Mitochondria
outer membrane
Matrix
inner membrane
intermembrane space
Mitochondria are double membrane enclosed organelles that specialize in the synthesis of ATP, using energy derived from electron transport and oxidative phosphorylation. They contain an internal matrix space and intermembrane space.
Proteins imported into the matrix of mitochondria are typically taken up from the cytosol within seconds to minutes of their release from ribosomes. Most of these proteins bound for mitochondia have a signal sequence at their N terminus which has the common feature of an amphipathic alpha helix in which positive residues are clustered on one side and uncharged hydrophobic residues are clustered on the other side.
Protein translocation across the membranes is mediated by multisubunit protein complexes. TOM complex functions as a tranlocase on the outer membrane and 2 TIM complexes as well as an OXA complex which function on the inner membrane. These complexes contain components that act as receptors as well as for the translocation channel.
Protein import requires ATP hydrolysis as well as an electrochemical H+ gradient across the inner membrane.
Mitochondrial precursor proteins remain unfolded in the cytosol through interactions with chaperone proteins of the hsp70 family. This prevents folding of the proteins before they engage with the TOM complex. Mitochondrial hsp70 also binds tightly to an imported protein as soon as it emerges in the matrix and is crucial for the import of the mitochondrial proteins. 2 models have been proposed to explain how this works. In the thermal ratchet model, the emerging chain slides back and forth in the TIM23 translocation channel and each time a sufficiently long portion of the chain is exposed, an hsp70 molecule binds to it thereby translocating it into the matrix.
After
the precursor protein is imported into the mitchondrial matrix, its signal
sequence is removed by a signal peptidase in the mitochondrial matrix.
For proteins that are to be integrated into the inner mitochondrial membrane
other pathways occur. In one pathway, the signal sequence of the imported
protein is cleaved which unmasks an adjacent hydrophobic signal sequence at the
new N terminus. This signal then directs the protein into the inner membrane
probably by an OXA dependent pathway. In some cases, this hydrophobic
sequence, however, can bind to the TIM23 translocator in the inner membrane
which stops translocation. The remainder of the protein is then pulled into the
inermemebrane space through the TOM translocator in the otter membrane.