Translation of the mitochondrial genetic material to bioenergetic proteins is performed by distinct mitoribosomes with specific factors. How those elements come together to initiate translation is not known. In the study led by Joanna Rorbach and SciLifeLab Fellow Alexey Amunts, mitoribosomal complexes involved in a non-canonical translation pre-initiation in human mitochondria were investigated using the structural biology approach.

 

To answer the question which specific states lead to the initiation, the postdoctoral researcher Anas Khawaja developed a system of pulling down intact complexes directly from mitochondria. Then, by applying cryo-EM, Yuzuru Itoh, the Marie Curie fellow from Amunts’ lab, discovered two previously unknown states, involving the small mitoribosomal subunit with initiation factors mtIF2 and mtIF3. Next, the researchers applied computational single component analysis to the head movement in the cryo-EM reconstructions, which revealed interdependence between mtIF3 and mitochondria-specific elements prevent that premature association of the messenger and transfer RNAs. To confirm that the structures represent a physiological state, Rorbach’s lab performed functional experiments using fluorescence cross-correlation spectroscopy and single-molecule imaging, which illustrated that mechanistic displacement of mtIF3 is essential in order to assemble the complete initiation complex.

 

Based on these data, it can be concluded that the newly identified complexes represent previously unknown states in translation pre-initiation. Since no equivalent states have been reported in any other translation system, it appears to be a distinct feature of human mitochondria.

 


Schematic of translation initiation steps in human mitochondria (top) and bacteria (bottom). In the mitochondrial pathway mtIF3 and mS37 stabilize the small subunit head for accommodation of mtIF2. Then joining of the large subunit results in the conformational change of mtIF2 and GTPase activation that lead to the messenger RNA accommodation.

 

Further references:

Paper in Nature Communication https://rdcu.be/b4MqX