Figure 3, Key Figure.

The proposed mechanism of SidE-catalyzed ubiquitination. (A) Key residues are included (residue numbers pertain to SdeA). (B) Structure of SidE bound to NAD⁺ and Ub R42A, depicting a likely pre-catalytic state analogous to ADP ribosylation of R42, corresponding to the scheme in A. (C) Structure of SdeD bound to ADPR-Ub, analogous to the phospho-transfer step of the proposed mechanism catalyzed by the PDE domain. (SdeA residue numbers are included in parentheses). Abbreviations: ADPR-Ub, ADP ribosylated ubiquitin; Nic, nicotinamide; Ub, ubiquitin.