Translational Control of Protein Synthesis

The frenetic pace of research in protein biosynthesis has led to a reasonable understanding of the transcriptional process and its attendant controls. However, the situation is not so clear regarding translational controls but one can expect an improved understanding of that process to emerge during the next few years. 

Here we wish to consider the role of hemin in translational control of globin synthesis. The subject of hemin control has been considered in further detail by Lodish (1976), Nienhaus and Benz (1977), Revel and Groner (1978), and Safer and Anderson (1978). The main phenotypic protein of reticulocytes is hemoglobin, which accounts for about 90% 

Evidence reviewed by Lodish (1976) indicates that both human and rat /3-globin mRNAs are able to initiate protein synthesis more efficiently than a-globin mRNA. These differences may result from a combination of intrinsic differences in the primary and secondary structure of these mRNAs, as well as from differences in the affinity for binding either to the 40 S-Met-tRNAf initiation complex or to eIF-2. 

It appears that a protein inhibitor termed hemin controlled repressor (HCR) forms in hemin-deficient cells from a latent prorepressor. This inhibitor appears to function as a protein kinase, adding one to two phosphorus atoms to the small subunit of eIF-2. Whether this activity is cAMP dependent (Datta et al., 1978) or not (Gross, 1978 Safer and Anderson, 1978) has not been resolved. The effect of the HCR appears to be to decrease the amount of initiator Met-tRNAf which binds to the 40 S subunit. The ability of eIF-2 to counteract the effect of HCR, in conjunction with the data on phosphorylation, strongly supports a role for HCR as inhibitor of the action of eIF-2. However, the actual means by which phosphorylation of the initiator factor leads to inhibition, or how hemin works, is not clear. Clarification of this phenomenon should prove illuminating not only for hemoglobin synthesis but also for nonerythroid cells, since the effect of hemin is seen with extracts from other cells as well. 

Coordination of heme and globin synthesis is the rule during erythropoiesis. While heme is synthesized in mitochondria, protein s)mthesis occurs in the endoplasmic reticulum. Control between compartments may have more far-reaching effects than merely coordinated hemoglobin manufacture. Indeed, there is evidence from studies with deprivation of amino acids, glucose, or oxygen that protein synthesis is coupled in some manner to the metabolic economy. So conceived, the role of hemin in the cytochromes may serve as the nexus between the 

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Storti, R.V., Scott, M.P., Rich, A. Pardue, M.L. (1980). Translational control of protein synthesis in response to heat shock in D. melanogaster cells. Cell, 22,825-34. 

Indeed, inhibitors of protein synthesis have been used in the in vivo rabbit [60] and in the rat LangendorfT models [61]. Cycloheximide or actinomycin D did not reverse the PC effect [60]. This is probably due to non-complete inhibition of protein synthesis. The latter study [61] showed that the PC effect was abolished only with cycloheximide, but not with actinomycin D, indicating that PC protection is regulated at the post-transcriptional level. Recent findings [62] have reconfirmed the translational control of protein synthesis in ischemic (but not pharmacological) PC. These are in accord with our findings concerning de novo synthesis of ferritin. 

Chottiner EG, Cloft HJ, Tartaglia AP, Mitchell BS. Elevated adenosine deaminase activity and hereditary hemolytic anemia. Evidence for abnormal translational control of protein synthesis. J Clin Invest 1987 79 1001-5. 

Evidence for Translational Control of Protein Synthesis in Bacteria. 187... 

The rabbit reticulocyte, which synthesizes about 90 of its protein as hemoglobin, offers one of the best examples of translational control of protein synthesis in animal cells. In the complete absence of a nucleus (which is extruded from the cell during its maturation) it coordinates the production of the a and R chains of globin, both with each other and with the supply of heme. However, the exact mechanism of this translational control is not yet fully understood, and only a partial description of what appears to be a surprisingly complex process can be given below. 

To what extent reversible modiflcations of ribosomal constituents are involved in translational control of protein synthesis is uncertain. Although phosphorylation of ribosomal protein S6 increases with cell proliferation, it is not known whether this change is directly related to the accompanying increase in protein synthesis by an effect on the translation rate. 

Kimball SR, Farrell PA, Jefferson LS. Invited review role of insulin in translational control of protein synthesis in skeletal muscle by amino acids or exercise. J Appl Physiol 2002 93 1168-1180 (review). 

Austin, S. A., and Clemens, M. J., 1981, The effects of haem on translational control of protein synthesis in cell-free extracts from fed and lysine-deprived Ehrlich ascites cells, Eur. J. Biochem. 117 601. 

Lodish, H. F., 1976, Translational control of protein synthesis, Annu. Rev. Biochem. 45 39. 

Kimball, S.R. andLS. Jefferson, 2005. Role of amino acids in the translational control of protein synthesis in mammals. Sem. Cell. Dev. Biol. 16, 21-27. 

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. 

In prokaryotes such as E. coli, most of the control of protein synthesis occurs at the level of transcription. (Refer to Section 18.3 for a discussion of the principles of prokaryotic transcriptional control.) This circumstance makes sense for several reasons. First, transcription and translation are directly coupled that is, translation is initiated shortly after transcription begins (Figure 19.8). Second, the lifetime of prokaryotic mRNA is usually relatively short. With half-lives of between 1 and 3 minutes, the types of mRNA produced in a cell can be quickly altered as environmental conditions change. Most mRNA molecules in E. coli are degraded by two exonucleases, referred to as RNase II and polynucleotide phosphorylase. 

The control of protein synthesis, either by regulation of the amount of mRNA available for translation or by the efficiency with which it is translated, is important in cell growth and development as a factor determining the level of cellular and extracellular proteins. Subversion of this control occurs in cells infected by viruses when the viral... 

Balkow, K., Hunt, T., and Jackson, R. J., 1975, Control of protein synthesis in reticulocyte lysates The effect of nucleotide triphosphates on formation of the translational repressor. Biochem. Biophys. Res. Commun. 67 366. 

Ernst, V., Levin, D. H., Ranu, R. S., and London, I. M., 1976, Control of protein synthesis in reticulocyte lysates Effects of cyclic AMP, ATP and GTP on inhibitions induced by heme-deficiency, dsRNA and a reticulocyte translational inhibitor. Proc. Natl. Acad. Sci. USA 73 1112. 

Gross, M., and Mendelewski, J., 1978, Control of protein synthesis by hemin. An association between the formation of the hemin-controlled translational repressor and the phosphorylation of a 100,000 molecular weight protein, Biochim. Biophys. Acta 520 650. 

From the discussion of the timing of the various groups of prereplicative phage proteins and the transition from prereplicative to late transcription, it would appear that most of the control of protein synthesis after T4 infection occurs on the transcriptional level. However, translational controls have been implicated in the shut-off of host protein synthesis and in the shut-off of early" protein synthesis late in the infectious cycle. 

Candelas, G. C., and Iverson, R. M. (1966). Evidence for translational level control of protein synthesis in the development of sea urchin eggs. Biochim. Biophys. Res. Commun. 24, 867-871. 

Ferritin, an iron-binding protein, prevents ionized iron (Fe ) from reaching toxic levels within cells. Elemental iron stimulates ferritin synthesis by causing the release of a cytoplasmic protein that binds to a specific region in the 5 nontranslated region of ferritin mRNA. Disruption of this protein-mRNA interaction activates ferritin mRNA and results in its translation. This mechanism provides for rapid control of the synthesis of a protein that sequesters Fe +, a potentially toxic molecule. 

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