ABSTRACT
This is a modication of the polypeptide chain in which parts of the chain are cleaved off by proteolytic enzymes and the chain is consequently shortened. The modication can serve various purposes, but one type of this modication is associated with the targeting of proteins. All proteins that are synthesized to be excreted from the cell, and most of those produced to enter specic organelle, contain a signal peptide. Membrane proteins can also contain a signal peptide; this can be as short as three amino acids or longer, containing over 50 amino acids, and is typically composed of several different regions. Out of these regions, one is a sequence of hydrophobic amino acids, and, in another, there are normally amino acids with a basic character. After translocation, the signal peptide is removed from the protein by the proteolytic action of signal peptide peptidases, which are transmembrane aspartyl proteases. Details about signal peptide cleavage are given in Chapters 7 and 24 and will not be further elaborated here.
