MitoRUSH as a tool to study the efficiency of mitochondrial import in complex I-deficient cells

Most mitochondrial proteins are imported through the actions of the presequence translocase of the inner membrane, the TIM23 complex, which requires energy in the form of the electrochemical potential of the inner membrane and ATP. Conversions of energy in mitochondria are disturbed in mitochondrial disorders that affect oxidative phosphorylation. Despite the widely accepted dependence of protein import into mitochondria on mitochondrial bioenergetics, effects of mitochondrial disorders on biogenesis of the mitochondrial proteome are poorly characterized. Here, we describe molecular tools that can be used to explore mitochondrial protein import in intact cells, the mitoRUSH assay, and a novel method based on labeling of nascent proteins with an amino acid analog and click chemistry. Using these orthogonal approaches, we discovered that defects in the electron transport chain and manipulating the expression of TIMM23, as well as the TIMM17A or TIMM17B paralogs, in human cells are associated with a decrease in protein import into mitochondria. We postulate that in the absence of a functional electron transfer chain, the mechanisms that support electrochemical potential of the inner membrane and ATP production are insufficient to sustain the import of proteins to mitochondria.