The COP9/Signalosome (CSN) is an evolutionary conserved macromolecular complex that regulates the Cullin-RING Ligase (CRL) class of E3 ubiquitin ligases primarily by removing the ubiquitin-like protein Nedd8 from the cullin subunit. In the C. elegans embryo, the CSN controls degradation of the microtubule-severing protein MEI-1 through CUL-3 deneddylation. However, the molecular mechanisms of CSN function and its subunit composition remain to be elucidated. Here, using a proteomic approach, we have characterized the CSN and CUL-3 complexes from C. elegans embryos. We show that the CSN physically interacts with the CUL-3-based CRL and regulates its activity by counteracting the autocatalytic instability of the substrate-specific adaptor MEL-26. Importantly, we identified the uncharacterized protein K08F11.3/CIF-1 (COP9/Signalosome - eIF3) as a stoichiometric and functionally important subunit of the CSN complex. CIF-1 appears to be the only ortholog of Csn7 encoded by the C. elegans genome, but also exhibits extensive sequence similarity with eIF3m family members, which are required for initiation of protein translation. Indeed, CIF-1 binds eIF-3.F, and inactivation of cif-1
impairs translation in vivo. Taken together, our results indicate that CIF-1 is a shared subunit of the CSN and eIF3-complexes, and may therefore link protein translation and degradation.