Translation - Regulation of EIF2 activity

Regulation of eIF2 activity

Eukaryotic translation initiation factors (eIF2) is a G-protein that is composed of 3 non-identical subunits, eIF2S1, eIF2S2 and eIF2S3, and catalyzes the first regulated step of protein synthesis initiation, promoting the binding of the initiator tRNA to 40S ribosomal subunits. Phosphorylation of eIF2S1 modulates rate of formation of the complex eIF2/GTP/tRNA [1].

Key pathways of eIF2 activity regulation are phosphorylation of eIF2S1 and phosphorylation of Eukaryotic initiation factor 2 (eIF2B) by some protein kinases.

Four distinct protein kinases inhibit protein synthesis in eukaryotic cells via phosphorylation of eIF2S1 at serine-51. There are Eukaryotic translation initiation factor 2-alpha kinase 1, 2, 3 and 4 (EIF2AK1, PKR, EIF2AK3 and GCN2), that belong to the serine/threonine family of protein kinases. Phosphorylation of eIF2S1 results in shutdown of protein synthesis.

EIF2AK1 is activated under conditions of heme deficiency, predominantly in immature erythroid cells, and its activity is inhibited by heme [2].

PKR acts as an antiviral machinery of type I Interferons. Expression of PKR is induced by interferon, and its kinase activity is stimulated by low concentrations of double-stranded RNA. PKR-mediated inhibition is neutralizated via direct dephosphorylation and monomerization of PKR by Alpha catalytic subunit protein phosphatase 1 (PP1) [3]. Also, PP1 can dephosphorylates eIF2S1 thereby activating eIF2 formation [4].

EIF2AK3 is activated under conditions of Endoplasmic reticulum (ER) stress. ER stress is caused by unfolded or misfolded proteins, which are accumulared by extracellular or intracellular stimuli [5].

GCN2 is activated by amino acid starvation phosphorylates. GCN2 inhibites EIF2S1. It is one of the eukaryotic initiation factors that have a role in eukaryotic peptide chain initiation process [6].

Exchange of GDP for GTP on eIF2 is stimulated by eIF2B. eIF2B is a heteromeric guanine nucleotide exchange factor that plays an important role in regulating mRNA translation. It is composed of 5 subunits termed eIF2B1-5 in order of increasing size [7].

eIF2B has multiple phosphorylation sites in the largest, catalytic, subunit eIF2B5.

Kinases, which phosphorylate of the eIF2B5-subunit of eIF2B, are Casein kinase I and II, Glycogen synthase kinase 3 alpha and beta isoforms (GSK3 alpha/beta), and Dual specificity tyrosine-phosphorylated and -regulated kinases (DYRK1a and DYRK2). elF2B5 phosphorylation by casein kinases enhances eIF2B activity, whereas phosphorylation by GSK3 has an inhibitory effect [7]. Phosphorylation by GSK3 requires previous elF2B5 phosphorylation that is catalyzed by DYRKs [8].

Phosphotase PP1 deviates inhibitory effect of GSK3 via dephosphorilation of eIF2B [9]. Activity of PP1 and GSK3 is regulated by different extracellular stimulus.

For example, Insulin receptor and Epidermal growth factor receptor (EGFR) activate the enzymatic activity of Phosphatidylinositol 3-kinase class I via recruitment of regulatory subunit (PI3K reg) either directly or via adaptor proteins (e.g. Insulin receptor substrates 1 and 2 (IRS-1 and IRS-2), GRB2-associated binding protein 1 (GAB1) or Signal transduction protein CBL (c-Cbl)) [10], [11], [12].

Active Phosphatidylinositol 3-kinase class I catalytic (PI3K cat) converts Phosphatidylinositol 4,5-biphosphate (PtdIns(4,5)P2) to Phosphatidylinositol 3,4,5-triphosphate (PtdIns(3,4,5)P3) [13]. PtdIns(3,4,5)P3 is a second messenger. It recruits and activates V-akt murine thymoma viral oncogene homolog 1 (AKT) and Phosphoinositide dependent protein kinase-1 (PDK) to membrane via PH domain [14].AKT, in turn, negatively regulates GSK3 by phosphorylation thereby abrogate inhibitory effect GSK3 to eIF2B activity [7], [15].

