Remodeling of the SCF complex-mediated ubiquitination system by compositional alteration of incorporated F-box proteins.

Ubiquitination regulates not only the stability but the localization and activity of substrate proteins involved in a plethora of cellular processes. The Skp1-Cullin-F-box protein (SCF) complexes constitute a major family of ubiquitin protein ligases, in each member of which an F-box protein serves as the variable component responsible for substrate recognition, thereby defining the function of each complex. Here we studied whether the composition of F-box proteins in the SCF complexes is remodeled under different conditions. We exploited stable isotope labeling and MS for relative quantification of F-box proteins in the SCF complexes affinity-purified en masse from budding yeast cells at log and post-diauxic phases, and revealed an increment of Saf1, an F-box protein involved in entry into quiescence, during the diauxic shift. Similarly, we found that Met4 overexpression induces a specific increment of Met30, the F-box protein responsible for ubiquitination of Met4. These results illustrate a cellular response to environmental and genetic perturbations through remodeling of the SCF complex-mediated ubiquitination system. Compositional alteration of incorporated F-box proteins may redirect the activity of this system toward appropriate substrates to be ubiquitinated under individual conditions for the maintenance of cellular homeostasis.