Signal sequences target proteins for secretion

Proteins destined to be secreted from the cell have an amino-terminal sequence that is rich in hydrophobic residues. This signal sequence binds to proteins in the endoplasmic reticulum (ER) membrane that draw the remainder of the protein through the lipid bilayer. The signal sequence is cleaved from the protein in this process by an enzyme called signal peptidase (not shown). Once inside the endoplasmic reticulum, the protein is directed to the cell membrane, from which it will be secreted.