Essential component of the COP9 signalosome complex (CSN), a complex involved in various cellular and developmental processes. The CSN complex is an essential regulator of the ubiquitin (Ubl) conjugation pathway by mediating the deneddylation of the cullin subunits of E3 ligase complexes, leading to modify the Ubl ligase activity. Component of the CSN complex. The holocomplex is comprised of 8 subunits csn1-8. In the complex, it probably interacts directly with csn2 and csn5. The PCI domain is necessary and sufficient for the interactions with other CSN subunits of the complex. Belongs to the CSN1 family.