COP9 (Constitutive photomorphogenesis 9) signalosome (CSN) is a protein complex with isopeptidase activity. It catalyses the hydrolysis of NEDD8 protein from the cullin subunit of Cullin-RING ubiquitin ligases (CRL). Therefore, it is responsible for CRL deneddylation â at the same time, it is able to bind denedyllated cullin-RING complex and retain them in deactivated form. COP9 signalosome thus serves as a sole deactivator of CRLs. The complex was originally identified in plants, and subsequently found in all eukaryotic organisms including human. Human COP9 signalosome (total size ~350 kDa) consists of 8 subunits - CSN1, CSN2, CSN3, CSN4, CSN5, CSN6, CSN7 (COPS7A, COPS7B), CSN8. All are essential for full function of the complex and mutation in some of them is lethal in mice.
Given the essential functions of the COP9 signalosome, the complex has been explored as a target for drug discovery. Preclinical studies showed that inhibiting COP9 resulted in death of cancer cells and medically important parasite.