Protein synthesis in eukaryotes

Nicchitta, C. V., Lerner, R. S., Stephens, S. B., Dodd, R. D., and Pyhtila, B. (2005). Pathways for compartmentalizing protein synthesis in eukaryotic cells The templatepartitioning model. Biochem. Cell Biol. 83, 687—695. 

Ered Sanger, a double Nobel Prize winner, sequenced the human mitochondrial genome back in 1981. This genome codes for 13 proteins and the mitochondrion possesses the genetic machinery needed to synthesize them. Thus, the mitochondria are a secondary site for protein synthesis in eukaryotic cells. It turns out that the 13 proteins coded for by the mitochondrial genome and synthesized in the mitochondria are critically important parts of the complexes of the electron transport chain, the site of ATP synthesis. The nuclear DNA codes for the remainder of the mitochondrial proteins and these are synthesized on ribosomes, and subsequently transported to the mitochondria. 

This drug has a direct amebicidal effect against trophozoites E. histolytica in tissues, and it is not active against cysts in either the lumen or intestinal walls, or in other organs. The mechanism of action of emetine consists of the blockage of protein synthesis in eukaryotic (but not in prokaryotic) cells. It inhibits the process of polypeptide chain formation. Protein synthesis is inhibited in parasite and mammalian cells, but not in bacteria. 

Jim6nez, A., Santos, A., Alonso, G. and Vazquez, D. 1976. Inhibitors of protein synthesis in eukaryotic cells. Comparative effects of some Amaryllidaceae alkaloids. Biochimica et Biophysica Acta, 425 342-348. 

Initiation of protein synthesis in eukaryotic cells requires a much more complex spectrum of initiation factors, abbreviated elF. At least nine separate factors have been identified, some of which are composed of as many as 11 different peptide subunits. The exact function of only a few... 

Formation of the initiation complex for protein synthesis in eukaryotes. The reaction begins with the small subunit held apart from the large subunit by an antiassociation factor and ends with the hydrolysis of GTP and joining of the large subunit as in prokaryotes. The intervening reactions are different. A much more complex spectrum of initiation factors (elFs) is involved, and the exact function of only a few of these factors is known with certainty. The... 

Narciclasine (215) is an antitumor agent which exerts an antimitotic effect during metaphase by immediately terminating protein synthesis in eukaryotic cells at the step of peptide bond formation (97,101,141,142), apparently by interaction with the ansiomycin area of the ribosomal peptidyl transferase center (142). The alkaloid has also been found to inhibit HeLa cell growth and to stabilize HeLa cell polysomes in vivo (97). Although DNA synthesis was retarded by narciclasine, RNA synthesis was practically unaffected (97,142). Sev-... 

Pretazettine (395) has been the subject of numerous biological studies, and it has been shown to exhibit a number of interesting activities (96,97,101,178-187). For example, 395 was found to inhibit HeLa cell growth as well as protein synthesis in eukaryotic cells by interfering with the peptide bond formation step (97,101). Furthermore, pretazettine inhibited the purified RNA-dependent DNA polymerase (reverse transcriptase) from avian myeloblastosis virus, a typical C-type virus (178), in an unusual fashion since it physically combined with the polymerase enzyme itself rather than interacted with the nucleic acid template. Pretazettine also exhibited antiviral activity against the Rauscher leukemia virus in mouse embryo cell cultures by suppressing viral replication (179). 

Although known for its toxicity, but unlike antimony and arsenic, selenium is an essential element which has been identified as part of several prokaryotic and eukaryotic proteins in the form of the amino acid, selenocysteine. Selenocysteine has been referred to as the 21st amino acid since gene products required for its incorporation into protein were discovered in bacteria (Stadtman, 1996). Aspects of the mechanism of selenocysteine insertion during protein synthesis in eukaryotes are currently being investigated (Low and Berry, 1996). The two strands of current selenium research are... 

The cap protects the 5 end of the primary transcript against attack by ribonu-cleases that have specificity for 3 5 phosphodiester bonds and so cannot hydrolyze the 5 5 bond in the cap structure. In addition, the cap plays a role in the initiation step of protein synthesis in eukaryotes. Only RNA transcripts from eukaryotic protein-coding genes become capped prokaryotic mRNA and eukaryotic rRNA and tRNAs are uncapped. 

Initiation The overall mechanism of protein synthesis in eukaryotes is basically the same... 

Initiation of protein synthesis in eukaryotes requires at least nine distinct eukaryotic initiation factors (elFs) (see Table 1) compared to the three initiation factors (IFs) in prokaryotes (see Topic H2). 

The mode of action of the harringtonines has been investigated. All inhibit protein synthesis in eukaryotic cells (190-192). The principal effect of harringtonine was inhibition of protein biosynthesis in HeLa cells (193). Homoharringtonine, a potential antineoplastic alkaloid (191), was cytotoxic in HeLa, KB, and L cells growing in monolayer cell cultures (194). 

