The same mechanism that transports proteins into the matrix space using the TOM and TIM23 translocators (see Figure 12–22) also mediates the initial translocation of many proteins that are destined for the inner mitochondrial membrane or the intermembrane space. In the most common translocation route, only the N-terminal signal sequence of the transported protein actually enters the matrix space (Figure 12–25A). A hydrophobic amino acid sequence, strategically placed after the N-terminal signal sequence, acts as a stop-transfer sequence, preventing protease intermembrane reduced protein to TRANSLOCATION ACROSScleavage space protein be imported SH SH SH SHSHSH S S S SH S S S S S SH RECOGNITION AND OXIDATION BY Mia40 REOXIDATION BY RESPIRATORY CHAIN OUTER MEMBRANE REDUCTION OF Mia40, OXIDATION OF IMPORTED PROTEINMia40 TOM complex
Figure 12–25 Protein import from the cytosol into the inner mitochondrial membrane and intermembrane space. (a) The n-terminal signal sequence (red) initiates import into the matrix space (see figure 12–22). a hydrophobic sequence (blue) that follows the matrix-targeting signal sequence binds to the Tim23 translocator (orange) in the inner membrane and stops translocation. The remainder of the protein is then pulled into the intermembrane space through the Tom translocator in the outer membrane, and the hydrophobic sequence is released into the inner membrane anchoring the protein there. (B) a second route for protein integration into the inner membrane first delivers the protein completely into the matrix space. Cleavage of the signal sequence (red) used for the initial translocation unmasks an adjacent hydrophobic signal sequence (blue) at the new n-terminus. This signal then directs the protein into the inner membrane, using the same oXa-dependent pathway that inserts proteins that are encoded by the mitochondrial genome and translated in the matrix space. (C) some soluble proteins of the intermembrane space also use the pathways shown in (a) and (B) before they are released into the intermembrane space by a second signal peptidase, which has its active site in the intermembrane space and removes the hydrophobic signal sequence. (D) some soluble intermembrane-space proteins become oxidized by the mia40 protein (mia = mitochondrial intermembrane space assembly) during import. mia40 forms a covalent intermediate through an intermolecular disulfide bond, which helps pull the transported protein through the Tom complex. mia40 becomes reduced in the process, and then is reoxidized by the electron transport chain, so that it can catalyze the next round of import. (e) multipass inner membrane proteins that function as metabolite transporters contain internal signal sequences and snake through the Tom complex as a loop. They then bind to the chaperones in the intermembrane space, which guide the proteins to the Tim22 complex. The Tim22 complex is specialized for the insertion of multipass inner membrane proteins.
further translocation across the inner membrane. The remainder of the protein then crosses the outer membrane through the TOM complex into the intermembrane space; the signal sequence is cleaved off in the matrix, and the hydrophobic sequence, released from TIM23, remains anchored in the inner membrane.