F UBE2R2 R149E is still defective in the transfer of the *LRLRGG peptide to an acceptor Ub compared to WT, probed by fluorescent scanning (top) and its quantitation (bottom). Statistical comparison was performed using an unpaired Welch's t-test (** p ≤ 0.01, n=5 independent, technical replicates). Error bars: SEM. Data normalized to the maximal *LRLRGG-Ub formation by WT UBE2R2. 