Figure 12–21 The protein translocators in the mitochondrial membranes. The Tom, Tim, sam, and oXa complexes are multimeric membrane protein assemblies that catalyze protein translocation across mitochondrial membranes. The protein components of the Tim22 and Tim23 complexes that line the import channel are structurally related, suggesting a common evolutionary origin of both Tim complexes. on the matrix side, the Tim23 complex is bound to a multimeric protein complex containing mitochondrial hsp70, which acts as an import aTpase, using aTp hydrolysis to pull proteins through the pore. in animal cells, subtle variations exist in the subunit composition of the translocator complexes to adapt the mitochondrial import machinery to the particular needs of specialized cell types. sam = sorting and assembly machinery; oXa = cytochrome oXidase activity; Tim = Translocator of the inner mitochondrial membrane;  Tom = Translocator of the outer membrane.
mitochondria. The n-terminal signal sequence of the mitochondrial precursor protein is recognized by receptors of the Tom complex. The protein is then translocated through the Tim23 complex so that it transiently spans both mitochondrial membranes (Movie 12.3). The signal sequence is cleaved off by a signal peptidase in the matrix space to form the mature protein. The free signal sequence is then rapidly degraded (not shown).
Although the TOM and TIM complexes usually work together to translocate precursor proteins across both membranes at the same time, they can work independently. In isolated outer membranes, for example, the TOM complex can translocate the signal sequence of precursor proteins across the membrane. Similarly, if the outer membrane is experimentally disrupted in isolated mitochondria, the exposed TIM23 complex can efficiently import precursor proteins into the matrix space.
aTp hydrolysis and a membrane potential Drive protein import into the matrix space
Directional transport requires energy, which in most biological systems is supplied by ATP hydrolysis. ATP hydrolysis fuels mitochondrial protein import at two discrete sites, one outside the mitochondria and one in the matrix space. In addition, protein import requires another energy source, which is the membrane potential across the inner mitochondrial membrane (Figure 12–23).
The first requirement for energy occurs at the initial stage of the translocation process, when the unfolded precursor protein, associated with chaperone proteins, interacts with the import receptors of the TOM complex. As discussed in Chapter 6, the binding and release of newly synthesized polypeptides from the chaperone proteins requires ATP hydrolysis.
Figure 12–23 The role of energy in protein import into the mitochondrial matrix space. (1) Bound cytosolic hsp70 chaperone is released from the precursor protein in a step that depends on aTp hydrolysis. after initial insertion of the signal sequence and of adjacent portions of the polypeptide chain into the Tom complex translocation channel, the signal sequence interacts with a Tim complex. (2) The signal sequence is then translocated into the matrix space in a process that requires the energy in the membrane potential across the inner membrane. (3) Mitochondrial hsp70, which is part of an import aTpase complex, binds to regions of the polypeptide chain as they become exposed in the matrix space, pulling the protein through the translocation channel, using the energy of aTp hydrolysis.
662 Chapter 12: intracellular Compartments and protein sorting