Fig. 2. Structures of E2-fold proteins. (A–C) Ribbon representations of Tsg101 UEV (A), Mms2 (B) and Ubc13 (C). Secondary structure elements are colored as in Figure 1. Black arrows indicate residues that correspond to the active site cysteine of Ubc13. (D) Superposition of the three structures. Tsg101 is colored yellow, Mms2 green and Ubc13 purple. The three major structural differences between Tsg101 and the other two proteins are highlighted by arrows: the extra N-terminal helix (red arrow), the β-hairpin ‘tongue’ (blue arrow) and the missing C-terminal helices (green arrow).