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Destined to matrix:
- - The precursor protein, with its matrix targeting sequence is synthesized in the cytoplasm
- - In the cytosol, it will bind to a chaperone, cytosolic hsp70, which will cause unfolding with the help of ATP
- - once unfolded, it will be presented to the outer membrane of the mitochondrion, will bind to a receptor on the outer membrane (near the site of contact with the inner membrane) and will translocate the protein through the TOM complex on the outer membrane with the help of a proton-motive force
- - Once past the TOM complex, it will continue translation through a complex on the inner membrane, called the TIM complex.
- - Once translocated (again, with the help of a proton motive force), it will once again bind to hsp70 to prevent folding.
- - It will only fold when reaching hsp60, a chaperonin, that catalyzes folding.
- - After folding, the uptake targeting sequence will be cleaved off by a mitochondrial processing peptidase.
Three methods of sorting to the intermembrane space
- leaky OMM
- a. Similar to the beginning, a matrix-targeting sequence is going to guide it to hsp70, which will cause unfolding and binding to the receptor on the outer membrane. Translocation will occur in the same way that it does in import to the matrix, through both complexes where the outer and inner membrane are very close in contact
- b. instead of catalysis of folding, as in the original model, the uptake targeting sequence will be cleaved, revealing an intermembrane-space targeting sequence. The protein is still unfolded at this point, bound to hsp70
- c. This sequence will target the protein to the inner membrane by binding to a receptor on the matrix side of the membrane, initiating translocation of the protein across the inner membrane through a protein-lined channel and into the intermembrane space
- d. in the intermembrane space, the sequence is cleaved by a specific protease that is related to the ER signal peptidase, and heme is added, enabling the cytochromes to fold into its mature configuration.
- a. Similar to the beginning, the uptake targeting sequence will guide the protein to hsp70, causing unfolding and translocation through the TOM complex, similar to the beginning.
- b. However, unlike matrix bound proteins and the conservative model, when the polypeptide attempts to go through the TIM complex, its intermembrane-space-targeting sequence acts as a stop-transfer, remaining anchored in the inner membrane, preventing translocation of the C-terminus.
- c. The protein causes disassembly of the channel, the rest of the protein moves across the outer membrane intot he intermembrane space, and the protein diffuses, within the inner membrane, away from the translocation site.
- d. cleavage of its intermembrane-space targeting sequence and heme groups are added
3rd method of transport to the intermembrane space
3) A different channel is used that allows the transport of small particles (not very often)