The COP9 (Constitutive photomorphogenesis 9) signalosome (CSN), a large 
multiprotein complex that resembles the 19S lid of the 26S proteasome, plays a 
central role in the regulation of the E3-cullin RING ubiquitin ligases (CRLs). 
The catalytic activity of the CSN complex, carried by subunit 5 (CSN5/Jab1), 
resides in the deneddylation of the CRLs that is the hydrolysis of the 
cullin-neural precursor cell expressed developmentally downregulated gene 8 
(Nedd8)isopeptide bond. Whereas CSN-dependent CSN5 displays isopeptidase 
activity, it is intrinsically inactive in other physiologically relevant forms. 
Here we analyze the crystal structure of CSN5 in its catalytically inactive form 
to illuminate the molecular basis for its activation state. We show that CSN5 
presents a catalytic domain that brings essential elements to understand its 
activity control. Although the CSN5 active site is catalytically competent and 
compatible with di-isopeptide binding, the Ins-1 segment obstructs access to its 
substrate-binding site, and structural rearrangements are necessary for the 
Nedd8-binding pocket formation. Detailed study of CSN5 by molecular dynamics 
unveils signs of flexibility and plasticity of the Ins-1 segment. These analyses 
led to the identification of a molecular trigger implicated in the 
active/inactive switch that is sufficient to impose on CSN5 an active 
isopeptidase state. We show that a single mutation in the Ins-1 segment restores 
biologically relevant deneddylase activity. This study presents detailed 
insights into CSN5 regulation. Additionally, a dynamic monomer-dimer equilibrium 
exists both in vitro and in vivo and may be functionally relevant.