Figure 2. Crystal structure of E. coli Ubl₃.

(a) Domain structure of E. coli Ubl₃. (b) Crystal structure of an E. coli Ubl₃ dimer, with one protomer colored dark orange/light orange/light yellow and the second protomer colored dark blue/medium blue/light blue. Bound Ca²⁺ ions are shown as gray spheres. (c) Structure of H. sapiens ubiquitin (PDB ID 1UBQ) ³⁶, colored as a rainbow from N-terminus (blue) to C-terminus (red) and with β-strands 1–5 labeled. (d) Structures of E. coli Ubl₃ β-grasp domains 1–3, colored as in panel (b) and aligned with the structure of H. sapiens ubiquitin. (e) Close-up of the intra-dimer Ca²⁺ ion bound by E. coli Ubl₃ indicated by a dotted box in panel (b). See Figure S3a for a sequence alignment of Ubl₃ β-grasp domains 1–2 showing conservation of this site.