E Similar to B, the role of the "closed" conformation in UBE2S-dependent Ub chain elongation of the APC/C substrate Ub-CycB*, monitored by the incorporation of C118A-substituted UBE2S variants (top). Quantification of ubiquitinated Ub-CycB* was assessed (bottom) and statistical significance by one-way ANOVA between C118A and WT, C118A/E139R, and C118A/E143R (** p ≤ 0.01, **** p ≤ 0.0001, n=3 independent, technical replicates). Error bars: SEM. Data normalized to WT UBE2S 4+ Ub additions. 