Segments several amino acids long in proteins that target them to their final destinations. For example, the nuclear-targeting signal is four to eight amino acids in length, and it contains several positively charged residues and usually one or more prolines. The targets can occur at a variety of places in different nuclear proteins. The peroxisomal-targeting signal is usually located near the carboxy terminus of the protein, and it consists of three amino acids (serine, lysine, and leucine). There are also targeting signals that cause specific proteins to be retained in the ER or Golgi apparatus or to be targeted to lysosomes. In animals like Caenorhabditis or Drosophila, about 5% of the proteins contain sorting signals that direct them to mitochondria. However, in plants, like Arabidopsis, nearly 25% of the nuclear genes direct encoded proteins to either chloroplasts or mitochondria. See protein sorting.
Subjects: Genetics and Genomics.