B Schematic of METRIS assay set-up and parameter analysis to assess the binding between UBE2R2 and Ub. In brief, biotinylated Ub is linked to a magnetic bead that is introduced to a magnetic field while on a UBE2R2 functionalized coverslip. The movement of each bead is quantified, and the rolling parameter correlates to binding affinities between Ub and E2. 