Targeting sequences (also known as localization sequences) usually comprise a length of 3-80 amino acids that are recognized in the cytosol by specific receptors that then guide the protein to the correct site and make contact with the appropriate translocation machinery. Once the protein has been imported into the new location the targeting sequence is often removed by enzymes that break the peptide bond between the targeting sequence and the rest of the protein. Some targeting sequences have been characterized better than others. The targeting sequence encoding import into the endoplasmic reticulum consists of about 5-15 mostly hydrophobic amino acids at the N terminus of the protein called the signal sequence. The import signal for mitochondria is a stretch of 20-80 amino acids in which positively charged side chains stick out on one side of the helix and hydrophobic side chains stick out on the other, a so-called amphipathic helix. A cluster of about five positively charged amino acids located within the protein sequence targets a protein to the nucleus, while the best known peroxisomal targeting sequence is the C-terminal tripeptide Ser-Lys-Leu-COOH.
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