Catalytic component of the COP9 signalosome (CSN) complex that acts as an regulator of the ubiquitin (Ubl) conjugation pathway by mediating the deneddylation of the cullin subunit of SCF-type E3 ubiquitin-protein ligase complexes. The CSN complex is involved in the regulation of the mating pheromone response. Component of a COP9 signalosome-like (CSN) complex, composed of at least RRI1/CSN5, CSN9, RRI2/CSN10, PCI8/CSN11, CSN12 and CSI1. Within this complex it probably interacts directly with CSN12. Also interacts with RPN5 (By similarity). Nuclear localization requires the formation of the CSN complex. 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.