Structure and Function of the Eukaryotic Ribosome and Elongation Factors

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Translational Control in Biology and Medicine

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The translation of the genetic message is performed by the ribosome, a large macromolecular assembly made of ribosomal RNA (rRNA) and proteins (rps). In eukaryotes, it is composed of a small 40S subunit (SSU) and a large 60S subunit (LSU), which perform decoding and peptidyl transfer, respectively. In bacteria, the ribosome has a molecular mass of 2.5 MD and is composed of three rRNA molecules and 51 proteins (Chapter 2). Due to additional rps and rRNA expansion segments, the complete eukaryotic 80S ribosome is significantly larger (> 3 MD) than its bacterial counterpart. Functionally, however, the ribosome is relatively conserved throughout all domains of life. The initiation of translation places an mRNA, with a Met-tRNAiMet, at the start codon in the 80S ribosome (Chapter 4). The subsequent elongation phase of protein synthesis is a cycle of aminoacyl-tRNA (aa-tRNA) delivery and peptide bond formation repeated hundreds of times during the synthesis of an average protein. This process is facilitated by the soluble eukaryotic elongation factors (eEFs), which have an important role in ensuring the accuracy of gene expression. The eEFs differ in many ways from their bacterial homologs, and they are posttranslationally modified and regulated (Chapter 21). The higher-order cellular structure of eukaryotes also necessitates highly specific interactions of the ribosome with membranes and its active role in protein transport. In the past decade, significant advances in the structural analysis of the eukaryotic ribosomes, eEFs, and their complexes have allowed new insights into the dynamic processes of translation elongation.