The cullin-based ubiquitin ligases regulate a broad spectrum of biological processes such as cell cycle progression, cell growth, tumor suppression, and transcriptional control, by promoting ubiquitination and subsequent degradation of a large number of key regulatory proteins. Deregulations of cullin-mediated protein ubiquitination have been implicated in tumorigenesis, cancer, and other human diseases. Among the six human cullins, Cul4A has recently been shown to interact with the damaged-DNA binding protein, DDB1, which recognizes UV-damaged DNA in nucleotide excision repair. Together, the two proteins form a ubiquitin ligase complex distinct from the Cul1-based SCF and Cul2-based SCF-like complexes and play a role in DNA repair. Despite the differences in subunit composition, all cullin-based ubiquitin ligases are regulated by the small ubiquitin-like protein Nedd8 and possibly by CAND1. In SCF, for example, the complex assembly is inhibited by CAND1 binding to Cul1 and the inhibitory effect can be reversed by the Nedd8 modification of Cul1. This proposal focuses on structure-function studies of both the recently identified Cul4A-DDB1 ubiquitin ligase and in general how the cullin-based ubiquitin ligases are regulated by Nedd8 and CAND1. The following specific aims are proposed: Aim 1: Structural analyses of DDB1 and the Cul4A-Rbx1-DDB1 complex. Crystal structures of DDB1, the Cul4A-DDB1 complex, and the DDB1-DDB2- DNA complex will be determined and analyzed to elucidate the structural mechanism underlying the Cul4A-DDB1 ubiquitin ligase activity in DNA repair. Aim 2: Structural analyses of Cullin-Rbx1-CAND1 interactions. Crystal structures of CAND1, the CAND1-CuI1-Rbx1 complex, and the neddylated-Cul1-Rbx1 complex will be solved to reveal how CAND1 and Nedd8 modulate the assembly and activity of the ubiquitin ligase. [unreadable] [unreadable]