NEDD8 conjugation of Cullin has an important role in ubiquitin-mediated protein degradation.The COP9 signalosome,of which CSN5 is the major catalytic subunit,is a major Cullin deneddylase.Another deneddylase,Deneddylase 1,has also been shown to process the Nedd8 precursor.In Drosophila,the DEN1 mutants do not have increased levels of Cullin neddylation,but instead show a significant decrease in neddylated Cullin.This characteristic decrease in neddylated Cullins in the DEN1null background can be rescued by UAS-dDEN1 WT overexpression but not by overexpression of mature NEDD8,indicating that this phenotype is distinct from the NEDD8-processing function of DEN1.We examined the role of DEN 1-CSN interaction in regulating Cullin neddylation.Overexpression of DEN1 in a CSN5hypo background slightly reduced unneddylated Cullin levels.The CSN5,DEN1 double mutation partially rescues the premature lethality associated with the CSN5 single mutation.These results suggest that DEN1 regulates Cullin neddylation by suppressing CSN deneddylase activity.