Polypeptide synthesis, ribosome cycle

Elongation factors Non-ribosomal protein factors that are necessary participants in the chain-elongation cycle of polypeptide synthesis they interact with the ribosome-mRNA complex or with other major cycle participants. 

Fio. 4. The bacterial ribosome cycle in polypeptide synthesis (courtesy of Dr. S. Odioa). 

Thus, at the end of the ribosome cycle the coding sequence of messenger RNA has been translated to produce a particular polypeptide chain, and all the components involved become available for re-use in another round of the cycle (Fig. 7h). Usually, several ribosomes become attached to one mRNA molecule, giving rise to polyribosomes (also called polysomes Fig. 9). In eukaryotic cells the efficiency of protein synthesis is stimulated by factor eIF4-G (Table III), which interacts with both factor eIF-4E and a poly A-binding protein. The resultant circularized polysomes show an enhanced ability to re-initiate after release of the ribosomal subunits from the messenger RNA at the end of a round of translation. 

Throughout the ribosome cycle, dynamic protein-mRNA interactions are functionally important in the initiation, elongation, and termination of polypeptide synthesis. In addition, more stable associations between proteins and mRNAs have been observed, particularly in eukaryotic cells. These messenger ribonucleoprotein complexes (mRNPs) occur both in polyribosomes and free in the cytosol, some of the latter being either temporarily or permanently unavailable for translation. Thus, protein-mRNA interactions contribute to the efficiency with which mRNAs are translated. 

The cycle of peptide-chain elongation continues until one of the three stop codons (UAA, UAG, UGA) is reached. There is no aminoacyl-tRNA complementary to these codons, and instead a termination factor or a release factor (RF) with bound GTP binds to the ribosome and induces hydrolysis of both the aminoacyl-linkage and GTP, thereby releasing the completed polypeptide chain from the ribosome. The 475 amino acid-long sequence of rabbit liver RF has been deduced from its cDNA sequence, and it shows 90% homology with mammalian trypto-phanyl-tRNA synthetase (Lee et al., 1990). It has also been reported that for efficient and accurate termination, an additional fourth nucleotide (most commonly an A or a G) after the stop codon is required (Tate and Brown, 1992). The exact role of the fourth nucleotide in the termination of protein synthesis is not fully understood at present. 

Three key steps in the elongation stage of protein synthesis are required for the addition of each amino acid. Because these steps are repeated for each peptide bond formed, this is sometimes called the elongation cycle. The central theme in elongation is that the fully assembled ribosomal complex functions as a ribonucleoprotein machine which rapidly moves 50 to 30 down the mRNA, much like a ratchet. At the center of this complex are two binding sites which line up over a pair of triplet codons, as shown in Figure 26.11. These two sites are called the P site, for peptidyl (or polypeptide), and the A site, for aminoacyl (or acceptor). A third site, called the E site for tRNA exit site, is also a functional component of the ribosome, but for reasons of clarity, it is not included in the figures. 

A peptide bond is formed between the new amino acid and the growing polypeptide chain. (4) The amino acid is cleaved from the tRNA, which can be cycled back to form another complex with an amino acid for a later synthesis. (5) The growing polypeptide forms a fiber-like tendril. (6) The ribosome essentially moves along the mRNA, reopening the initiation site for additional protein synthesis. In this way, proteins are synthesized by several ribosomes acting on the same mRNA molecule. 

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. 

The mechanism whereby RNA is translated into protein is complex, and the cell devotes considerable resources to the translational machinery. The components include 20 different amino acids, transfer RNAs, aminoacyl-tRNA synthetases, ribosomes, and a number of protein factors which cycle on and off the ribosomes and facilitate various steps in initiation of translation, elongation of the nascent polypeptide chain, and termination of synthesis with release of the completed polypeptide from the ribosome. The process depends on a supply of energy provided by ATP and GTP. The rate of protein synthesis is typically in the range of 6 (immature red blood cells of the rabbit) to 20 Escherichia coli growing optimally) peptide bonds per sec. at 37°C. 

After translocation the ribosomal P site is occupied by dipeptidyl-tRNA and the vacant A site contains the third mRNA codon. Entry of the next aminoacyl-tRNA, selected as before by the codon-anticodon interaction, into the A site (Fig. If) enables peptide bond synthesis to continue and repeated operation of the elongation-translocation cycle gives rise to a stepwise elongation of the nascent polypeptide chain, each complete cycle elongating the chain by one amino acid residue and moving the mRNA by one codon in the 5 to 3 direction. When the end of the coding sequence is reached and one of the termination (or stop) codons has entered the A site, translation stops and the completed polypeptide chain is released. 

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