Signal peptides

Abstract

Signal peptides are amino-terminal (N-terminal) extensions of polypeptides that target them from the cytosol to the cytoplasmic (plasma) membrane of prokaryotes and to the membrane of the endoplasmic reticulum (ER) of eukaryotes. After the targeting, they direct the linked proteins to translocate the membrane and are usually cleaved after the translocation. Signal peptides are also called signal sequences or leader sequences; however, the term “signal sequence” can sometimes be confusing because of the broader meaning of “protein-sorting signal.” Signals for the translocation of the mitochondrial inner membrane and the chloroplast thylakoid membrane are homologs of signal peptides. Although most signal peptides are likely to be recognized by specific molecules within the cell, some of them spontaneously interact with the membrane and are inserted into it, at least in vitro. Thus, they are closely related to so-called cell-penetrating peptides, the main theme of this book. In addition, understanding signal peptides should be useful for a wide range of application areas such as biotechnology and clinical investigation.