Figure 7.

Recognition of the Start Codon in the PIC

Relative to its conformation in the 80S ribosome (I) (Ben-Shem et al., 2011), the head of the 40S in the PIC-2 complex (II) is rotated by 8 degrees. The disordered NTT of eIF1A is shown as a dashed line. The rotation of the head allows TC to bind to the P site in a P_OUT conformation as it scans the 5′ UTR for the start codon (III). For clarity, only the eIF2α subunit is shown. Recognition of the start codon is stabilized by the tRNA_i P_IN state (IV), which involves a movement of the ASL deeper into the P site. The rRNA G1575/A1576 bases (depicted in red) makes interactions with the minor groove of the conserved G:C pairs in the ASL, the NTT of eIF1A directly engaged with codon:anticodon duplex, and eIF2α-D1 interacting with the −3 position of mRNA in the E site. The head undergoes a further rotation compared to PIC-2 complex.