Large-scale movement of eIF3 domains during translation initiation modulate start codon selection

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• The eukaryotic initiation factor 3 (eIF3) complex plays a crucial role in translation initiation.
• Its precise molecular functions and structural dynamics within the preinitiation complex (PIC) remain incompletely understood.

Purpose of the Study:

• To elucidate the structural organization of the yeast 48S ribosomal preinitiation complex (PIC) during the scanning phase of translation initiation.
• To investigate the dynamic behavior and functional significance of the eIF3 subunits, particularly eIF3b and eIF3i, within the PIC.

Main Methods:

• Single particle electron cryomicroscopy (cryo-EM) was employed to reconstruct the yeast 48S PIC in an open conformation.
• Reprocessing of existing cryo-EM data and genetic analysis were utilized to understand subunit relocation and functional dependencies.

Main Results:

• A cryo-EM reconstruction revealed the yeast 48S PIC in an open conformation, with eIF3b positioned on the 40S interface near the decoding center, interacting with the ternary complex.
• eIF3b and eIF3i were observed to relocate from solvent interface positions, with eIF3i showing no direct 40S contacts in this state.
• Analysis suggests the entire eIF3b-3i-3g-3a-Cter module relocates during initiation, and eIF3b's 40S interactions are critical for high-fidelity start codon selection.

Conclusions:

• The dynamic relocation of eIF3 subunits, particularly eIF3b, is integral to the translation initiation process.
• eIF3b's interactions with the 40S subunit interface are essential for regulating PIC conformation and ensuring accurate start codon selection.
• Understanding these structural dynamics provides insights into the regulation of protein synthesis and fidelity.