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.