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 the SCF-type E3 ligase complexes, leading to decrease the Ubl ligase activity of SCF. 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 (By similarity). The CSN complex plays an essential role in embryogenesis and oogenesis and is required to regulate microtubule stability in the early embryo. Mediates mei-3/katanin targeting for degradation at the meiosis to mitosis transition via deneddylation of cul-3. May stabilize glh-1 protein levels by antagonizing kgb-1 (PubMed:17699606). Component of the CSN complex, probably composed of csn-1, csn-2, csn-3, csn-4, csn-5, csn-6 and csn-7. Within the complex it probably interacts directly with csn-1 (PubMed:12781129). Interacts with glh-1 and glh-3 (PubMed:12435362). Interacts with lag-1 (PubMed:10615043). Interacts with kgb-1 (PubMed:17699606). 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.