Initiation factors phosphorylation

Initiation factor phosphorylation. The phosphorylation of eIF-2 in response to certain circumstances (e.g., heat shock, viral infections, and growth factor deprivation) has been observed to decrease protein synthesis generally. However, the translation of certain mRNA increases. For example, hsp (heat shock protein) synthesis increases in response to heat shock and other stressful conditions. The specific mechanisms are unknown. 

The major differences between prokaryotic and eukaryotic translation control mechanisms are related to the complexity of eukaryotic gene expression. Features that distinguish eukaryotic translation include mRNA export (spatial separation of transcription and translation), mRNA stability (the half-lives of mRNA can be modulated), negative translational control (the translation of certain mRNAs can be blocked by the binding of specific repressor proteins), initiation factor phosphorylation (mRNA translation rates are altered by certain circumstances when eIF-2 is phosphorylated), and translational frame-shifting (certain mRNAs can be frame-shifted so that a different polypeptide is synthesized). 

Farrell, P. J., Balkow, K., Hunt, T., Jackson, R. J., andTrachsel, H. (1977). Phosphorylation of initiation factor eIF-2 and the control of reticulocyte protein synthesis. Cell 11, 187-200. 

Kimball, S. R., Fabian, J. R., Pavitt, G. D., Hinnebusch, A. G., and Jefferson, L. S. (1998). Regulation of guanine nucleotide exchange through phosphorylation of eukaryotic initiation factor eIF2alpha. Role of the alpha- and delta-subunits of eIF2B. J. Biol. Chem. 273, 12841-12845. 

Matts, R. L., Levin, D. H., and London, I. M. (1983). Effect of phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 on the function of reversing factor in the initiation of protein synthesis. Proc. Natl. Acad. Sci. USA 80, 2559—2563. 

Panniers, R., and Henshaw, E. C. (1983). A GDP/GTP exchange factor essential for eukaryotic initiation factor 2 cycling in Ehrlich ascites tumor cells and its regulation by eukaryotic initiation factor 2 phosphorylation. J. Biol. Chem. 258, 7928—7934. 

Wang, X., Paulin, F. E., Campbell, L. E., Gomez, E., O Brien, K., Morrice, N., and Proud, C. G. (2001). Eukaryotic initiation factor 2B Identification of multiple phosphorylation sites in the epsilon-subunit and their functions in vivo. EMBO J. 20,... 

Mazroui, R., Sukarieh, R., Bordeleau, M. E., Kaufman, R. J., Northcote, P., Tanaka, J., Gallouzi, I., and Pelletier, J. (2006). Inhibition of ribosome recruitment induces stress granule formation independendy of eukaryotic initiation factor 2alpha phosphorylation. Mol. Biol. Cell 17, 4212-4219. 

McEwen, E., Kedersha, N., Song, B., Scheuner, D., Gilks, N., Han, A., Chen, J. J., Anderson, P., and Kaufman, R. J. (2005). Heme-regulated inhibitor (HRI) kinase-mediated phosphorylation of eukaryotic translation initiation factor 2 (eIF2) inhibits translation, induces stress granule formation, and mediates survival upon arsenite exposure. J. Biol. Chem. 280, 16925—16933. 

Ueda, T., Watanabe-Fukunaga, R., Fukuyama, H., Nagata, S., and Fukunaga, R. (2004). Mnk2 and Mnkl are essential for constitutive and inducible phosphorylation of eukaryotic initiation factor 4E but not for cell growth or development. Mol. Cell Biol. 24, 6539-6549. 

Wang, X., Flynn, A., Waskiewicz, A. J., Webb, B. L. J., Vries, R. G., Baines, I. A., Cooper, J., and Proud, C. G. (1998). The phosphorylation of eukaryotic initiation factor eIF4E in response to phorbol esters, cell stresses, and cytokines is mediated by distinct MAP kinase pathways. J. Biol. Chem. 273, 9373-9377. 

Wang, X., Li, W., Parra, J.-L., Beugnet, A., and Proud, C. G. (2003). The C-terminus of initiation factor 4E-binding protein 1 contains multiple regulatory features that influence its function and phosphorylation. Mol. Cell. Biol. 23, 1546—1557. 

Human eukaryotic translation initiation factor 4G (eIF4G) recruits mnkl to phosphorylate eIF4E. EMBO J. 18, 270-279. 

IFN-a, -P and -y are all known to induce the enzyme in various animal cells. However, in human epithelial cells the kinase is induced only by type I interferons, whereas none of the interferons seem capable of inducing synthesis of the enzyme in human fibroblasts. The purified kinase is highly selective for initiation factor eIF-2, which it phosphorylates at a specific serine residue. 

