9 Mechanism of Translation Initiation in the Yeast Saccharomyces cerevisiae

Abstract

Our understanding of the translation initiation pathway is based largely on biochemical analysis of purified mammalian factors, as described in Chapter 4. Genetic and biochemical analysis of the yeast Saccharomyces cerevisiae is helping to elucidate molecular mechanisms in the pathway, and genetic studies in yeast provide the best opportunity to establish the physiological relevance of conclusions reached using cell-free systems, thus providing the impetus for this review. According to the current model (Fig. 1), the pathway begins with binding to the free 40S subunit of a ternary complex (TC) comprising eIF2, GTP, and Met-tRNA_i^Met, in a reaction promoted by eIF1, eIF1A, and eIF3. The 43S preinitiation complex (PIC) thus formed binds to the mRNA, with the assistance of eIF4F, -4A, -4B, and poly(A)-binding protein (PABP), and scans the mRNA for the start codon. Base-pairing of Met-tRNA_i^Met with AUG stimulates hydrolysis of the GTP bound to eIF2, dependent on the GTPase-activating protein (GAP) eIF5 and 40S subunit, the eIF2-GDP and other factors are released, and eIF5B-GTP stimulates 60S subunit joining. The eIF2-GDP must be recycled to eIF2-GTP by the guanine nucleotide exchange factor (GEF) eIF2B for a new round of initiation, and this reaction is inhibited under stress conditions by phosphorylation of the α subunit of eIF2 on Ser-51.

Many advances in our knowledge of the mechanism and regulation of Met-tRNA_i^Met recruitment have come from genetic analysis of translational control of yeast GCN4 mRNA by the eIF2α kinase GCN2 (general control non-derepressible 2). Activation of GCN2 by amino acid starvation produces...