Toward the mechanism of eIF4F-mediated ribosomal attachment to mammalian capped mRNAs

Area of Science:

• Molecular Biology
• Biochemistry
• Genetics

Background:

• Ribosomal binding to mammalian capped mRNAs is mediated by a complex interaction chain involving eukaryotic initiation factor 4E (eIF4E), eIF4G, eIF3, and the 40S ribosomal subunit.
• The precise mechanism by which mRNA enters the 40S subunit's mRNA-binding channel during translation initiation remains poorly understood.

Purpose of the Study:

• To elucidate the mechanism of mRNA entry into the 40S ribosomal subunit during translation initiation.
• To investigate the role of eukaryotic initiation factor 4E (eIF4E) in mRNA binding and initiation complex formation.

Main Methods:

• Reconstituted translation system used to recapitulate initiation on capped mRNAs in vitro.
• Investigated initiation complex formation at 5'-terminal AUG codons.
• Assessed the impact of omitting eIF4A or disrupting eIF4E-eIF4G-eIF3 interactions on initiation.

Main Results:

• Initiation complex formation followed the 'first AUG' rule and occurred even at the mRNA's 5' end, indicating inspection from the first nucleotide.
• eIF4E dissociated from the cap in assembled initiation complexes.
• eIF4A omission or disruption of eIF4E-eIF4G-eIF3 interactions specifically inhibited initiation on capped mRNAs.

Conclusions:

• A model is proposed where eIF4E-eIF4G-eIF3-40S interactions position eIF4E at the leading edge of the 40S subunit, facilitating mRNA threading.
• eIF4E likely dissociates from the cap before mRNA entry to overcome steric hindrance, allowing Met-tRNAi (Met) to inspect mRNA from the start.
• The m(7)G cap specifically interacts with eIF3l.