The COP9 signalosome counteracts the accumulation of cullin SCF ubiquitin E3 RING ligases during fungal development

Summary Defects in the COP9 signalosome (CSN) impair multi- cellular development, including embryonic plant or animal death or a block in sexual development of the fungus Aspergillus nidulans. CSN deneddylates cullin-RING ligases (CRLs), which are activated by covalent linkage to ubiquitin-like NEDD8. Deneddyla- tion allows CRL disassembly for subsequent reassem- bly. An attractive hypothesis is a consecutive order of CRLs for development, which demands repeated cycles of neddylation and deneddylation for reassem- bling CRLs. Interruption of these cycles could explain developmental blocks caused by csn mutations. This predicts an accumulation of neddylated CRLs exhibit- ing developmental functions when CSN is dysfunc- tional. We tested this hypothesis in A. nidulans, which tolerates reduced levels of neddylation for growth. We show that only genes for CRL subunits or neddylation are essential, whereas CSN is primarily required for development. We used functional tagged NEDD8, recruiting all three fungal cullins. Cullins are associ- ated with the CSN1/CsnA subunit when deneddylation is defective. Two CRLs were identified which are spe- cifically involved in differentiation and accumulate during the developmental block. This suggests that an active CSN complex is required to counteract the accu- mulation of specific CRLs during development.