Mitochondrial calcium (Ca2+) uptake plays fundamental roles in various signaling processes. Entry of Ca2+ into the mitochondrial matrix is tightly controlled by the mitochondrial Ca2+ uniporter (MCU) in a higher order complex regulated by inter- and intra-molecular protein-protein interactions. However, the precise mechanisms controlling MCU function and regulation remain largely unknown. I identified a well-folded N-terminal MCU region from residues 72-189, and its crystal structure revealed a β-grasp-like fold with a cluster of acidic residues that facilitates interactions with dibasic cations. Binding of Ca2+ or Mg2+ destabilize this fold and shift the protein towards monomer. Single mutants disrupting the acidic face reduced the cation sensitivity in vitro. Together, my data reveal that the β-grasp-like matrix region of MCU harbors a distinct acidic patch that modulates stability and protein-protein interaction equilibria in response to Mg2+ and Ca2+ binding, suggesting that these matrix cations play an important role in controlling MCU activity
