In vitro activity and interactions of 13 yeast SCF complexes

SCF complexes are multisubunit ubiquitin ligases that have a variable component called an F-box. In yeast there are three characterised SCF complexes with different F-box proteins, and each acts in concert with the E2 Cdc34 to ubiquitinate target proteins. The majority of other yeast F-box proteins remain uncharacterised. 13 yeast SCF complexes were reconstituted in vitro, and their activity and biochemistry investigated. A characterised SCF reaction, ubiquitination of phospho-Sic1 by SCFCdc4, was found to be reconstituted with Ubc4 as well as Cdc34. 12 of the purified SCF complexes ubiquitinated their F-box proteins in vitro. The autoubiquitination of each F-box was in some cases catalyzed only by Cdc34, and in other cases preferentially catalyzed by Ubc4. Ubc4 thus interacts with multiple SCFs in vitro, and the combinatorial interactions of the SCF and E2 components of the ubiquitination machinery may allow further diversification of the roles of SCFs in vivo.