Two groups have recently reported ubiquitin-deconjugating activities associated with the COP9 signalosome (CSN). Together with a previously identified deneddylation activity, CSN now appears to possess biochemical activities towards the cullin-containing ubiquitin ligases (e.g. deneddylation) and ubiquitinated protein substrates in the proteasome-mediated degradation processes (e.g. deubiquitination). Here, we discuss how these findings support a central role for CSN in ubiquitination of substrate proteins and their proteolysis.