Gribskov, M. (1992). Translational initiation factors IF-1 and eIF-2 alpha share an RNA-binding motif with prokaryotic ribosomal protein SI and polynucleotide phosphoryl-ase. Gene 119, 107-111. 

Fig. 1.32. Phosphorylation of the C-terminal domain of RNA polymerase II and the beginning of transcription. The transition from the initiation complex to actual begin of transcription is regulated via phosphorylation of the C-terminal domain (CTD) of RNA polymerase II. In the above model it is assumed that initially a complex is formed between TFIID and a holoenzyme of RNA polymerase consisting of RNA polymerase II and associated factors (mediators, SRB proteins) and the basal transcription factors. Phosphorylation of the C-terminal domain effects the dissociation of the RNA polymerase from the initation complex and the transition to the elongation phase. A protein kinase, which is part of TFIIH, is responsible for the phosphorylation. The nature of the signal that induces phosphorylation of RNA polymerase II remains unknown. SRB suppressor of RNA polymerase B. After Koleske and Young (1995).

The de novo synthesis of proteins can be varied in response to external stimuli, such as hormones or heat stress. The regulation of protein biosynthesis ocems primarily via phosphorylation of translation initiation factors. The regulatory points in eucaryotes are, above all, the translation factors eIF-2 and elF-4. 

The regulation of translation is accomplished in this system via a specific inhibitory protein and an initiation factor of translation. The binding activity of the inhibitor protein is regulated by protein phosphorylation, and thus, by protein kinases. The activity of protein kinases can be regulated in a multitude of ways. A signal initiating from insulin, for example, can activate the PI3-kinase and the Akt kinase pathway (see 6.6.2), resulting in phosphorylation of 4E-BP1. 

Initiation factors are subject to phosphorylation by a number of protein kinases. The phosphorylated forms are often less active and cause a general depression of translation in the cell. 

The MAPK cascade also has direct effects upon protein synthesis, i.e., on the translation of mRNA messages. For example, insulin stimulates phosphorylation of proteins that regulate a translation initiation factor, a protein called eIF-4E (see Chapter 29). Phosphorylation of inhibitory proteins allows them to dissociate from the initiation factor so that protein synthesis can proceed 485/486... 

Many antibiotics, which inhibit protein synthesis, do not bind to ribosomes but block any of a variety of vital chemical processes needed for growth. Among them are pseudomonic acid, which inhibits isoleucyl-tRNA synthetase from many gram-positive bacteria.1111/VV Rapamycin, best known as an immunosuppressant (Box 9-F), inhibits phosphoinositide-3-kinase and also phosphorylation of the cap-binding protein 4G, a component of the eukaryotic initiation factor complex (Fig. 29-11 ).ww The bacterial enzyme peptide deformylase, which is absent from the human body, has been suggested as a target for design of synthetic antibiotics. 01... 

Hinnebusch, A. G., Involvement of an initiation factor and protein phosphorylation in translational control of GCN4 mRNA. Trends Biochem. Sci. 15 148-152, 1990. 

High concentrations of hemin inhibit the transport of ALA synthase into the mitochondria, where one of the substrates, succinyl-CoA, is formed. Thus, heme synthesis is inhibited until enough globin is made to react with any heme already formed. Low concentrations, or the absence, of hemin is the signal that globin is not needed this protein (and, therefore, globin) synthesis is inhibited. In the absence of hemin, a protein kinase is activated this phosphorylates an initiation factor of (eukaryotic) protein synthesis, eIF-2, which then inhibits polypeptide chain initiation (Chap. 17) and hence inhibits globin synthesis. 

Globin is synthesized in reticulocytes (see Chap. 1, Prob. 1.1). which have no nucleus and therefore cannot utilize transcriptional and other potential modes of control. Control of globin synthesis from the pool of globin-enriched mRNA is geared to the concentration of hemin (Fe(III)-protoporphyrin]. which has the ability to inactivate a translational inhibitor of protein synthesis. The inhibitor is a protein kinase that phosphorylates and inactivates one of the initiation factors involved in initiation of translation. When the concentration of hemin is high, it binds to a regulatory subunit of the kinase and. as a result, initiation of globin synthesis can proceed. 

Translation can be inhibited through the phosphorylation of eukaryote initiation factor 2 (eIF2) by dsRNA-dependent PK (activated by viral dsRNA as a consequence of viral infection), by hemin-inhibited PK (activated in the absence of hemin in reticulocytes) and by GCN2 kinase (general control non-derepressible kinase) (activated by amino acid starvation and excess free tRNA). Phosphorylation of RNA polymerase II is a key process in the regulation of transcription (Chapter 9). 

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