Transitpeptider

Transitpeptider, or transit peptides, are short amino-terminal sequences found on many nucleus-encoded proteins destined for organelles such as chloroplasts and mitochondria. They function as targeting information that directs precursors from the cytosol to the appropriate import machinery and are typically removed after successful import.

Chloroplast transit peptides are generally rich in serine and threonine and contain few acidic residues, with no strongly conserved sequence motif. They vary in length, usually around 20 to 100 amino acids, and guide proteins to the chloroplast through the TOC/TIC translocon complexes in the chloroplast envelope. Mitochondrial targeting peptides, or presequences, are often 15 to 60 amino acids long, form an amphipathic α-helix with positively charged residues, and facilitate import via the TOM/TIM complexes at the mitochondrial outer and inner membranes. Both types of peptides are usually cleaved by organellar processing peptidases after import, yielding mature, functional proteins.

The import process typically requires cytosolic chaperones to keep precursors in an import-competent, partially unfolded state, followed by recognition by receptor components of the respective translocons and translocation into the organelle matrix or stroma. The efficiency and specificity of import depend on peptide properties beyond any simple motif, including charge distribution, hydrophobicity, and overall structure.

In research and biotechnology, transit peptides are used to direct engineered proteins to plastids or mitochondria, enabling targeted metabolic engineering, protein production, or studying organelle biogenesis. Prediction tools and experimental assays continue to refine understanding of how these signals determine organelle localization.