Probable protease subunit of the COP9 signalosome complex (CSN), a complex involved in various cellular and developmental processes such as photomorphogenesis and auxin and jasmonate responses. 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 is involved in repression of photomorphogenesis in darkness by regulating the activity of COP1-containing Ubl ligase complexes. The complex is also required for degradation of PSIAA6 by regulating the activity of the Ubl ligase SCF-TIR complex. Not involved in CSN's deneddylation/derubylation activity (PubMed:15486099, PubMed:17307927). Essential for the structural integrity of the CSN holocomplex (PubMed:17307927). Component of the CSN complex, probably composed of CSN1, CSN2, CSN3, CSN4, CSN5 (CSN5A or CSN5B), CSN6 (CSN6A or CSN6B), CSN7 and CSN8 (PubMed:9811788, PubMed:12615944, PubMed:15486099). CSN5A or CSN5B are present within distinct CSN complexes each containing only one copy of CSN5 (PubMed:15486099). Interacts with itself (PubMed:9811788). In the complex, it is located in the center and probably interacts directly with CSN4 and CSN6A or CSN6B (PubMed:9811788, PubMed:12615944). Also exists as monomeric form (PubMed:9811788). Interacts with CYT1 in vitro and in planta (PubMed:23424245). Interacts with FLZ3 (Ref.11). Ubiquitously expressed. Highly expressed in flowers and roots. Expressed at lower level in seedlings and siliques. 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). No visible phenotype and no effect on the derubylation of CUL1 (PubMed:15486099, PubMed:17307927). Csn5a and csn5b double mutants are lethal at the seedling stage (PubMed:17307927). Belongs to the peptidase M67A family. CSN5 subfamily.