Elucidating the Mechanism Underlying UBA7•UBE2L6 Disulfide Complex Formation

We elucidate cryo-EM structure and formation of the ubiquitin-associated bovine UBA7•UBE2L6 disulfide complex, shedding light on a highly specific and evolutionarily conserved mechanism governing ISG15 conjugation, a pivotal process in the immune response. UBA7 displays a unique capacity to recognize UBE2L6, distinct from this latter’s homolog UBE2L3, highlighting the intricacies of cellular regulation. Inter-species interactions of the resulting complex further underscore its significance. We characterize three crucial factors that influence UBA7•UBE2L6 disulfide complex formation: (1) strong binding affinity and specificity; (2) conformational differences in the catalytic cysteine capping loop (CCL); and (3) increased thiolate/thiol ratios at catalytic cysteines. Modification of any of these factors profoundly impacts complex activation and the ISG15 transfer cascade. This redox-sensitive complex implies a link between oxidative stress and regulation of the immune response, highlighting a potential therapeutic target for modulating immune reactions arising from infections and inflammatory conditions.