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 csn1, csn3, csn5, csn6, csn7 and csn8. Belongs to the CSN2 family.