It also helps to insert some proteins that are initially transported into the matrix by the TOM and TIM complexes. A third protein translocator in the inner mitochondrial membrane, the OXA complex, mediates the insertion of inner membrane proteins that are synthesized within the mitochondria. The TIM22 complex mediates the insertion of a subclass of inner membrane proteins, including the carrier protein that transports ADP, ATP, and phosphate. The TIM23 complex then transports some of these proteins into the matrix space, while helping to insert transmembrane proteins into the inner membrane. It initially transports their signal sequences into the intermembrane space and helps to insert transmembrane proteins into the outer membrane. The TOM complex is required for the import of all nucleus-encoded mitochondrial proteins. These complexes contain some components that act as receptors for mitochondrial precursor proteins and other components that form the translocation channel. TOM and TIM stand for translocase of the outer and inner mitochondrial membranes, respectively. Protein translocation across mitochondrial membranes is mediated by multi- subunit protein complexes that function as protein translocators: the TOM complex functions across the outer membrane, and two TIM complexes, the TIM23 and TIM22 complexes, function across the inner membrane ( Figure 12-24). Cytochrome oxidase is a large multiprotein complex located in the inner mitochondrial membrane, where it functions as the terminal enzyme in the electron-transport chain (discussed in Chapter 14). This configuration-rather than a precise amino acid sequence-is recognized by specific receptor proteins that initiate protein translocation.Ī signal sequence for mitochondrial protein import. Sequence comparisons and physical studies of different matrix signal sequences suggest that their common feature is the propensity to fold into an amphipathic α helix, in which positively charged residues are clustered on one side of the helix, while uncharged hydrophobic residues are clustered on the opposite side ( Figure 12-23). The signal sequences are both necessary and sufficient for import of the proteins that contain them: through the use of genetic engineering techniques, these signals can be linked to any cytosolic protein to direct the protein into the mitochondrial matrix. Most of the mitochondrial precursor proteins have a signal sequence at their N terminus that is rapidly removed after import by a protease (the signal peptidase) in the mitochondrial matrix. Thus, in contrast to the protein translocation into the ER described later, mitochondrial proteins are first fully synthesized as precursor proteins in the cytosol and then translocated into mitochondria by a posttranslational mechanism. Proteins imported into the matrix of mitochondria are usually taken up from the cytosol within seconds or minutes of their release from ribosomes. Translocation into the Mitochondrial Matrix Depends on a Signal Sequence and Protein Translocators
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