Structural basis for the transition from translation initiation to elongation by an 80S-eIF5B complex

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• Ribosome function is critical for protein synthesis, with start codon recognition by initiator tRNA setting the mRNA reading frame.
• Eukaryotic translation initiation factor 5B (eIF5B) plays a key role in positioning the initiator methionine tRNA (Met-tRNAiMet) on the ribosome, preceding the elongation phase.
• Recent single-molecule fluorescence studies revealed a prolonged residence time of eIF5B on the ribosome.

Purpose of the Study:

• To determine the cryo-electron microscopy (cryo-EM) structure of the ribosome complexed with eIF5B and Met-tRNAiMet.
• To investigate the structural basis of eIF5B's function in translation initiation and its interaction with the ribosome.
• To elucidate the mechanism of the fidelity checkpoint preceding the transition to elongation.

Main Methods:

• Cryo-electron microscopy (cryo-EM) to visualize the ribosome complex.
• Single-molecule fluorescence measurements to characterize eIF5B residence time.
• Structural analysis of the ribosome-eIF5B-Met-tRNAiMet complex.

Main Results:

• The cryo-EM structure reveals the ribosome complex with eIF5B and Met-tRNAiMet at the pre-elongation stage.
• The structure uncovers a eukaryotic-specific, dynamic fidelity checkpoint involving eIF5B and large ribosomal subunit components.
• The long residence time of eIF5B facilitates this checkpoint mechanism.

Conclusions:

• eIF5B is essential for establishing the correct reading frame by ensuring accurate initiator tRNA placement.
• A novel, dynamic fidelity checkpoint mediated by eIF5B operates in eukaryotes before translation elongation.
• This checkpoint mechanism ensures the fidelity of protein synthesis initiation.