Mitochondrial Precursor Proteins

A presequence is a cleavable N-terminal amphipathic signal sequence that directs most precursor proteins to mitochondria. This alpha-helical sequence contains hydrophobic residues on one side and hydrophilic residues on the other, allowing it to interact with both the lipid bilayer core and polar phosphate heads. Import receptors recognize presequences to facilitate translocation through mitochondrial translocons.

Cytosolic chaperones, including heat shock proteins 70 (Hsp70) and mitochondrial import stimulation factors (MSFs), use ATP hydrolysis energy to bind and stabilize unfolded precursors. These complexes keep precursors in a disaggregated state, preventing spontaneous folding into their native conformation. This maintains precursors in an import-competent form until they reach mitochondrial translocons.

TOM20 receptors recognize precursors bound to Hsp70 chaperones carrying cleavable amphipathic presequences, while TOM70-TOM37 receptors recognize precursors bound to MSFs with non-cleavable internal import signals. TOM20's shallow binding groove accommodates only amphipathic alpha-helices, distinguishing mitochondrial precursors from ER precursors and ensuring targeting specificity.

Internal import signals are non-cleavable sequences found in precursors targeted to mitochondrial membranes or intermembrane space, whereas presequences are cleavable N-terminal sequences. Unlike presequences, internal import signals remain part of the mature protein after import. Both signal types are recognized by import receptors to direct protein translocation through mitochondrial translocons.

Stop-transfer sequences are internal hydrophobic signals that arrest translocation of proteins destined for the inner mitochondrial membrane. Once the presequence is cleaved by mitochondrial processing peptidases, these sequences anchor the protein in the membrane while the rest of the polypeptide is pulled into the intermembrane space, enabling direct membrane insertion.

Mitochondrial processing peptidases cleave presequences from proteins targeted to the inner membrane or intermembrane space. Matrix proteases cleave presequences from matrix-targeted proteins. Inner membrane proteases process intermembrane space proteins. After cleavage, chaperones assist processed precursors in folding into their native conformation, and mature proteins are released as functional molecules.

Hydrophilic positively charged residues on presequences enhance precursor affinity toward negative charges on the matrix side of the inner membrane. These electrostatic interactions provide the necessary energy to drive protein translocation across the lipid bilayer and initiate the import process. This charge-based mechanism complements hydrophobic interactions with the bilayer core.