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. The CSN complex plays an essential role in oogenesis and embryogenesis and is required for proper photoreceptor R cell differentiation and promote lamina glial cell migration or axon targeting. It also promotes Ubl-dependent degradation of cyclin E (CycE) during early oogenesis. Also involved in regulation of axis formation by checkpoint-dependent, translational control of Gurken. Component of the CSN complex, probably composed of CSN1b, alien/CSN2, CSN3, CSN4, CSN5, CSN6, CSN7 and CSN8. Interacts directly with CSN2. Also exists as monomeric form. Interacts via its MPN domain with Trc8. Expressed in the optic lobe neuropil. 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.