Unique role for the UbL-UbA protein Ddi1 in turnover of SCF^Ufo1 complexes

SCF complexes are E3 ubiquitin-protein ligases that mediate degradation of regulatory and signaling proteins and control G₁/S cell cycle progression by degradation of G₁ cyclins and the cyclin-dependent kinase inhibitor, Sic1. Interchangeable F-box proteins bind the core SCF components; each recruits a specific subset of substrates for ubiquitylation. The F-box proteins themselves are rapidly turned over by autoubiquitylation, allowing rapid recycling of SCF complexes. Here we report a role for the UbL-UbA protein Ddi1 in the turnover of the F-box protein, Ufo1. Ufo1 is unique among F-box proteins in having a domain comprising multiple ubiquitin-interacting motifs (UIMs) that mediate its turnover. Deleting the UIMs leads to stabilization of Ufo1 and to cell cycle arrest at G₁/S of cells with long buds resembling skp1 mutants. Cells accumulate substrates of other F-box proteins, indicating that the SCF pathway of substrate ubiquitylation is inhibited. Ufo1 interacts with Ddi1 via its UIMs, and Δddi1 cells arrest when full-length UFO1 is overexpressed. These results imply a role for the UIMs in turnover of SCF^Ufo1 complexes that is dependent on Ddi1, a novel activity for an UbL-UbA protein.