CSN7B defines a variant COP9 signalosome complex with distinct function in DNA damage response.

Mammalian COP9 signalosome (CSN) exists as two variant complexes containing either CSN7A or CSN7B paralogs of unknown functional specialization. Constructing knockout cells, we found that CSN7A and CSN7B have overlapping functions in the deneddylation of cullin-RING ubiquitin ligases. Nevertheless, CSNCSN7B has a unique function in DNA double-strand break (DSB) sensing, being selectively required for ataxia telangiectasia mutated (ATM)-dependent formation of NBS1S343p and γH2AX as well as DNA-damage-induced apoptosis triggered by mitomycin C and ionizing radiation. Live-cell microscopy revealed rapid recruitment of CSN7B but not CSN7A to DSBs. Resistance of CSN7B knockout cells to DNA damage is explained by the failure to deneddylate an upstream DSB signaling component, causing a switch in DNA repair pathway choice with increased utilization of non-homologous end joining over homologous recombination. In mice, CSN7B knockout tumors are resistant to DNA-damage-inducing chemotherapy, thus providing an explanation for the poor prognosis of tumors with low CSN7B expression.

Overexpression of COP9 signalosome subunits, CSN7A and CSN7B, exerts different effects on adipogenic differentiation.

The COP9 signalosome (CSN) is an essential regulator of cullin-RING-ubiquitin (Ub) ligases (CRLs), which ubiquitinate important cellular regulators and target them for degradation by the Ub proteasome system (UPS). The CSN exhibits deneddylating activity localized on subunit CSN5, which removes the ubiquitin-like protein Nedd8 from the cullins of CRLs. CSN-mediated deneddylation is an important step in the process of CRL remodeling, in which new substrate recognition units are incorporated into Ub ligases to meet changed requirements for proteolysis in cells. For instance, extensive CRL remodeling occurs during adipogenic differentiation when new CRL3s are formed. Diversification of CSN complexes during evolution is most likely another adaptation to meet different cellular requirements. Best known CSN variants are formed by different CSN subunit isoforms. For instance, in plant cells, isoforms have been identified for the MPN-domain subunits CSN5 (CSN5A and CSN5B) and CSN6 (CSN6A and CSN6B) which form four distinct CSN variants. In mammalian cells CSNCSN7A and CSNCSN7B variants are generated by CSN7 isoforms. We demonstrate that the two variants coexist in human LiSa-2 cells and in mouse embryonic fibroblasts. During adipogenic differentiation of LiSa-2 cells CSN7B increases in parallel with an elevation of the total CSN complex. Permanent overexpression of Flag-CSN7B but not of Flag-CSN7A accelerates adipogenesis in LiSa-2 cells indicating a specific function of the CSNCSN7B variant in stimulating adipogenesis. Silencing of CSN7A as well as of CSN7B in LiSa-2 cells and in mouse embryonic fibroblasts (MEFs) reduces adipogenic differentiation demonstrating that both CSNCSN7A and CSNCSN7B variants are involved in the process.