The backside β-turn is a key structural element of Rad6-family E2 ubiquitin-conjugating enzymes.

Protein ubiquitination regulates diverse cellular processes, and its dysregulation contributes to human disease, including cancer. E2 ubiquitin-conjugating enzymes share a conserved UBC fold in which surface loops fine-tune catalysis and partner interactions, yet the roles of individual loops remain incompletely defined. Here, we identify loop 3-a component of the "backside" β2-β3 hairpin-as a conserved structural and allosteric element in Rad6-family E2s. Structural and bioinformatic analyses of yeast Rad6 and its human homologs (UBE2A/UBE2B) reveal that loop 3 forms an overlapping triple β-turns, with variable first turn and a highly conserved second/third turn that links catalytic regulation to E3 ligase engagement. Systematic mutagenesis of the yeast Rad6 backside β-turn (residues 42-51) shows that this element is required in vivo for Bre1-dependent histone H2B Lys123 monoubiquitination, Rad18-dependent PCNA monoubiquitination, and Ubr1/Ubr2-dependent polyubiquitination and degradation of Sml1 and N-end rule substrates, and related biological processes. Charge-reversal mutations at backside β-turn Glu49 and Asp50 disrupt E3 binding, whereas cancer-relevant substitutions in kink-inducing prolines (Pro43/Pro47) impair mono- and polyubiquitination without abolishing E3 interactions. Certain backside β-turn mutations, including cancer-relevant variants, compromise steady-state levels following DNA damage, revealing them as conditional null or loss-of-function alleles. Nuclear magnetic resonance spectroscopy demonstrates that Pro43/Pro47 mutations induce long-range structural perturbations from backside β-turn into the front-face catalytic pocket, correlating with reduced in vitro ubiquitination activity. Deletion or alanine replacement of the β-turn destabilizes yeast Rad6 and human UBE2A/UBE2B. Together, these findings establish the loop 3/backside β-turn as a critical structural element of Rad6-family enzymes.