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 IAA6 by regulating the activity of the Ubl ligase SCF-TIR complex. In the complex, it plays a central role in CSN assembly. 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 and (via PCI domain) with CSN7 (via PCI domain). In the CSN complex, it probably interacts directly with CSN2, CSN3, CSN4 and CSN5B. Interacts with the 26S proteasome subunit RPN6. Interacts (via N-terminal domain) with TSA1 (via C-terminal domain). Binds to the translation initiation factors TIF3C1, TIF3E1 and TIF3H1 (PubMed:9849901, PubMed:19704582, PubMed:15548739). Expressed in leaves, flowers, immature siliques, and light-grown roots. The PCI domain is necessary and sufficient for the interactions with other CSN subunits of the complex. The N-terminal part (1-211), which is not required for deneddylating activity and CSN complex formation, is nevertheless essential for other aspects of CSN complex function. Belongs to the CSN1 family.