ERMA (TMEM94) is a P-type ATPase transporter for Mg²⁺ uptake in the endoplasmic reticulum

Intracellular Mg²⁺ (_iMg²⁺) is bound with phosphometabolites, nucleic acids, and proteins in eukaryotes. Little is known about the intracellular compartmentalization and molecular details of Mg²⁺ transport into/from cellular organelles such as the endoplasmic reticulum (ER). We found that the ER is a major _iMg²⁺ compartment refilled by a largely uncharacterized ER-localized protein, TMEM94. Conventional and AlphaFold2 predictions suggest that ERMA (TMEM94) is a multi-pass transmembrane protein with large cytosolic headpiece actuator, nucleotide, and phosphorylation domains, analogous to P-type ATPases. However, ERMA uniquely combines a P-type ATPase domain and a GMN motif for _ERMg²⁺ uptake. Experiments reveal that a tyrosine residue is crucial for Mg²⁺ binding and activity in a mechanism conserved in both prokaryotic (mgtB and mgtA) and eukaryotic Mg²⁺ ATPases. Cardiac dysfunction by haploinsufficiency, abnormal Ca²⁺ cycling in mouse Erma^+/− cardiomyocytes, and ERMA mRNA silencing in human iPSC-cardiomyocytes collectively define ERMA as an essential component of _ERMg²⁺ uptake in eukaryotes.