A Multifactor Complex of eIF1, eIF2, eIF3, eIF5, and tRNAiMet Promotes Initiation Complex Assembly and Couples GTP Hydrolysis to AUG Recognition

Translation initiation in mammalian cells requires theformation of an 80S initiation complex consisting of the80S ribosome with methionyl initiator tRNA (MettRNAiMet) bound to the P site and base-paired with theAUG start codon in mRNA. Formation of this complex iscatalyzed by ~30 polypeptides comprising 11 differenteukaryotic initiation factors (eIFs). According to currentmodels, the first step in the initiation pathway involvesdissociation of 80S ribosomes into free 40S and 60S subunits, and is stimulated by eIF1A and the multisubuniteIF3. The eIF3 remains bound to the 40S subunit and participates in subsequent reactions. The second step involves the transfer of Met-tRNAiMet to the 40S ribosomein a ternary complex (TC) with the heterotrimeric factoreIF2 and GTP, producing the 43S complex. This step isstimulated by eIFs 1, 1A, and 3 (Fig. 1). Binding ofmRNA to the 43S complex is catalyzed by eIF4F, containing the m7G cap-binding protein (eIF4E), RNA helicase eIF4A, and the scaffolding subunit eIF4G. TheeIF4F binds at the cap, and the helicase activity of eIF4Aremoves secondary structure from the mRNA leader, dependent on the accessory factors eIF4B and eIF4H. Interaction of the 43S complex with the unstructured mRNAleader, producing the 48S complex, is facilitated by physical interaction between eIF4G and eIF3. It is also stimulated by the mRNA poly(A) tail and poly(A)-binding protein (PABP), which can additionally interact with eIF4G.The 48S complex scans the mRNA for an AUG triplet ina reaction facilitated by eIFs 1 and 1A. On base-pairingbetween AUG and the anticodon of Met-tRNAiMet, theGTP in the TC is hydrolyzed and most (if not all) eIFs arereleased, yielding the 40S initiation complex. The hydrolysis of GTP is dependent on eIF5, functioning as aGTPase activating protein (GAP). In the final step, the60S subunit joins the 40S complex in a reaction stimulated by eIF5B and requiring hydrolysis of a secondmolecule of GTP, yielding the 80S initiation complex(Fig. 1) (for review, see Hershey and Merrick 2000)...