4 Binding of Initiator Methionyl-tRNA to Ribosomes

Abstract

Among the important events in the expression of a gene is the selection of the correct codon on the messenger RNA to initiate the synthesis of the encoded protein. This is achieved in the multistep pathway of translation initiation. In these reactions, a ribosome binds the mRNA and the first amino acid to be incorporated into the future polypeptide chain. All organisms use the amino acid methionine to initiate translation and a specific translation initiation factor to carry it in the form of methionyl-tRNA to the ribosome. In eukaryotes, the main steps of the translation initiation pathway include binding of initiator methionyl-tRNA (Met-tRNA_i) to the small (40S) ribosomal subunit, selection of an AUG codon by the (40S·Met-tRNA_i) complex on mRNA, and joining of a large (60S) ribosomal subunit with the 40S initiation complex to form an 80S ribosome competent for translation elongation (for details, see Merrick and Hershey, this volume). This chapter describes the mechanism and regulation of Met-tRNA_i binding to ribosomes.

COMPONENTS INVOLVED IN MET-tRNA BINDING
Most of the proteins that catalyze specific steps of translation initiation in eukaryotes were originally purified from the rabbit reticulocyte ribosomal salt wash fraction, and their functions were determined by reconstitution of the initiation pathway in vitro. From these early studies, it could be shown that some of the reaction intermediates of the pathway identified in vitro, such as the (40S·Met-tRNA_i) complex, also existed in extracts derived from rabbit reticulocytes and wheat germ. They were therefore considered to be physiologically relevant (Safer and...