Crystal structure of human USB1. (A-B) Ribbon representation of human USB1. The active site residues H208, S210, H120, and S122 are shown. The 2 conformers for H120 and S122 observed in the electron density map are indicated. The terminal (magenta) and transit (cyan) lobes are labeled. Figures were prepared using the program PyMOL (http://www.pymol.org). (C-D) Representation of the electrostatic surface potential of human USB1, calculated by the program APBS,⁴²  and colored using a linear color ramp from − 7.0 kT (red) to + 7 kT (blue). The orientation of USB1 in panels C and D is identical to panels A and B, respectively. White arrow indicates the active site. (E) Section of the σ_A-weighted, solvent flattened 2Fo-Fc electron density map contoured at 0.75 σ for residues F119 (left) to L123 (right), indicating the presence of 2 conformations for the H120 and S122 side-chains. The figure was generated using CCP4mg.⁴³  (F) Schematic diagram of USB1 secondary structure topology. Secondary structure elements are shown as arrows (β-strands) and cylinders (α-helices). Transit lobe is colored cyan and the terminal lobe, magenta.