transitpeptide

A transit peptide is an N-terminal signal sequence found in many nuclear-encoded proteins that directs their delivery to specific organelles, most commonly chloroplasts or mitochondria. These peptides are synthesized in the cytosol as part of the precursor protein and are removed after successful import, yielding the mature, functional protein. Transit peptides enable post-translational targeting and do not form part of the final folded protein.

Transit peptides are typically short, variable in sequence, and lack a strongly conserved motif. They are generally rich in hydroxylated residues (serine and threonine), with substantial amounts of alanine, and they usually lack acidic residues. They rarely form long hydrophobic cores. Because of their low sequence conservation, transit peptides are identified more reliably by prediction algorithms that assess overall composition, hydrophobicity profiles, and potential cleavage sites rather than by a fixed sequence motif.

In chloroplasts, transit peptides are recognized by the Toc/Toc receptor complex and guided through the Toc and Tic translocons into the plastid, where they are cleaved by stromal processing peptidase and other processing enzymes. In mitochondria, they are recognized by the Tom/Tim complexes and transported across the outer and inner membranes, where mitochondrial processing peptidase cleaves the peptide. Some proteins can be directed to more than one organelle, and transit peptides can sometimes be engineered to alter subcellular localization for research or biotechnological applications.