Assembly and Regulation of CRL Ubiquitin Ligases

Cullin-RING ubiquitin ligases (CRLs) determine the substrate specificity of ubiquitination reactions, and substrates are recruited to the cullin core through binding to their cognate substrate receptor modules. Because a family of substrate receptors compete for the same cullin core, the assembly and activity of CRLs are dynamically regulated to fulfill the needs of the cell to adapt to the changing pool of proteins demanding ubiquitination. Cullins are modified by NEDD8, a ubiquitin-like protein. This process, referred to as neddylation, promotes the E3 activity of CRLs by inducing conformational rearrangement in the Cullin-RING catalytic core. Cand1 is a cullin-associated protein whose binding is excluded by cullin neddylation. Although early biochemical studies suggested that Cand1 inhibits CRL activity, genetic studies revealed its positive role in ubiquitination. Emerging evidence from kinetic and quantitative proteomic studies demonstrated that Cand1 stimulates assembly of new Skp1-Cul1-F-box protein (SCF) complexes by exchanging the Skp1-F-box protein substrate receptor modules. Furthermore, aided by refined experimental design as well as computational simulation, an attractive model has been developed in which substrate, neddylation cycle and Cand1-mediated "adaptive exchange" collaborate to maintain the dynamics of the cellular SCF repertoire. Here, we review and discuss recent advances that have deepened our understanding of CRL regulation.