Probable protease subunit of the COP9 signalosome complex (CSN), a complex involved in various cellular and developmental processes such as photomorphogenesis and response to hormones (By similarity). The CSN complex is an essential regulator of the ubiquitin (Ubl) conjugation pathway by mediating the deneddylation of the cullin subunits of SCF-type E3 ligase complexes, leading to decrease the Ubl ligase activity of SCF (By similarity). Involved in early response to iron deficiency (Probable). Component of the CSN complex, probably composed of CSN1, CSN2, CSN3, CSN4, CSN5, CSN6, CSN7 and CSN8 (By similarity). Interacts with MCM2 (PubMed:18755073). Down-regulated during the early stage of iron deficiency (at protein level). Belongs to the peptidase M67A family. CSN5 subfamily.