine-labeled proteins (462 isoelectric focusing, IEF; 98 nonequilibrium pH gradient electrophoresis, NEPHGE), out of the 3038 recorded in the master keratinocyte database, were excised from dry, silver-stained gels of normal proliferating primary keratinocytes and K14 cells and the radioactivity was determined by liquid scintillation counting. Two hundred and thirty five proteins were found to be either up- (177) or down-regulated (58) in the transformed cells by 50% or more, and of these, 115 corresponded to known proteins in the keratinocyte database (J.E. Celis et al., Electrophoresis 1993, 14, 1091-1198). The lowest abundance acidic protein quantitated was present in about 60,000 molecules per cell, assuming a value of 10(8) molecules per cell for total actin. The results identified individual, and groups of functionally related proteins that are differentially regulated in K14 keratinocytes and that play a role in a variety of cellular activities that include general metabolism, the cytoskeleton, DNA replication and cell proliferation, transcription and translation, protein folding, assembly, repair and turnover, membrane traffic, signal transduction, and differentiation. In addition, the results revealed several transformation sensitive proteins of unknown identity in the database as well as known proteins of yet undefined functions. Within the latter group, members of the S100 protein family--whose genes are clustered on human chromosome 1q21--were among the highest down-regulated proteins in K14 keratinocytes. Visual inspection of films exposed for different periods of time revealed only one new protein in the transformed K14 keratinocytes and this corresponded to keratin 18, a cytokeratin expressed mainly by simple epithelia. Besides providing with the first global overview of the functional changes associated with the transformed phenotype of human keratinocytes, the data strengthened previous evidence indicating that transformation results in the abnormal expression of normal genes rather than in the expression of new ones.
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Affiliation(s)
- J E Celis
- Institute of Medical Biochemistry, Aarhus University, Denmark
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39
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40
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Redpath NT, Proud CG. Molecular mechanisms in the control of translation by hormones and growth factors. BIOCHIMICA ET BIOPHYSICA ACTA 1994; 1220:147-62. [PMID: 8312359 DOI: 10.1016/0167-4889(94)90130-9] [Citation(s) in RCA: 84] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Affiliation(s)
- N T Redpath
- Department of Biochemistry, School of Medical Sciences, University of Bristol, UK
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41
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Rhoads RE, Joshi B, Minich WB. Participation of initiation factors in the recruitment of mRNA to ribosomes. Biochimie 1994; 76:831-8. [PMID: 7880899 DOI: 10.1016/0300-9084(94)90184-8] [Citation(s) in RCA: 67] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
The step of protein synthesis which is normally rate limiting, formation of the 48S initiation complex, is catalyzed by the group 4 initiation factors. Collectively they recognize the 7-methylguanosine-containing cap of mRNA, unwind mRNA secondary structure, and allow scanning for the initiation codon by the small ribosomal subunit. The activities of the eIF-4 polypeptides are modulated by phosphorylation. Recent studies shed new light on the mechanism of assembly of the 48S initiation complex and the effect of phosphorylation of one of the eIF-4 polypeptides, the cap-binding protein eIF-4E.
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Affiliation(s)
- R E Rhoads
- Department of Biochemistry and Molecular Biology, Louisiana State University Medical Center, Shreveport 71130
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42
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Abstract
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.
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Affiliation(s)
- N Price
- Department of Biochemistry, School of Medical Sciences, University of Bristol, UK
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43
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Abstract
The in vitro activities of recombinant gene products of the vaccinia virus E3L and K3L genes have been compared. These proteins are both potent inhibitors of the dsRNA activated protein kinase (PKR) as assayed in cell-free translation systems or with purified PKR. The two gene products function at similar molar concentrations. Both proteins are expressed early in vaccinia virus infection suggesting that vaccinia virus maintains redundant mechanisms for the down regulation of PKR. The K3L gene product can be shown to be associated with PKR in vaccinia virus infected cells. The activities of the vaccinia virus PKR inhibitors are compared with other viral protein inhibitors of PKR. A variety of cellular proteins have also been identified by their ability to inhibit PKR activity or to prevent PKR activation. These cellular PKR interacting proteins have been uncovered from the studies of viral strategies to prevent PKR activation, as well as from studies looking at the effects of growth control, growth factors or oncogene expression on PKR activity. A picture emerges of PKR fulfilling a complex regulatory role in cell function with the regulation of its activity as part of a complex cascade interfacing with the signal transduction/cell cycle control machinery.
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Affiliation(s)
- R Jagus
- Center of Marine Biotechnology, University of Maryland Biotechnology Center, Baltimore 21202, USA
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Abstract
This review focuses on how cells establish the levels of initiation factors, within the broader context of determining levels of the translational machinery. Most initiation factor polypeptides are moderately abundant proteins with concentrations approaching those of ribosomes. eIF4A and eIF5A are more abundant than ribosomes, whereas eIF4F alpha and eIF2B are considerably less abundant than the other factors. The cloning of cDNAs generates hybridization probes for monitoring the levels and activities of factor mRNAs, and the cloning of their genes is just beginning to provide insight into promoter structures and regulation. Initiation factor gene expression appears to be coordinately regulated in many cases, and preferential synthesis is seen in mitogen-activated T-cells. The gene for eIF2 alpha has been best characterized, and mechanisms that provide for the coordinated synthesis of eIF2 subunits are emerging. Recombinant DNA methods also allow investigators to manipulate the levels of expression of specific factor genes by overexpression or antisense repression. Such approaches provide a means to investigate in vivo the mechanisms of action of the initiation factors and their roles in regulating translation rates.
