Component of the COP9 signalosome (CSN) complex that acts as an regulator of the ubiquitin (Ubl) conjugation pathway by mediating the deneddylation of the cullin subunit of SCF-type E3 ubiquitin-protein ligase complexes (By similarity). Required, indirectly, for activation of ribonucleotide reductase through the degradation of the protein spd1, thereby supplying deoxyribonucleotides for DNA replication and repair. Component of the COP9 signalosome (CSN) complex composed of at least csn1, csn2, csn4 and csn5 subunits. Belongs to the CSN1 family.