Structural and mutational insights define ERMA as the ER Mg²⁺ ATPase and reservoir gatekeeper

Magnesium (Mg²⁺) is the most abundant divalent cation in cells, yet the mechanisms mediating its organellar transport remain poorly defined. We identify endoplasmic reticulum (ER) Mg²⁺ adenosine triphosphatase (ATPase) (ERMA) as the transporter that drives Mg²⁺ uptake into the ER lumen, establishing the ER as a bi-ionic intracellular reservoir. MagFRET biosensors targeted to the ER demonstrate that ERMA mediates dynamic ER Mg²⁺ storage and robust adenosine 5'-triphosphate-dependent Mg²⁺ uptake reaching 15 to 30 millimolar. Cryo-electron microscopy structures of human and mouse ERMA reveal a P-type ATPase fold with an unwound transmembrane 4 (TM4) that coordinates Mg²⁺ via the unique PILP backbone and the TM5 residue Q1110, whose mutation markedly impairs ERMA-mediated Mg²⁺ uptake. Functional reconstitution of domain mutants, ERMA-SERCA chimeras, and pathogenic variants confirm ERMA as an ER-resident Mg²⁺ pump and gatekeeper of ER Mg²⁺ ionic equilibrium.