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 ligasew 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. Csn5 is essential for growth or survival. Component of the CSN complex. The holocomplex is comprised of 8 subunits csn1-8. In the complex, it probably interacts directly with csn1, csn2, csn3, csn4, csn6 and csn8. 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). Belongs to the peptidase M67A family. CSN5 subfamily.