Structural basis of mitochondrial protein import by the TIM23 complex

Area of Science:

• Mitochondrial biology
• Molecular cell biology
• Protein import mechanisms

Background:

• Mitochondria import thousands of proteins from the cytosol.
• The TIM23 complex is essential for importing proteins into the mitochondrial matrix and inner membrane.
• The precise mechanism of protein translocation by TIM23 subunits, particularly Tim23 and Tim17, remained structurally undefined.

Purpose of the Study:

• To elucidate the structural basis of protein translocation mediated by the TIM23 complex.
• To resolve the roles of Tim23 and Tim17 in forming the protein-conducting channel.
• To propose a revised model for mitochondrial protein import.

Main Methods:

• Cryo-electron microscopy (cryo-EM) to determine the structure of the core TIM23 complex.
• Biochemical analyses to investigate protein translocation pathways.

Main Results:

• The cryo-EM structure of the heterotrimeric Tim17-Tim23-Tim44 complex was determined.
• Contrary to prior models, Tim23 and Tim17 do not form a single channel; they possess separate, opposing cavities.
• The cavity within Tim17, not Tim23, functions as the protein translocation path, with Tim23 likely playing a structural role.
• The subunit Mgr2 was observed to seal the lateral opening of the Tim17 cavity during polypeptide translocation.

Conclusions:

• The prevailing model of Tim23 and Tim17 forming a direct protein channel is incorrect.
• Tim17 forms the functional translocation pore, while Tim23 likely serves a structural or regulatory function.
• A new model for TIM23-mediated protein import is proposed, involving distinct roles for subunits and potential sealing by Mgr2, advancing our understanding of mitochondrial biogenesis.