"The COP9 Signalosome (CSN) is an eight-subunit molecular machine conserved in eukaryotes. Defects in CSN function are reported in human disorders and a CSN-related pathway recently emerged as a promising cancer drug target. CSN regulates embryonic development, DNA repair, cell cycle checkpoint progression and autophagy. The remarkable pleiotropy of CSN is partially explained through its enzymatic activity towards Cullin-RING E3 ubiquitin ligases (CRLs), which are activated by neddylation and inactivated by CSN-mediated deneddylation. However, loss of CSN function results in loss of CRL activity in cells: an observation which has not been fully explained and is known as the 'CSN paradox'. Intriguingly, we recently observed stable CSN-CRL complexes, irrespective of CRL neddylation and our preliminary results suggest a novel model of multimodal regulation of CRL architecture and function by CSN.Our key research objectives are to structurally characterize CSN-CRL complexes, to determine their functional significance and to dissect the mechanisms, which regulate the interaction. Owing to our team’s complementary expertise and the already established molecular tools, we are in a unique position to achieve these experimentally challenging aims. Our hypotheses are guided by 3D structural data and will be validated in established in vitro assays. All required protein complexes can be purified in large amounts from recombinant systems. In parallel, testing specific predictions and activities in vivo will benefit from already available human cell lines. Our findings will be complemented by an unbiased, large-scale proteomic analysis to characterize distinct CRL-CSN complexes and identify relevant post-translational modifications that regulate complex formation.We expect our results to have important implications for the fundamental understanding of CSN function and CRL regulation, and these may reveal new opportunities to develop specific inhibitors of clinical relevance."