Mitochondrial Protein Sorting

Mitochondrial precursor proteins are unfolded or loosely folded polypeptides encoded by the nuclear genome and synthesized in the cytosol. Each contains a signal sequence that targets it to the mitochondria. Cytosolic chaperones like Hsp90 and Hsc70 bind to precursors to prevent aggregation or degradation until transport begins through specialized translocons.

The translocase of the outer membrane (TOM) complex is a multisubunit protein complex in the mitochondrial outer membrane containing five transmembrane proteins forming a barrel-like structure. TOM20 and TOM70 import receptors recognize signal sequences on precursors, transferring them through the central TOM40 channel into the intermembrane space using ATP hydrolysis energy.

The translocase of the inner membrane (TIM) complex facilitates protein translocation across the inner mitochondrial membrane. The TIM23 receptor recognizes incoming polypeptides and uses the electrochemical potential across the inner membrane to shuttle precursors through the TIM22 channel, releasing them into the mitochondrial matrix for further processing.

The five pathways are: presequence pathway for cleavable N-terminal presequences through TOM40 and TIM23; carrier pathway for inner membrane proteins; beta-barrel pathway for outer membrane integration via SAM complex; MIA pathway for cysteine-rich intermembrane space proteins; and MIM pathway for multipass outer membrane proteins with hydrophobic anchor signals.

Precursors with cleavable N-terminal presequences are translocated through TOM40 and TIM23 channels into the matrix. The presequence translocase associated motor (PAM) uses ATP hydrolysis energy to transport the complete peptide. Inside the matrix, matrix processing peptidases cleave the presequence, and the processed protein folds into its active form.

Carrier proteins with non-cleavable internal import signals are translocated by the TOM complex on the outer membrane and carrier translocases TIM22 on the inner membrane. This carrier pathway allows inner membrane carrier proteins to bypass the matrix and integrate directly into the inner membrane for their specific functions.

The beta-barrel pathway translocates precursors through the TOM complex and integrates them into the outer membrane via the sorting and assembly machinery (SAM) complex. The MIA pathway targets cysteine-rich proteins to the intermembrane space using the mitochondrial import and assembly machinery (MIA) complex, which pulls precursors across the TOM channel.