Inhibition of protein synthesis in eukaryotic cells by the Cephalotaxus alkaloids harringtonine, homoharringtonine, and isoharringtonine has been studied.16 In model systems, these alkaloids were found not to inhibit any of the initiation steps but to block certain parts of the elongation phase of translation. 

Carter, C.J., Cannon, M. (1977). Stmctural requirements for the inhibitory action of 12,13-epoxytrichothecenes on protein synthesis in eukaryotes. Biochem. J. 166 399 09. 

Transfer RNA molecules (tRNAs), messenger RNA, and many proteins participate in protein synthesis along with ribosomes. The link between amino acids and nucleic acids is first made by enzymes called aminoacyl-tRNA synthetases. By specifically linking a particular amino acid to each tRNA, these enzymes implement the genetic code. This chapter focuses primarily on protein synthesis in prokaryotes because it illustrates many general principles and is relatively well understood. Some distinctive features of protein synthesis in eukaryotes also are presented. 

The basic plan of protein synthesis in eukaryotes and archaea is similar to that in bacteria. The major structural and mechanistic themes recur in all domains of life. However, eukaryotic protein synthesis entails more protein components than does prokaryotic protein synthesis, and some steps are more intricate. Some noteworthy similarities and differences are as follows ... 

Diphtheria Toxin Blocks Protein Synthesis in Eukaryotes by Inhibiting Translocation... 

Figure 29.35. Blocking of Translocation by Diphtheria Toxin. Diphtheria toxin blocks protein synthesis in eukaryotes by catalyzing the transfer of an ADP-ribose unit from NAD+ to diphthamide, a modified amino acid residue in elongation factor 2 (translocase). Diphthamide is formed by a posttranslational modification (blue) of a histidine residue.

The basic plan of protein synthesis in eukaryotes is similar to that of prokaryotes, but there are some significant differences between them. Eukaryotic ribosomes (808) consist of a 408 small subunit and a 608 large subunit. The initiating amino acid is again methionine, but it is not formylated. The initiation of protein synthesis is more complex in... 

Which of the following is NOT required for the elongation reactions of protein synthesis in eukaryotes ... 

The mechanism for termination of protein synthesis in eukaryotes requires... 

The translocation step is probably the point at which prokaryotic and eukaryotic secretion differ most. The energy for this process may derive from different sources from the energy of protein synthesis in eukaryotes, and from protein synthesis and/or ATP hydrolysis and/or the membrane potential in prokaryotes. [In fact there is evidence for more than one secretion pathway in E. coli. The degree of coupling between translation and translocation may also be different in prokaryotes and eukaryotes (Section V,C).]... 

V. M. Pain Initiation of protein synthesis in eukaryotic cells. European Journal of Biochemistry 236, 747 (1996). 

The answer is c. (Murray, pp 452-467. Scriver, pp 3-45. Sack, pp 1-40. Wilson, pp 101-120.) In a general sense, the mechanism of protein synthesis in eukaryotic cells is similar to that found in prokaryotes however, there are significant differences. Cycloheximide inhibits elongation of proteins in eukaryotes, while erythromycin causes the same effect in prokaryotes. Thus, one is an antibiotic beneficial to humans, while the other is a poison. Cytoplasmic ribosomes of eukaryotes are larger, sedimenting at SOS instead of 70S. While eukaryotic cells utilize a specific tRNA for initiation, it is not formylated as in bacteria. Finally, eukaryotic mRNA always specifies only one polypeptide, as opposed to prokaryotic mRNA, which may specify the synthesis of more than one gene product per mRNA. 

Since protein synthesis in eukaryotes occurs both in the cytoplasm and in certain cellular organelles such as mitochondria (and chloroplasts in the case of plants), the mechanisms of cytoplasmic protein synthesis are described first, followed by a discussion of the similarities and differences in protein synthesis in the organelles, particularly the mitochondria. 

Figure 1. Schematic diagram showing the major steps of the initiation of protein synthesis in eukaryotic cells. The formation of an 80S initiation complex involves the joining of the 40S subunit bound to mRNA and met-tRNAi, and the 60S subunit. Numbers in represent various initiation factors, o represents tRNA, and a represents the amino acid. (From Johansen and Rattan, 1993). 

A variety of processing events follow protein synthesis. In eukaryotes, many of these modifications are associated with specific subcellular compartments. For example, glycosylation of proteins requires enzymes that are present both in the endoplasmic reticulum and the Golgi apparatus. Other proteins need to be localized to specific regions in the cell, for example, the localization of integral membrane proteins to the plasma membrane, or nuclear -encoded mitochondrial proteins that are synthesized in the cytosol but need to be imported into the mitochondria. 

Messenger RNA (mRNA) The RNA transcript that serves as the template for protein synthesis in eukaryotes, mRNA is fully processed hnRNA. 

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