The molecular basis of CRL4DDB2/CSA ubiquitin ligase architecture, targeting, and activation.

The DDB1-CUL4-RBX1 (CRL4) ubiquitin ligase family regulates a diverse set of cellular pathways through dedicated substrate receptors (DCAFs). The DCAF DDB2 detects UV-induced pyrimidine dimers in the genome and facilitates nucleotide excision repair. We provide the molecular basis for DDB2 receptor-mediated cyclobutane pyrimidine dimer recognition in chromatin. The structures of the fully assembled DDB1-DDB2-CUL4A/B-RBX1 (CRL4DDB2) ligases reveal that the mobility of the ligase arm creates a defined ubiquitination zone around the damage, which precludes direct ligase activation by DNA lesions. Instead, the COP9 signalosome (CSN) mediates the CRL4DDB2 inhibition in a CSN5 independent, nonenzymatic, fashion. In turn, CSN inhibition is relieved upon DNA damage binding to the DDB2 module within CSN-CRL4DDB2. The Cockayne syndrome A DCAF complex crystal structure shows that CRL4DCAF(WD40) ligases share common architectural features. Our data support a general mechanism of ligase activation, which is induced by CSN displacement from CRL4DCAF on substrate binding to the DCAF.