Fig. 1. Domain architecture and structural features of MCU.

(See also Fig. S1, Fig. S3, Fig. S7 and Table S1).

(A) Domain architecture of human MCU (NCBI, Q8NE86). Residue ranges and the prevailing topology are indicated at top. The relative location of the signal peptide (SP, blue), transmembrane domains (TM1, TM2, cyan), CC domains (CC1, CC2, yellow) and MCU_72-189 (green) are indicated.

(B) Backbone structure of MCU_72-189. The β-strands (β1-β6, arrows) and loop regions (L1-L6) are colored green while the α-helices (α1, α2) are pink. L2 is bounded by a dashed box.

(C) Zoomed view of L2. The R93 and E95 side chains are shown as red and blue sticks, respectively. The C97 and S92 residues are shown as yellow sticks.

(D) Electrostatic surface potential of MCU_72-189. The gradient shown is from −3 (acidic, red) to +3 (basic, blue) kT/e. Locations of acidic and basic residues are indicated.

(E) Location of the Mg²⁺ atom (yellow sphere) relative to the backbone structure and acidic residues (red sticks). The dashed box bounds the acidic patch region.

(F) Zoomed view of the acidic patch shown in (E) and centrally located Mg²⁺ atom (yellow sphere).

(G) Mg²⁺ atom coordination geometry. The 2Fo-Fc (2.0 σ) electron density map (grey mesh), protein chain (sticks), water molecules (orange spheres) and Mg²⁺ atom (yellow sphere) are shown. The Fo-Fc (9.0 σ) omit map (magenta mesh) shows unaccounted for density in the absence of the Mg²⁺ atom. Distances between the Asp (red sticks) Cγ atoms are indicated. The coordination geometry of Mg²⁺ is shown with distances to oxygen atom indicated.

(H) Electron density map of MCU_72-189 solved in the presence of LiSO₄. The 2Fo-Fc (2.0 σ) map (grey mesh) of the same region shown in (G) does not exhibit evidence for a cation, and the Fo-Fc (2.5 σ) map (magenta mesh) shows no unaccounted for density.