Protein synthesis initiation

Fig. 8.4 Outline of the main events in protein synthesis initiation, elongation, translocation and termination. AUG is an initiation codon on the mRNA it codes for Af-fomiylmelhionine and initiates the formation of the 70S rihosome. UAG is a termination codon it does not code for any amino acid and brings about termination of protein synthesis.

Analysis of Protein Synthesis Initiation on ER Membrane-Bound... 

Konieczny, A., and Safer, B. (1983). Purification of the eukaryotic initiation factor 2-eukaryotic initiation factor 2B complex and characterization of its guanine nucleotide exchange activity during protein synthesis initiation. J. Biol. Chem. 258, 3402—3408. 

In vitro analysis of protein synthesis initiation on membrane-bound ribosomes... 

Stem, B. D., Wilson, M., and Jagus, R. (1993). Use of nonreducing SDS-PAGE for monitoring renaturation of recombinant protein synthesis initiation factor, eIF-4 alpha. Protein Expr. Purif. 4, 320-327. 

Lamphear, B. J., Kirchweger, R., Skem, T., and Rhoads, R. E. (1995). Mapping of functional domains in eukaryotic protein synthesis initiation factor 4G (eIF4G) with picornaviral proteases. Implications for cap-dependent and cap-independent translational initiation. J. Biol. Chem. 270, 21975—21983. 

The binding of interferons to their receptors induces a rapid increase in the transcription of particular genes and synthesis of corresponding proteins.196 202 One of the proteins induced is a double-stranded RNA-activated 2 -5 A synthase, which polymerizes ATP to a series of 2 -5 linked oligonucleotides containing triphosphates at the 5 termini.202-204 Double-stranded RNA is uncommon except in replicating viruses, and it is thought that the activation by dsRNA is related to establishment of an antiviral state. Another interferon-induced enzyme is the small subunit of eukaryotic protein synthesis initiation factor eIF-2. 

BJ Lamphear, R Yan, F Yang, D Waters, HD Liebig, H Klump, E Kuechler, T Skern, RE Rhoads. Mapping the cleavage site in protein synthesis initiation factor elF-4 gamma of the 2A proteases from human Coxsackievirus and rhinovirus. J Biol Chem 268 19200-19203, 1993. 

M. Bushell, W. Wood, M.J. Clemens, and S.J. Morley. 2000. Changes in integrity and association of eukaryotic protein synthesis initiation factors during apoptosis Eur. J. Biochem. 267 1083-1091. (PubMed)... 

There are also codons for protein synthesis initiation (AUG) and polypeptide chain termination (UAG, UGA, and UAA). 

In this section, we describe the three basic stages of protein synthesis initiation, elongation, and termination. These three processes are fairly similar between prokaryotes and eukaryotes, with the two exceptions being that more protein factors have been identified as necessary for eukaryotic protein synthesis, and that transcription and translation are physically linked in prokaryotes but not in eukaryotes. Note that the reactions will be schematized as a single ribosome transversing the mRNA, but as shown in Figure 26.3, translation actually occurs on polyribosomes. 

FIGURE 49-3 Interferon-mediated antiviral activity occurs via mub le mechanisms. The binding of IFN to specific cell surface receptor molecules signals die cell to produce a series of antiviral proteins. The stages of viral replication diat are inhibited by various IFN-induced antiviral proteins are shown. Most of these act to inhibit the translation of viral proteins (mechanism 2), but odier steps in viral replication also are affected (mechanisms 1,3, and 4). The roles of diese mechanisms in the other actions of IFNs are under study. Key IFN = interferon mRNA = messenger RNA Mx = specific cellular protein tRNA = transfer RNA RNase L = latent cellular endoribonuclease 2 5 A = 2 -5 -oligoadeny-lates eIF-2a = protein synthesis initiation factor. 

Only two compounds presently in the clinic act on unconventional targets oxazolidinones (which inhibit bacterial protein synthesis initiation by binding the 50S ribosomal subunit) and cationic peptides (which permeabilize bacterial membranes). 

To this point, you have become familiar with the molecules that participate in protein synthesis and the genetic code, the language that directs the synthesis. We now investigate the actual process by which polypeptide chains are assembled. There are three major stages in protein synthesis initiation of the polypeptide chain, elongation of the chain, and termination of the completed polypeptide chain. 

Roles of Eukaryotic Initiation Factor 2 Ancillary Factors in the Regulation of Eukaryotic Protein Synthesis Initiation Naba K. Gupta... 

VAUGHAN, M.H., PA L0 SKI, P.J. and FORGHHAMMER, J. Regulation of protein synthesis initiation in HeLa cells deprived of single essential amino acids. Proc. Natl. Acad. Sci. U.S.A. 

The association of ( S) methionyl-tRRA with the 4O-S subunit was strongly inhibited by poly(l) poly(c) in extracts of interferon-treated cells. In extracts of control cells poly(l) poly(c) had no significant effect upon the formation of methionyl-tRHA 40-S subunit complexes. Addition of crude rabbit reticulocyte initiation factors greatly reduced the poly(l) poly(c)-induced inhibition of (55s) methionyl-tRRA 4O-S subunit complex formation. The effect was most marked when the initiation factors were added during the incubation with dsRRA, strongly suggesting that in interferon-treated cell extracts dsRHA acts at the level of a protein synthesis initiation factor which binds methionyl-tRNA to native 4O-S ribosomal subunits, a situation analogous to that of the rabbit reticulocyte lysate (20-22 and chapter 10 of this volume). 

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