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Affiliation(s)
- J W Hershey
- Department of Biological Chemistry, School of Medicine, University of California, Davis 95616
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Chevesich J, Chaudhuri J, Maitra U. Characterization of mammalian translation initiation factor 5 (eIF-5). Demonstration that eIF-5 is a phosphoprotein and is present in cells as a single molecular form of apparent M(r) 58,000. J Biol Chem 1993. [DOI: 10.1016/s0021-9258(20)80775-3] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
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46
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Wolthuis RM, Cremers AF, Kasperaitis MA, van der Mast C, Voorma HO, Boonstra J. Epidermal growth factor stimulates phosphorylation of eukaryotic initiation factor 4B, independently of protein kinase C. BIOCHIMICA ET BIOPHYSICA ACTA 1993; 1177:160-6. [PMID: 8388736 DOI: 10.1016/0167-4889(93)90035-n] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Abstract
We demonstrate that exposure of human epidermoid carcinoma A431 cells to epidermal growth factor (EGF) results in phosphorylation of eIF-4B within minutes after addition of EGF. The EGF-induced phosphorylation of eIF-4B is not caused by the EGF receptor tyrosine kinase itself, since no tyrosine-phosphorylated eIF-4B could be detected upon immunoprecipitation using an anti-phosphotyrosine antibody. Enhanced phosphorylation of eIF-4B was also detected upon exposure of the cells to phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C (PKC), suggesting that eIF-4B may be a substrate of PKC. However, down-regulation of PKC did not influence the EGF-induced eIF-4B phosphorylation, which indicates that eIF-4B is phosphorylated by an as yet unknown kinase, activated early in the EGF-induced signal transduction cascade.
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Affiliation(s)
- R M Wolthuis
- Department of Molecular Cell Biology, University of Utrecht, The Netherlands
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47
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Ray MK, Chakraborty A, Datta B, Chattopadhyay A, Saha D, Bose A, Kinzy TG, Wu S, Hileman RE, Merrick WC. Characteristics of the eukaryotic initiation factor 2 associated 67-kDa polypeptide. Biochemistry 1993; 32:5151-9. [PMID: 8098621 DOI: 10.1021/bi00070a026] [Citation(s) in RCA: 28] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
A eukaryotic initiation factor 2 (eIF-2) associated 67-kDa polypeptide (p67) protects the eIF-2 alpha-subunit from eIF-2 kinase(s) catalyzed phosphorylation, and this promotes protein synthesis in the presence of active eIF-2 kinase(s), [Datta, B., et al. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 3324-3328]. This report presents the results of studies related to characteristics of p67 action and the mechanism of p67:eIF-2 interaction: (1) p67 antibodies inhibited protein synthesis in hemin-supplemented rabbit reticulocyte lysates, and such inhibition was reversed by preincubation of the antibodies, specifically with p67. (2) p67 inhibited HRI- and dsI-catalyzed phosphorylations of the eIF-2 alpha-subunit and histones, but it did not inhibit casein kinase catalyzed phosphorylation of the eIF-2 beta-subunit. (3) p67 bound specifically to the eIF-2 gamma-subunit. p67 co-immunoprecipitated with the eIF-2 subunits when a p67/eIF-2 mixture was treated with p67 or eIF-2 subunit antibodies and protein A agarose. However, when eIF-2 was preincubated specifically with the eIF-2 gamma-subunit antibodies, subsequent co-immunoprecipitation of p67 with the eIF-2 subunits was completely inhibited. Similarly, preincubation of p67 and p67 antibodies prevented subsequent p67 binding to eIF-2. Preincubation of eIF-2, with either eIF-2 alpha- or beta-subunit antibodies, had no effect on p67 co-immunoprecipitation with the eIF-2 subunits. (4) p67:eIF-2 interaction is necessary for p67 activity to protect the eIF-2 alpha-subunit from eIF-2 kinase(s) catalyzed phosphorylation.(ABSTRACT TRUNCATED AT 250 WORDS)
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Affiliation(s)
- M K Ray
- Department of Chemistry, University of Nebraska, Lincoln 68588
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Wieloch T, Bergstedt K, Hu BR. Protein phosphorylation and the regulation of mRNA translation following cerebral ischemia. PROGRESS IN BRAIN RESEARCH 1993; 96:179-91. [PMID: 8332740 DOI: 10.1016/s0079-6123(08)63266-5] [Citation(s) in RCA: 23] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Affiliation(s)
- T Wieloch
- Department of Neurobiology, Lund Hospital, Sweden
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Luis A, Izquierdo J, Ostronoff L, Salinas M, Santarén J, Cuezva J. Translational regulation of mitochondrial differentiation in neonatal rat liver. Specific increase in the translational efficiency of the nuclear-encoded mitochondrial beta-F1-ATPase mRNA. J Biol Chem 1993. [DOI: 10.1016/s0021-9258(18)53935-1] [Citation(s) in RCA: 26] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
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50
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Phosphorylation of eukaryotic initiation factor (eIF) 2 alpha and inhibition of eIF-2B in GH3 pituitary cells by perturbants of early protein processing that induce GRP78. J Biol Chem 1992. [DOI: 10.1016/s0021-9258(18)41842-x] [Citation(s) in RCA: 93] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
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