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Abstract
The vast majority of mitochondrial proteins are encoded in the nuclear genome and synthesized on cytosolic ribosomes as precursor proteins with specific mitochondrial targeting signals. Mitochondrial targeting signals are very diverse, however, about 70% of mitochondrial proteins carry cleavable, N-terminal extensions called presequences. These amphipathic helices with one positively charged and one hydrophobic surface target proteins to the mitochondrial matrix with the help of the TOM and TIM23 complexes in the outer and inner membranes, respectively. Translocation of proteins across the two mitochondrial membranes does not take place independently of each other. Rather, in the intermembrane space, where the two complexes meet, components of the TOM and TIM23 complexes form an intricate network of protein–protein interactions that mediates initially transfer of presequences and then of the entire precursor proteins from the outer to the inner mitochondrial membrane. In this Mini Review, we summarize our current understanding of how the TOM and TIM23 complexes cooperate with each other and highlight some of the future challenges and unresolved questions in the field.
Highlights
Eukaryotic cells are defined by the presence of different membrane-enclosed compartments, cell organelles, that contain specific sets of proteins and provide specific chemical milieus
Translocation of presequence-containing precursor proteins across the two mitochondrial membranes is mediated by the TOM and TIM23 complexes in the outer and inner membranes, respectively (Figure 1)
Experiments performed in intact mitochondria and with isolated recombinant proteins showed that the TIM23 complex interacts with the trans site of the TOM complex even in the absence of protein translocation (Figure 2A)
Summary
Eukaryotic cells are defined by the presence of different membrane-enclosed compartments, cell organelles, that contain specific sets of proteins and provide specific chemical milieus. Translocation of presequence-containing precursor proteins across the two mitochondrial membranes is mediated by the TOM and TIM23 complexes in the outer and inner membranes, respectively (Figure 1). Though initial data suggested that Tom70 is involved in recognition of internal targeting signals within mitochondrial proteins, recent work shows that its predominant function is in tethering cytosolic chaperones to the surface of mitochondria (Backes et al, 2021), suggesting a more general role for Tom70 in protein translocation into mitochondria.
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