Component 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 SCF-type E3 ligase complexes, leading to decrease the Ubl ligase activity of SCF. It 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. 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. Interacts with itself. In the complex, it probably interacts directly with CSN4 and CSN5A or CSN5B. Interacts with CSN7 (via C-terminal tail). Binds to the translation initiation factors TIF3E1 (PubMed:19704582). No visible phenotype when grown under normal conditions, due to the redundancy with CSN6A. Csn6a and csn6b double mutants are lethal at the seedling stage. Although strongly related to metalloprotease proteins, it lacks the JAMM motif that probably constitutes the catalytic center. Its function as protease is therefore unsure. Belongs to the peptidase M67A family. CSN6 subfamily.