Essential component of the COP9 signalosome complex (CSN), a complex involved in various cellular and developmental processes. 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-type complexes such as SCF, CSA or DDB2. The complex is also involved in phosphorylation of p53/TP53, c-jun/JUN, IkappaBalpha/NFKBIA, ITPK1 and IRF8/ICSBP, possibly via its association with CK2 and PKD kinases. CSN-dependent phosphorylation of TP53 and JUN promotes and protects degradation by the Ubl system, respectively. Suppresses G-protein- and mitogen-activated protein kinase-mediated signal transduction (By similarity). Component of the CSN complex, composed of COPS1/GPS1, COPS2, COPS3, COPS4, COPS5, COPS6, COPS7 (COPS7A or COPS7B), COPS8 and COPS9 (PubMed:9707402). In the complex, it probably interacts directly with COPS2, COPS3, COPS4 and COPS5 (By similarity). Interacts directly with inositol kinase ITPK1 (By similarity). Interacts with CAPN8 (PubMed:16476741). Expressed in the base region of the oxyntic and pyloric mucosae. The PCI domain is necessary and sufficient for the interactions with other CSN subunits of the complex (By similarity). Mediates the interaction with CAPN8. The N-terminal part (1-196), which is not required for deneddylating activity and CSN complex formation, is nevertheless essential for other aspects of CSN complex function, such as repression of c-fos/FOS expression. Belongs to the CSN1 family.