Precursor convertases in the secretory pathway, cytosol and extracellular milieu

Precursor proteins that transit through the secretory pathway often require processing at specific sites in order to release their bioactive entities. The most prevalent limited proteolysis occurs at single or paired basic residues, and is achieved by one or more of the seven subtilisin-like proprotein convertases (PCs); Furin, PC1, PC2, PACE4 (paired basic amino acid converting enzyme 4), PC4, PC5 and PC7. Other types of cleavages occur at hydophobic residues, some of which are performed by subtilisin/kexin-like isozyme-1 (SKI-1), which is also known as site-1 protease. Together, the PCs and SKI-1 regulate the activity of a large variety of cellular proteins, including growth factors, neuropeptides, receptors, enzymes and even toxins and glycoproteins from infectious retroviruses. These processing events are exquisitely regulated by multiple zymogen-activation steps, as well as by specific subcellular localization signals. The above mentioned convertases are implicated in a number of pathologies such as cancer, neurodegenerative diseases, endocrine disorders and inflammation. Recently, it was recognized that the metalloendopeptidase N-arginine dibasic convertase (NRDc; nardilysin), which cleaves at the N-terminus side of basic residues in dibasic pairs, is localized both in the cytosol and at the cell surface or in the extracellular milieu. While NRDc binds heparin-binding epidermal growth factor (HB-EGF) at the cell surface and potentiates its physiological effect, HB-EGF potently inhibits the NRDc's activity. NRDc could represent the equivalent of the PCs in the cytosol or the extracellular space.