Ribbon representations of the A and B subunits of UBA5. a, catalytic cysteine (Cys250), ATP, and boundaries of partially disordered loop regions (β2–α3 and β6–α6) are labeled. Helices, strands, and loops are colored cyan, green, and yellow, respectively. b, stereoscopic representation of the catalytic cysteine and its environs. Van der Waals interactions are shown as dashed black lines. The disordered region of the crossover loop is shown as a dashed blue line, and the hydrogen bond between the catalytic cysteine sulfur and the Asn210 main chain carbonyl is shown as a dashed red line. Secondary structures are labeled; B subunit labels are primed. Water molecules are shown as red stars. N, N-terminal; C, C-terminal.