Protein elongation factor

The principal drawbacks to the technique are its labor-intensive nature and low protein yields, which in the best cases reach only a milligram or less. Competition with release factors at the amber stop codon often results in truncated protein as the primary product [16, 17]. Suppression efficiency rates are also affected by the character of the amino acid, which determines whether it is a good substrate for the ribosome and protein elongation factors. In addition, context effects variously ascribed to the influence of neighboring mRNA... 

As with initiation, elongation is a multistep process. In the first step, another GTP-binding protein, elongation factor Tu (EF-Tu), delivers the next aminoacyl tRNA to the A site on the ribosome. As this occurs, EF-Tu hydrolyzes GTP and leaves the ribosome (Fig. 23-5). 

A novel derivative of histidine present only in the eukaryotic protein elongation factor 2 (EF-2), which participates in the elongation step of protein biosynthesis. Diphtheria toxin inhibits eukaryotic protein synthesis by catalyzing a covalent modification of diphthamide (see Chapter 25). 

A charged tRNA is escorted to the A site in a complex with the protein elongation factor EF-Tu, which also carries a molecule of GTP. 

Translocation in eukaryotes involves another G protein, elongation factor EF2 (EF-G in prokaryotes) that complexes with GTP and binds to the ribosome, causing a conformational change that moves the mRNA and its base-paired tRNAs with respect to the ribosome. The uncharged tRNA moves from the P site and is released from the ribosome. The peptidyl-tRNA moves into the P site, and the next codon of the mRNA occupies the A site. During translocation, GTP is hydrolyzed to GDP, which is released from the ribosome along with the elongation factor (see Fig. 15.10). 

Chain elongation. Elongation includes the synthesis of all peptide bonds of a polypeptide chain. This is accomplished, with the assistance of a set of protein elongation factors, by a repetitive cycle of events in which successive aa-tRNA adds to the A site and the growing peptidyl-tRNA occupying the P site of the mRNA ribosome complex. 

It may be that we should be more concerned with why the bacterial machinery is relatively so simple. In rapidly growing bacterial cells a very substantial fraction of the total cellular protein is involved directly in protein biosynthesis (activating enzymes, ribosomal structural proteins, elongation factors etc). Evolutionary pressure to attain high growth rates may therefore have resulted in a streamlining of the bacterial protein synthesis mechanism to the bare minimum of components, in which case we should not look for some subtlety of regulation in every additional feature of complexity in the eukaryotic system. 

---