Fig. (1).

Schematic of the canonical translation pathway in eukaryotes with the ribosomal recycling and initiation phases shown in detail. This figure combines findings from both yeast and mammals and indicates potential differences. The terminating 80S ribosome is split into individual subunits with help of ABCE1/RLI1 and eIFs 1, 1A and 3. How eRFs 1 and 3 are recycled is not properly understood. The former eIFs either remain bound to the 40S subunit or dissociate prior to the initiation phase. In the former case, the Met-tRNAi ^Met•eIF2•GTP ternary complex (TC) and eIF5 join the existing 40S-eIF1-eIF1A-eIF3 post-recycling complex in a “stochastic” way (i) to form the 43S pre-initiation complex (PIC). In the latter case, the 43S PIC is formed in the “higher order” manner via simultaneous binding of all components of the Multifactor complex (eIFs 1, 3, 5 and the TC) and eIF1A. Upon binding, eIFs1 and 1A induce conformational change that opens the mRNA binding channel of the 40S ribosome for mRNA loading. As a part of this major rearrangement eIF1, if delivered to the ribosome in the MFC, must translocate from eIF3 to the P-site. mRNA is delivered to the 43S PIC in a complex with eIF4F (composed of eIF4A, E and G), eIF4B (and/or eIF4H in mammals) and PABP in an ATP-dependent reaction creating a “landing pad” close to the mRNA’s cap structure that is bound by eIF4E (the interaction between eIF4G and PABP is shown as a dotted line for simplicity). As a result, the 48S PIC is formed and scanning for AUG commences. The actual attachment of mRNA to the ribosome is believed to be mediated via the eIF4G – eIF3 interaction in mammals (dotted line “M”) that seems to be bridged via eIF5 in yeast (dotted line “Y”; this line is not shown in all cartoons for simplicity). During scanning, all secondary structures that could impede the movement of the PIC in the 5' to 3' direction are melted with help of helicase eIF4A and its co-factors eIF4B or eIF4H at the expense of ATP. Also, eIF5 stimulates GTP hydrolysis on eIF2 (GAP activity), however, the resulting Pi is not released until the AUG is located. Upon AUG recognition, eIF1 as a gatekeeper is either ejected from the ribosome or could move back to eIF3 to allow Pi release triggering reciprocal conformational switch to the closed form of the PIC that arrests scanning. eIF5B then promotes subunit joining that kicks out all interface-side-bound eIFs with the exception of eIF1A, and the solvent-side-bound eIF3 and eIF4F (interactions between eIF3 and two “solvent-side” ribosomal proteins RPS0 and RACK1/ASC1, based on [41,59,117], are indicated). GTP hydrolysis on eIF5B stimulated by the GTPase activating center (GAC) of the large subunit triggers coupled release of eIF5B and eIF1A rendering the resulting 80 initiation complex ready to elongate. It is believed that eIF3 and eIF4F can stay 80S-bound for at least a few elongation cycles thanks to their location on the back of the 40S subunit. eIF2•GDP is released in a binary complex with eIF5 that competes with and thus partially inhibits the action of the GEF eIF2B to exchange GDP for GTP on eIF2 (GDI activity). Upon this exchange, eIF2•GTP is ready to form a new TC that can enter the entire cycle all over again. See text for more details. Two “Translational control (TC) points” briefly mentioned in the main text are indicated by yellow arrows and the mechanism of their action by yellow cross lines; the first targets the eIF4E–eIF4G interaction and the other the GTP/GDP exchange on eIF2 by phosphorylating its α subunit.