Mammalian protein synthesis

Diphtheria toxin, an exotoxin of Corynebacterium diphtheriae infected with a specific lysogenic phage, catalyzes the ADP-ribosylation of EF-2 on the unique amino acid diphthamide in mammalian cells. This modification inactivates EF-2 and thereby specifically inhibits mammalian protein synthesis. Many animals (eg, mice) are resistant to diphtheria toxin. This resistance is due to inability of diphtheria toxin to cross the cell membrane rather than to insensitivity of mouse EF-2 to diphtheria toxin-catalyzed ADP-ribosylation by NAD. 

The common amino acids used in mammalian protein synthesis belong to the L-enantiomeric series. However, fungi also employ the o-enantiomers in the biosynthesis of some secondary metabolites. These are normally formed from the corresponding L-amino acid. Fungi can also make amino acids with structures that differ from those commonly found in mammalian proteins and in higher plants. These unusual amino acids are utilized for the synthesis of secondary metabolites and some peptides. 

Selective toxicity of these protein synthesis inhibitors against microorganisms may be explained by target differences. Chloramphenicol does not bind to the SOS ribosomal RNA of mammalian cells, though it can inhibit the functions of mitochotidrial ribosomes, which contain 70S ribosomal RNA. Tetracyclines have little effect on mammalian protein synthesis because an active efflux mechanism prevents their intracellular accumulation. 

THE MECHAHISM MD CYTOPLASMIC CONTROL OP MAMMALIAN PROTEIN SYNTHESIS... 

There are two predominant proteins (M.Wt. approximately 50>000 and 7Qf000) in polysomal mRNPs isolated from a wide variety of mammalian sources (reviewed in reference 59) Other proteins are present in minor amounts, and are far from consistent between different preparations. Those who wish to see complications at every level of mammalian protein synthesis like to hint that these minor components could be protein specific to particular mRNA species, perhaps involved in regulating the translation of the mRNA, whilst the 50,000 and J8,000 dalton proteins would be seen as invariant components common to all mRNAs. This idea seems unlikely to be true, at least as a generality, since there is no sign of the putative mRNA-specific proteins in mRNP preparations from cells... 

Pain, V. M., and Clemens, M. J., 1983, Assembly and breakdown of mammalian protein synthesis initiation complexes Regulation by guanine nucleotides and by phosphorylation of initiation factor eIF-2, Biochemistry 22 726. 

Schreier, M. H., Erni, B., and Staehelin, T., 1977, Initiation of mammalian protein synthesis. I. Purification and characterization of seven initiation factors, J. Mol. Biol. 116 727. 

Jagus, R., Anderson, W. F., and Safer, B., 1981, The regulation of initiation of mammalian protein synthesis, in Progress in Nucleic Acid Research and Molecular Biology, Vol. 25, (W. E. Cohn, and E. Volkin, eds.), pp. 127-185, Academic Press, New York. 

Puromycin. Puromycin (19), elaborated by S. alboniger (1—4), inhibits protein synthesis by replacing aminoacyl-tRNA at the A-site of peptidyltransferase (48,49). Photosensitive analogues of (19) have been used to label the A-site proteins of peptidyltransferase and tRNA (30). Compound (19), and its carbocycHc analogue have been used to study the accumulation of glycoprotein-derived free sialooligosaccharides, accumulation of mRNA, methylase activity, enzyme transport, rat embryo development, the acceptor site of human placental 80S ribosomes, and gene expression in mammalian cells (51—60). 

With the aid of cytosine permease, flucytosine reaches the fungal cell where it is converted by cytosine deaminase into 5-fluorouracil [51-21-8]. Cytosine deaminase is not present in the host, which explains the low toxicity of 5-FC. 5-Fluorouracil is then phosphorylated and incorporated into RNA and may also be converted into 5-fluorodeoxyuridine monophosphate, which is a potent and specific inhibitor of thymidylate synthetase. As a result, no more thymidine nucleotides are formed, which in turn leads to a disturbance of the DNA-synthesis. These effects produce an inhibition of the protein synthesis and cell repHcation (1,23,24). 5-Fluorouracil caimot be used as an antimycotic. It is poorly absorbed by the fungus to begin with and is also toxic for mammalian cells. 

