Once the signal sequence has passed through the TOM complex and is bound to a TIM complex, further translocation through the TIM translocation channel requires the membrane potential, which is the electrical component of the electrochemical H+ gradient across the inner membrane (see Figure 11–4). Pumping of H+ from the matrix space to the intermembrane space, driven by electron transport processes in the inner membrane (discussed in Chapter 14), maintains the electrochemical gradient. The energy in the electrochemical H+ gradient across the inner membrane therefore not only powers most of the cell’s ATP synthesis, but it also drives the translocation of the positively charged signal sequences through the TIM complexes by electrophoresis.
Mitochondrial hsp70 also plays a crucial part in the import process. Mitochondria containing mutant forms of the protein fail to import precursor proteins. The mitochondrial hsp70 is part of a multisubunit protein assembly that is bound to the matrix side of the TIM23 complex and acts as a motor to pull the precursor protein into the matrix space. Like its cytosolic cousin, mitochondrial hsp70 has a high affinity for unfolded polypeptide chains, and it binds tightly to an imported protein chain as soon as the chain emerges from the TIM translocator in the matrix space. The hsp70 then undergoes a conformational change and releases the protein chain in an ATP-dependent step, exerting a ratcheting/pulling force on the protein being imported. This energy-driven cycle of binding and subsequent release provides the final driving force needed to complete protein import after a protein has initially inserted into the TIM23 complex (see Figure 12–23).
After the initial interaction with mitochondrial hsp70, many imported matrix proteins are passed on to another chaperone protein, mitochondrial hsp60. As discussed in Chapter 6, hsp60 helps the unfolded polypeptide chain to fold by binding and releasing it through cycles of ATP hydrolysis.
Bacteria and mitochondria Use similar mechanisms to insert porins into their outer membrane
The outer mitochondrial membrane, like the outer membrane of Gram-negative bacteria (see Figure 11–17), contains abundant pore-forming β-barrel proteins called porins, and it is thus freely permeable to inorganic ions and metabolites (but not to most proteins). In contrast to other outer membrane proteins, which are anchored in the membrane through transmembrane α-helical regions, the TOM complex cannot integrate porins into the lipid bilayer. Instead, porins are first transported unfolded into the intermembrane space, where they transiently bind specialized chaperone proteins, which keep the porins from aggregating (Figure 12–24A). They then bind to the SAM complex in the outer membrane, which both inserts them into the outer membrane and helps them fold properly.
One of the central subunits of the SAM complex is homologous to a bacterial outer membrane protein that helps insert β-barrel proteins into the bacterial outer
Figure 12–24 Integration of porins into the outer mitochondrial and bacterial membranes. (a) after translocation through the Tom complex in the outer mitochondrial membrane, β-barrel proteins bind to chaperones in the intermembrane space. The sam complex then inserts the unfolded polypeptide chain into the outer membrane and helps the chain fold.
(B) a structurally related Bam complex in the outer membrane of Gram-negative bacteria catalyzes β-barrel protein insertion and folding (see figure 11–17).
membrane from the periplasmic space (the equivalent of the intermembrane space in mitochondria) (Figure 12–24B). This conserved pathway for inserting β-barrel proteins further underscores the endosymbiotic origin of mitochondria.
Transport into the inner mitochondrial membrane and intermembrane space occurs Via several routes