The COP9 signalosome (CSN) is a eukaryotic protein complex, which regulates a 
wide range of biological processes mainly through modulating the cullin 
ubiquitin E3 ligases in the ubiquitin-proteasome pathway. The CSN possesses a 
highly conserved deneddylase activity that centers at the JAMM motif of the Csn5 
subunit but requires other subunits in a complex assembly. The classic CSN is 
composed of 8 subunits (Csn1-8), yet in several Ascomycota, the complex is 
smaller and lacks orthologs for a few CSN subunits, but nevertheless contains a 
conserved Csn5. This feature makes yeast a powerful model to determine the 
minimal assemblage required for deneddylation activity. Here we report, that 
Csi1, a diverged S. cerevisiae CSN subunit, displays significant homology with 
the carboxyl terminal domain of the canonical Csn6, but lacks the amino terminal 
MPN(-) domain. Through the comparative and experimental analyses of the budding 
yeast and the mammalian CSNs, we demonstrate that the MPN(-) domain of the 
canonical mouse Csn6 is not part of the CSN deneddylase core. We also show that 
the carboxyl domain of Csn6 has an indispensable role in maintaining the 
integrity of the CSN complex. The CSN complex assembled with the carboxyl 
fragment of Csn6, despite its lack of an MPN(-) domain, is fully active in 
deneddylation of cullins. We propose that the budding yeast Csi1 is a functional 
equivalent of the canonical Csn6, and thus the composition of the CSN across 
phyla is more conserved than hitherto appreciated.