Probable protease subunit 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. In the complex, it probably acts as the catalytic center that mediates the cleavage of nedd8 from cullins. It however has no metalloprotease activity by itself and requires the other subunits of the CSN complex. Component of the CSN complex, probably composed of cops1, cops2, cops3, cops4, cops5, cops6, cops7, cops8 and cops9. The JAMM motif is essential for the protease activity of the CSN complex resulting in deneddylation of cullins. It constitutes the catalytic center of the complex (By similarity). The CSN complex is associated with some 'Lys-63'-specific deubiquitination. Such activity is however not mediated by the core CSN complex but by the brcc3/brcc36 component of the BRISC complex (By similarity). Belongs to the peptidase M67A family. CSN5 subfamily.