Also EGFR induces MAPK cascade. It was shown, that MEK/ Mitogen-activated protein kinase (ERK) pathway is required for activation of eIF2B. It was suggested that the active ERK activates phosphotase PP1. This leads to activation of eIF2B [9].

References:

1. Kimball SR
   Eukaryotic initiation factor eIF2. The international journal of biochemistry & cell biology 1999 Jan;31(1):25-9
2. Han AP, Yu C, Lu L, Fujiwara Y, Browne C, Chin G, Fleming M, Leboulch P, Orkin SH, Chen JJ
   Heme-regulated eIF2alpha kinase (HRI) is required for translational regulation and survival of erythroid precursors in iron deficiency. The EMBO journal 2001 Dec 3;20(23):6909-18
3. Tan SL, Tareen SU, Melville MW, Blakely CM, Katze MG
   The direct binding of the catalytic subunit of protein phosphatase 1 to the PKR protein kinase is necessary but not sufficient for inactivation and disruption of enzyme dimer formation. The Journal of biological chemistry 2002 Sep 27;277(39):36109-17
4. Mu?oz F, Martin ME, Manso-Tomico J, Berlanga J, Salinas M, Fando JL
   Ischemia-induced phosphorylation of initiation factor 2 in differentiated PC12 cells: role for initiation factor 2 phosphatase. Journal of neurochemistry 2000 Dec;75(6):2335-45
5. Schroder M, Kaufman RJ
   ER stress and the unfolded protein response. Mutation research 2005 Jan 6;569(1-2):29-63
6. Berlanga JJ, Santoyo J, De Haro C
   Characterization of a mammalian homolog of the GCN2 eukaryotic initiation factor 2alpha kinase. European journal of biochemistry / FEBS 1999 Oct;265(2):754-62
7. Wang X, Paulin FE, Campbell LE, Gomez E, O'Brien K, Morrice N, Proud CG
   Eukaryotic initiation factor 2B: identification of multiple phosphorylation sites in the epsilon-subunit and their functions in vivo. The EMBO journal 2001 Aug 15;20(16):4349-59
8. Woods YL, Cohen P, Becker W, Jakes R, Goedert M, Wang X, Proud CG
   The kinase DYRK phosphorylates protein-synthesis initiation factor eIF2Bepsilon at Ser539 and the microtubule-associated protein tau at Thr212: potential role for DYRK as a glycogen synthase kinase 3-priming kinase. The Biochemical journal 2001 May 1;355(Pt 3):609-15
9. Quevedo C, Salinas M, Alcazar A
   Initiation factor 2B activity is regulated by protein phosphatase 1, which is activated by the mitogen-activated protein kinase-dependent pathway in insulin-like growth factor 1-stimulated neuronal cells. The Journal of biological chemistry 2003 May 9;278(19):16579-86
10. Lecoq-Lafon C, Verdier F, Fichelson S, Chretien S, Gisselbrecht S, Lacombe C, Mayeux P
    Erythropoietin induces the tyrosine phosphorylation of GAB1 and its association with SHC, SHP2, SHIP, and phosphatidylinositol 3-kinase. Blood 1999 Apr 15;93(8):2578-85
11. Yarden Y, Sliwkowski MX
    Untangling the ErbB signalling network. Nature reviews. Molecular cell biology 2001 Feb;2(2):127-37
12. Kuemmerle JF
    IGF-I elicits growth of human intestinal smooth muscle cells by activation of PI3K, PDK-1, and p70S6 kinase. American journal of physiology. Gastrointestinal and liver physiology 2003 Mar;284(3):G411-22
13. Katso R, Okkenhaug K, Ahmadi K, White S, Timms J, Waterfield MD
    Cellular function of phosphoinositide 3-kinases: implications for development, homeostasis, and cancer. Annual review of cell and developmental biology 2001;17:615-75
14. Vanhaesebroeck B, Alessi DR
    The PI3K-PDK1 connection: more than just a road to PKB. The Biochemical journal 2000 Mar 15;346 Pt 3:561-76
15. Murata H, Hresko RC, Mueckler M
    Reconstitution of phosphoinositide 3-kinase-dependent insulin signaling in a cell-free system. The Journal of biological chemistry 2003 Jun 13;278(24):21607-14