2 The Pathway and Mechanism of Initiation of Protein Synthesis

Elucidation of the detailed molecular mechanism of protein synthesis is essential for understanding translational controls. This chapter is concerned with the prokaryotic and eukaryotic pathways of initiation, where most translational controls are found (Chapter 1). In general terms, it focuses on (1) how the initiation factors catalyze the binding of the initiator tRNA and mRNA to the small ribosomal subunit; (2) how the initiation codon is recognized; and (3) how the large ribosomal subunit joins to form an initiation complex capable of elongation. Considerable progress has been made during the past 5 years in refining our knowledge of the pathway and in determining the three-dimensional structures of some of the macromolecular components of initiation. Emphasis is placed on the structures of the initiation factors and how the factors function to promote and regulate the pathway. The reader is directed to other chapters in this volume for descriptions of elongation (Chapter 3) and termination (Chapter 11). The process of translation initiation was elucidated during the late 1960s through the 1970s primarily by biochemical studies that utilized radiolabeled amino acids and fractionated lysates derived from bacterial or mammalian cells. The major macromolecular components were identified by purifying proteins and nucleic acids required to reconstitute translation in vitro. The ribosome itself was viewed essentially as a “black box” that appeared to provide a rigid surface onto which the other translational components bind. Surprisingly, genetic approaches contributed only modestly to the identification of the 200 or more macromolecular components that comprise the translational apparatus.