Protein import across and insertion into mitochondrial membranes

Mitochondria are present in practically all eukaryotic cells, where they generate energy in the form of ATP among a number of other functions. Reflecting their evolution from Gram-negative bacteria, they possess two membranes surrounding them, complicating any import/export processes that occur. Furthermore, although mitochondria contain the machinery for protein translation, 99% of the 1500-2000 mitochondrial proteins are nuclear encoded and must be imported from the cytosol. An increasing number of high-resolution structures of the machinery required for this import across (and into) both the outer and inner mitochondrial membranes (OMM and IMM) have been determined. Using molecular dynamics simulations, we have investigated the conformational dynamics of key proteins involved in import, including in particular the SAM and TIM23 complexes. SAM is required for inserting β-barrels into the OMM; our work has revealed notable commonalities between the core component, Sam50, and the bacterial homolog BamA, such as the presence of a dynamic lateral gate facing the membrane core. Additionally, we have investigated protein import across the IMM by simulating the first structures of the TIM23 complex, finding that the membrane thins significantly at the pore of Tim17, which forms the protein translocation pathway. Taken together, these results highlight a critical interplay between the membrane and the embedded machinery that enables protein import and membrane insertion.