Translational control in eukaryotes

T. Preiss and M.W. Hentze. 1999. From factors to mechanisms Translation and translational control in eukaryotes Curr. Opin. Genet. Dev. 9 515-521. (PubMed)... 

Another example of translational control in eukaryotes is the inhibition of yeast GCN4 protein synthesis by stem-loop structures present in the 50 end of the mRNA. GCN4 control, and an analogous situation in bacteria, links amino-acid biosynthesis to ribosome pausing in the 50 end of the mRNA. This mechanism was first described for the tryptophan operon in E. coli and it is often referred to as attenuation. Transcriptional and translational control of the tryptophan biosynthetic enzymes are described in Chapter 28. 

Prokaryotes do not seem to make extensive use of control at the translational level, whereas eukaryotes use translational control much more widely. In part, translational control in eukaryotes occurs at the mRNA level. It may involve the sequestering of specific mRNAs by combining with specific mRNA-binding proteins and/or rapid degradation of mRNA so that they do not persist in inappropriate phases of the cell cycle. Other translational controls include the phosphorylation of factors involved in translation, as listed below ... 

Phosphorylation of initiation factors appears to be a general method for translational control in eukaryotes. 

Eukaryotes potentially have many more opportunities for control of gene expression than do bacteria. For example, the cell could take advantage of control at the level of the processing of primary transcripts. It is known that RNA is not transported across the nuclear membrane until all introns are excised. A more subtle form of control could involve alternative modes of splicing a particular transcript. There are now examples known where this occurs to yield different mRNA molecules. Perhaps one of the best-known examples of yet another level of control in eukaryotes is that of translational control of globin synthesis. 

TRANSLATION CONTROL MECHANISMS Eukaryotic translation control mechanisms are proving to be exceptionally complex, substantially more so than those observed in prokaryotes. In eukaryotes these mechanisms appear to occur on a continuum, from global controls (i.e., the translation of a wide variety of mRNAs is altered) to specific controls (i.e., the translation of a specific mRNA or small group of mRNAs is altered). Although most aspects of eukaryotic translational control are currently unresolved, the following features are believed to be important ... 

The separation of transcription and translation means there are many more opportunities for control in eukaryotes. Eor example, expression can be controlled at a posttranscriptional level. The cell can alter the rate at which RNA transcripts are... 

Regulation at the translational level and the role of informosomes. Many data indicate that regulation of gene expression takes place at the translational level in eukaryotic cells and that regulation of transcription is not the only mode of control of the kinds of proteins that are produced. Classic examples of translational control are the synthesis of such proteins as fibroin in the silk glands of the silkworm or hemoglobin production in reticulocytes. In both cases the synthesis of mRNA much precedes protein formation, and for a long period of time mRNA is accumulated without involvement in protein production. In fact, the peak of protein synthesis coincides with the period of depression of RNA production (Smirnov et al., 1964 see also review of Spirin, 1966). 

Post-translational modification of proteins plays a critical role in cellular function. For, example protein phosphorylation events control the majority of the signal transduction pathways in eukaryotic cells. Therefore, an important goal of proteomics is the identification of post-translational modifications. Proteins can undergo a wide range of post-translational modifications such as phosphorylation, glycosylation, sulphonation, palmitoylation and ADP-ribosylation. These modifications can play an essential role in the function of the protein and mass spectrometry has been used to characterize such modifications. 

The mDHFR protein complementation assay has been used to map a signal transduction network that controls the initiation of translation in eukaryotes (Remy and Michnick, 2001). A total of 35 different pairs of full-length proteins were analyzed and 14 interactions were identified using the survival selection of cells grown in the absence of nucleotides. In addition, the use of the fMTX reagent in combination with fluorescence microscopy was used to localize the protein complex within cells (Remy and Michnick, 2001). 

Polunovsky, V. A., Rosenwald, I. B., Tan, A. T., White, J., Chiang, L., Sonenberg, N., and Bitterman, P. B. (1996). Translational control of programmed cell death Eukaryotic translation initiation factor 4E blocks apoptosis in growth-factor-restricted fibroblasts with physiologically expressed or deregulated Myc. Mol. Cell Biol. 16, 6573-6581. 

Regulation of catalase expression in eukaryotes takes place as part of a generalized response mechanism. In yeast, promoter elements of the peroxisomal catalase CTA-1 respond to glucose repression and activation by fatty acids as part of organelle synthesis. The cytosolic catalase CTT-1 responds as part of a generalized stress response to starvation, heat, high osmolarity, and H2O2, and there is even evidence of translational control mediated by heme availability 26). 

Another way in which gene expression is regulated is by translational control, where the rate of protein synthesis is controlled at the point of transcription of mRNA into polypeptides (Appendix 5.6). Generally, the majority of the control mechanisms in bacteria is at the transcriptional level. Translational control is less well understood and appears to be a secondary mechanism in bacteria, but it is thought to be very important in eukaryotic organisms. 

Hershey, J. W. B., Protein phosphorylation controls translation rates. J. Biol. Chem. 264 20823, 1989. Describes how protein kinases are thought to regulate translation in eukaryotic systems. 

Inactivation of eukaryotic translation factors by covalent modification is one of the few mechanisms known to regulate the rate of translation. Specific protein kinases have been identified that phosphorylate and inactive both eIF-1 and EF-2. The significance of the phosphorylation of EF-2 as a regulatory mechanism of the elongation rate is still not clear, but the phosphorylation of eIF-2 appears to be a general mechanism for controlling translation initiation in many cells. 

Jay, F. T., Laughlin, C. A. and Carter, B. J. (1981). Eukaryotic translational control Adeno-associated virus protein synthesis is affected by a mutation in the adenovirus DNA-binding protein. Proc. Natl. Acad. Sci. USA 78, 2927-2931. 

The conversion of nucleic acid into protein information doesn t completely solve the problem of translation. Proteins must be targeted to their appropriate locations, either inside or outside the cell. In eukaryotes especially, proteins must be broken down at appropriate rates some proteins have longer half-lives than others do. These steps are all possible points for cellular control. 

The end products of gene expression are proteins, mainly enzymes, and it is essential that their levels be strictly controlled. There are many potential sites of control in both bacteria and eukaryotes. DNA or gene amplification (Chap. 16) in eukaryotes is one way of responding to the demand for more of the protein product if there arc more copies of the gene, then transcription can occur at a faster rate. More often, control is effected at the level of cither transcription or translation, with the former probably being more important for both bacteria and eukaryotes. Transcriptional control in bacteria is particularly effective because of the very short half-life (a few minutes) of mRNA in such cells the half-life is longer in eukaryotes. The prototype for transcriptional control is the lactose operon in E. coli. 

The translation of mRNA to protein concludes the gene expression cascade and links the proteome to the genome. Consequently, control of translation can be a direct and effective means to modulate the proteome [21, 30, 31]. In addition to transcript interactions with protein regulators, translation is also modulated by structural features or regulatory sequences appearing within the mRNA molecules. The 7-methylguanylate triphosphate nucleotidyl caps at the 5 end, poly-A tails, uORFs, and IRESs are examples of structures that affect the rate and efficiency of translation in eukaryotes [21]. 

Shi, Y, Vattem, K.M., Sood, R., An, J., Liang, J., Stramm, L. and Wek, R.C. (1998) Identification and characterization of pancreatic eukaryotic initiation factor 2 alpha-subunit kinase, PEK, involved in translational control. Mol. Cell Biol. 18, 7499-7509. 

Regulationof gene expression in eukaryotes proceeds primarily by control of transcription as in prokaryotes. Some systems are also regulated at the translational level. 

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