Initiation of protein synthesis

Cation control of equilibrium and rate processes in initiation of protein synthesis. M. Grunberg-Manago, H. B. Hoa, P. Douzou and A. Wishnia, Adv. Inorg. Biochem., 1981, 3,193-232 (78). 

Initiation of protein synthesis requires that an mRNA molecule be selected for translation by a ribosome. Once the mRNA binds to the ribosome, the latter finds the correct reading frame on the mRNA, and translation begins. This process involves tRNA, rRNA, mRNA, and at least ten eukaryotic initiation factors (elFs), some of which have multiple (three to eight) subunits. Also involved are GTP, ATP, and amino acids. Initiation can be divided into four steps (1) dissociation of the ribosome into its 40S and 60S subunits (2) binding of a ternary complex consisting of met-tRNAf GTP, and eIF-2 to the 40S ribosome to form a preinitiation complex (3) binding of mRNA to the 40S preinitiation complex to form a 43S initiation complex and (4) combination of the 43S initiation complex with the 60S ribosomal subunit to form the SOS initiation complex. 

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. 

Figure 38-6. Diagrammatic representation of the initiation of protein synthesis on the mRNA template containing a 5 cap (G" TP-5 ) and 3 poiy(A) terminai [3 (A)J. This process proceeds in three steps (1) activation of mRNA ...

Matts, R. L., Levin, D. H., and London, I. M. (1983). Effect of phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 on the function of reversing factor in the initiation of protein synthesis. Proc. Natl. Acad. Sci. USA 80, 2559—2563. 

Hershey, J. W. B., and Merrick, W. C. (2000). Pathway and mechanism of initiation of protein synthesis. In Translational Control of Gene Expression (N. Sonenberg, J. W. B. Hershey, and M. B. Mathews, eds.), pp. 33—88. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York. 

H. W. Dickerman, The Role of Folate Coenzyme in the Initiation of Protein Synthesis in Folate Antagonists as Chemotherapeutic Agents, Ann. N.Y. Acad. Sci. 186, 70 (1971). 

Linezohd (Zyvox) is an oxazolidinone, a tive-membered heterocychc ring that forms the core of the hnezohd structure. The approval of hnezohd by the FDA in 2000 marked the first new structural class of antibacterial introduced into medical practice in the United States in 40 years. It is notable for its activity against methicillin-resistant Staph aureus, MRSA, and vancomycin-resistant Enterococcus faecium, VRE. It is bacteriostatic rather than bactericidal but finds significant use in patients with an intact immune system. Like several other classes of antibacterials, linezolid is an inhibitor of protein synthesis. It interacts specifically with the RNA component of a bacterial ribosome subunit to prevent initiation of protein synthesis. 

Activation of a factor(s) that controls initiation of protein synthesis. 

L A. The aminoglycosides appear to act by binding to various sites on bacterial SOS ribosomal subunits and disrupting the initiation of protein synthesis. The other agents appear to have the capacity to directly inhibit bacterial cell-wall synthesis. 

As described in Section 27.2, initiation of protein synthesis in the cell is an elaborate process that relies on initiation codons and other signals in the mRNA. In retrospect, the experiments of Nirenberg and Khorana to identify codon function should not have worked in the absence of initiation codons. Serendipitously, experimental conditions caused the normal initiation require-... 

Initiation of protein synthesis involves formation of a complex between the 30S ribosomal subunit, mRNA, GTP, fMet-tRNAfMet, three initiation factors, and the 50S subunit GTP is hydrolyzed to GDP and P,. 

Initiation of protein synthesis involves the assembly of the components of the translation system before peptide bond formation occurs. These components include the two ribosomal subunits, the mRNA to be translated, the aminoacyl-tRNA specified by the first codon in the message, GTP (which provides energy for the process), and initiation factors that facilitate the assembly of this initiation complex (see Figure 31.13). [Note In prokaryotes, three initiation factors are known (IF-1, IF-2, and IF-3), whereas in eukary- otes, there are at least ten (designated elF to indicate eukaryotic origin).] There are two mechanisms by which the ribosome recognizes the nucleotide sequence that initiates translation ... 

Binds to the 30S subunit and distorts its structure, interfering with the initiation of protein synthesis. 

Figure 29-10 Initiation of protein synthesis on bacterial ribosomes. Images are not drawn to scale. Some details are indicated on the larger scale image at the left.

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... 

Pain, V. M Initiation of protein synthesis in mammalian cells. Biochem. J. 235 625, 1986. A comprehensive review of the complex process of translational initiation in mammalian systems, emphasizing the mechanism of the process and its regulation. 

Kozak, M. (1980). Evaluation of the Scanning Model for initiation of protein synthesis in eucaryotes. Cell 22, 7-8. 

McIntosh, B., Ramachandiran, V., Kramer, G., and Hardesty, B. (2000). Initiation of protein synthesis with fluorophore-Met-tRNA(f) and the involvement of IF-2. Biochimie 82, 167-174. 

Immediately after transcription, the 5 phosphate is removed, guanosyl transferase adds a G residue linked via a 5 -5 covalent bond, and this is methylated to form a 7-methylguanosine (m7G) cap (methylated in N-7 position of the base). The ribose residues of either the adjacent one or two nucleotides may also be methylated by methyl group addition to the 2 OH of the sugar. The cap protects the 5 end of the mRNA against ribonuclease degradation and also functions in the initiation of protein synthesis. 

Initiation of protein synthesis is catalyzed by proteins called initiation factors (IFs). In prokaryotes, three initiation factors (IF1, IF2 and IF3) are essential. Because of the complexity of the process, the exact order of binding of IF1, IF2, IF3, fMet-tRNAfMet and mRNA is still unclear. One current model is shown in Fig. 4 and is described below. 

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). 

---