Figure 3. Multiple routes to SCF-mediated substrate ubiquitylation.

At saturating conditions, all enzymes can prime SCF-bound substrates, but only UBE2D3 and ARIH1 can prime at physiological concentrations. (a) Quantitation of the fractions of Cyclin E substrate (S0) and all products taken from the 10 s time-points from the quench flow reactions in Figure 2. Solid bars represent levels for reactions where ARIH1, UBE2D3, and/or UBE2R2 were saturating for SCF; empty bars represent reactions containing the same enzymes at more physiological levels (Table 2). Notice that UBE2R2 is capable of generating substantial products with long poly-ubiquitin chains when saturating for SCF^FBW7, but produces almost no product when UBE2R2 levels are at estimated physiological levels. (b) Same as in (a), except with β-Catenin peptide substrate and SCF^βTRCP.