The antiviral activity of (5)-DHPA in vivo was assessed in mice inoculated intranasaHy with vesicular stomatitis vims ( 5)-DHPA significantly increased survival from the infection. (5)-DHPA did not significantly reduce DNA, RNA, or protein synthesis and is not a substrate for adenosine deaminase of either bacterial or mammalian origin. However, (5)-DHPA strongly inhibits deamination of adenosine and ara-A by adenosine deaminase. Its mode of action may be inhibition of Vadenosyl-L-homocysteine hydrolase (61). Inhibition of SAH hydrolase results in the accumulation of SAH, which is a product inhibitor of Vadenosylmethionine-dependent methylation reactions. Such methylations are required for the maturation of vital mRNA, and hence inhibitors of SAH hydrolase may be expected to block vims repHcation by interference with viral mRNA methylation. 

Induction of apoptosis has been reported in various mammalian cell lines. In previous studies, it has been reported that TBT induces apoptosis in isolated thymocytes at concentrations which are relevant to those causing thymus atrophy in vivo. TBT can also induce apoptosis in PC12 cells, and in human T-lymphoblastoid CEM cells. While the mechanism of TBT-induced apoptosis is still unknown, it has been reported that TBT stimulates thymocyte apoptosis by a mechanism independent of protein synthesis and under conditions where intracellular ATP levels are severely depleted. ... 

Group 2 includes some 80 sesquiterpene trichothecenes, which are particularly associated with fungi belonging to the group Fusarium. Fusarium species are widely known both as plant pathogens and contaminants of stored foods snch as maize. Trichothecenes are strong inhibitors of protein synthesis in mammalian cells. There have been many incidents of poisoning of farm animals cansed by contamination of their food by these componnds. 

Biochemical and genetic experiments in yeast have revealed that the b poly(A) tail and its binding protein, Pablp, are required for efficient initiation of protein synthesis. Further studies showed that the poly(A) tail stimulates recruitment of the 40S ribosomal subunit to the mRNA through a complex set of interactions. Pablp, bound to the poly(A) tail, interacts with eIF-4G, which in turn binds to eIF-4E that is bound to the cap structure. It is possible that a circular structure is formed and that this helps direct the 40S ribosomal subunit to the b end of the mRNA. This helps explain how the cap and poly(A) tail structures have a synergistic effect on protein synthesis. It appears that a similar mechanism is at work in mammalian cells. 

Schultz, R. M., and Wassarman, P. M. (1977). Biochemical studies of mammalian oogenesis protein synthesis during oocyte growth and meiotic maturation in the mouse. J. Cell Sci. 24 167-194. 

Iron homeostasis in mammalian cells is regulated by balancing iron uptake with intracellular storage and utilization. As we will see, this is largely achieved at the level of protein synthesis (translation of mRNA into protein) rather than at the level of transcription (mRNA synthesis), as was the case in microorganisms. This is certainly not unrelated to the fact that not only do microbial cells have a much shorter division time than mammalian cells, but that one consequence of this is that the half-life of microbial mRNAs is very much shorter (typically minutes rather than the hours or often days that we find with mammals). This makes it much easier to control levels of protein expression by changing the rate of specific mRNA synthesis by the use of inducers and repressors. So how do mammalian cells... 

Plant-based production systems are now being used commercially for the synthesis of foreign proteins [1-3]. Post-translational modification in plant cells is similar to that carried out by animal cells plant cells are also able to fold multimeric proteins correctly. The sites of glycosylation on plant-produced mammalian proteins are the same as on the native protein however, processing of N-linked glycans in the secretory pathway of plant cells results in a more diverse array of glycoforms than is produced in animal expression systems [4]. Glycoprotein activity is retained in plant-derived mammalian proteins. 

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