A role for Fbw7 dimerization. A: Dimerization-deficient Fbw7 strictly requires S384 phosphorylation for cyclin E turnover. Cyclin E/CDK2 or two S384 mutants of cyclin E (S384A and S384E) were co-transfected into 293A cells together with either wild-type or non-dimerizing Fbw7α as indicated. Lysates were probed for the steady-state expression of these proteins with an antibody cocktail. The asterisks indicate cross-reactions. B: Analysis of stable interaction between cyclin E and Fbw7α. 293A cells were transfected as indicated together with dominant-negative Cul1. Binding of Fbw7α to cyclin E was assayed by co-immunoprecipitation anti Flag. dnCul1 co-migrates with the Flag antibody heavy chain. The asterisks indicate heavy chain cross-reactions. The upper panel lysate membrane was incubated with a cocktail of antibodies to detect all relevant proteins.