Translational activators align mRNAs at the small mitoribosomal subunit for translation initiation

Area of Science:

• Mitochondrial biology
• Molecular genetics
• Structural biology

Background:

• Mitochondrial gene expression is crucial for cellular energy production via oxidative phosphorylation.
• Mitochondrial ribosomes (mitoribosomes) translate mitochondrial mRNAs, but their regulation is poorly understood.
• Nuclear-encoded mitochondrial translational activators (TAs) are known to enhance specific mRNA translation in yeast.

Purpose of the Study:

• To elucidate the mechanism by which pentatricopeptide repeat (PPR) domain-containing TAs regulate mitoribosome binding and translation initiation.
• To investigate the structural basis of TA-mediated transcript-specific translation.

Main Methods:

• Selective mitoribosome profiling to map ribosome positions on mRNAs.
• Cryo-electron microscopy (cryo-EM) to determine the structures of TA-mitoribosome-mRNA complexes.
• Biochemical analyses of TA-mRNA and TA-mitoribosome interactions.

Main Results:

• TAs demonstrate high selectivity for mitoribosomes engaged with their target mRNAs.
• TAs facilitate the recruitment of mitoribosomes to positions near the start codon.
• Cryo-EM structures reveal TAs binding to both the mRNA 5' UTR and the mitoribosome, positioning the mRNA for initiation.

Conclusions:

• TAs act as crucial adaptors, bridging specific mRNAs and mitoribosomes to ensure accurate translation initiation.
• This mechanism allows mitochondria to achieve transcript-specific translation without relying on general sequence elements for mitoribosome positioning.
• The findings provide a mechanistic framework for understanding mitochondrial translational control.