Translocation of Proteins into the Mitochondria

Proteins carrying presequences are threaded through the TOM/TIM complex. Mitochondrial Hsp70 binds the peptide as it exits the TIM channel and moves it into the matrix. Matrix proteases then cleave the presequence, releasing the active protein. This mechanism ensures proper targeting and processing of matrix-destined proteins.

Mia40 is an oxidoreductase that imports intermembrane space proteins lacking presequences. Oxidized Mia40 forms a transient disulfide bond with cysteine residues on the incoming polypeptide and pulls the nascent peptide through the TOM channel. Once the entire peptide is threaded through, Mia40 is reduced and recycled for another import cycle.

Inner membrane proteins with stop-transfer sequences are arrested during translocation across the TIM complex and inserted directly into the membrane. This mechanism prevents complete translocation to the matrix. Some of these proteins are first processed by signal peptidases before insertion, while others are recognized by OXA translocase, which embeds them in the membrane.

The stop-transfer pathway arrests precursors with internal hydrophobic patches within the TIM channel and laterally releases them onto the inner membrane. The conservative pathway translocates precursors completely into the matrix first, then assembles them into the membrane via OXA protein. Matrix proteases cleave presequences in the conservative route before proteins fold using chaperonin energy.

Erv1 is a flavin-dependent oxidoreductase that cooperates with Mia40 to oxidize precursor cysteine residues, facilitating folding and transport into the intermembrane space. Erv1 transfers electrons to the reduced CPC motif of Mia40 via cytochrome C and the respiratory chain. This redox cycle regenerates oxidized Mia40 for subsequent rounds of protein import.

Matrix proteins are cleaved by matrix processing peptidases after translocation through TIM23. Some precursors undergo further processing by mitochondrial intermediate peptidase or inner membrane protease I at additional presequence cleavage sites. Processed matrix proteins are then released into the matrix where chaperonins fold them using ATP hydrolysis energy.

Intermembrane space proteins carry an additional hydrophobic signal sequence that stops translocation across the TIM complex. Signal peptidases cleave this hydrophobic segment, releasing the active protein in the intermembrane space. This cleavage mechanism ensures proteins remain in the intermembrane space rather than continuing into the matrix.