16.11:
Energy to Drive Translocation
Mitochondrial protein translocation is fueled by two distinct energy sources: ATP hydrolysis and the electrochemical potential across the inner membrane.
As a chaperone-bound precursor passes through the TOM complex to reach the TIM channel, an electrochemical potential of 200 millivolts across the inner membrane pulls the positively charged presequence through.
In the matrix, mitochondrial Hsp70 binds the incoming peptide and translocates it by the thermal ratchet or cross-bridge ratchet model.
In the thermal ratchet model, an emerging polypeptide chain moves back and forth across the TIM channel. When the ATP bound to Hsp70 is hydrolyzed, it prevents further polypeptide backsliding. As multiple Hsp70s bind, the precursor is translocated forward to the mitochondrial matrix.
In the cross-bridge ratchet model, matrix Hsp70 binds the TIM complex near the mouth of its channel.
As ATP binds, Hsp70 undergoes a conformational change to latch on to the precursor as it exits the TIM channel. ATP hydrolysis secures the binding of Hsp70, pulling the rest of the peptide through the TIM channel.
Rebinding of ATP triggers the dissociation of Hsp70, following which the peptide is released into the matrix.
16.11:
Energy to Drive Translocation
Mitochondrial protein import is powered by two distinct energy sources: ATP hydrolysis and electrochemical potential across the inner membrane. Newly synthesized precursors are bound by cytosolic chaperones of the Hsp70 family, which guide them to the import receptors on the mitochondrial surface. Utilizing the energy of ATP hydrolysis, Hsp70 chaperones transfer these precursors to the TOM receptors on the mitochondrial outer membrane.
Generally, polypeptides are unfolded by two distinct mechanisms before being transported in the TOM/TIM import pathway: spontaneous global unfolding and catalyzed unfolding. Precursors with shorter presequences undergo spontaneous global unfolding. In contrast, precursors with longer positively charged presequences undergo local unfolding. The unstructured presequence then traverses the TOM/TIM import complexes and interacts with the inner membrane’s negative charges to reach the matrix. Mitochondrial Hsp70 (mtHsp70) associated with TIM44 recognizes the emerging polypeptide and translocates it entirely into the matrix by accelerating the unfolding process. mtHsp70 also traps any incoming loosely folded precursors without undergoing any conformational change and translocates them to the matrix without undergoing any conformational change.
In contrast, the translocation of tightly folded polypeptides induces ATP-dependent conformational changes in mtHsp70. mtHsp70 utilizes the ATP hydrolysis energy to pull the incoming peptide across the TIM translocase. Rebinding of ATP causes the opening of mtHsp70, and the precursor gets released into the matrix.
Two models can describe precursor translocation by the mtHsp70: the thermal ratchet model and the cross-bridge ratchet model. MtHsp70 translocates precursors by trap and release mechanism in the thermal ratchet model. mtHsp70 uses the ATP hydrolysis energy to bind the spontaneously unfolded precursors and trap them into the matrix, preventing further backward movement. In contrast, translocation by cross-bridge ratchet mechanism involves precursor unfolding coupled to an ATP-dependent conformational change of mtHsp70. mtHsp70 generates an ATP-dependent pulling force to transport precursors into the matrix. Accelerated precursor unfolding facilitates its unidirectional forward movement and transport into the matrix.
Suggested Reading
- Nikolaus Pfanner and Kaye N. Truscot. Powering mitochondrial protein import. Nature structural biology, volume 9 number 4, April 2002.
- Dejana Mokranjac, Energetics of mitochondrial protein sorting. Biochimica et Biophysica Acta 1777 (2008), 758-762
- Benjamin S Glick. Can Hsp70 proteins act as force-generating motors? Cell, Volume 80, Issue 1, 13 January 1995, Pages 11-14.