The conserved aspartate ring of MCU mediates MICU1 binding and regulation in the mitochondrial calcium uniporter complex.

The mitochondrial calcium uniporter is a Ca²⁺ channel that regulates intracellular Ca²⁺ signaling, oxidative phosphorylation, and apoptosis. It contains the pore-forming MCU protein, which possesses a DIME sequence thought to form a Ca²⁺ selectivity filter, and also regulatory EMRE, MICU1, and MICU2 subunits. To properly carry out physiological functions, the uniporter must stay closed in resting conditions, becoming open only when stimulated by intracellular Ca²⁺ signals. This Ca²⁺-dependent activation, known to be mediated by MICU subunits, is not well understood. Here, we demonstrate that the DIME-aspartate mediates a Ca²⁺-modulated electrostatic interaction with MICU1, forming an MICU1 contact interface with a nearby Ser residue at the cytoplasmic entrance of the MCU pore. A mutagenesis screen of MICU1 identifies two highly-conserved Arg residues that might contact the DIME-Asp. Perturbing MCU-MICU1 interactions elicits unregulated, constitutive Ca²⁺ flux into mitochondria. These results indicate that MICU1 confers Ca²⁺-dependent gating of the uniporter by blocking/unblocking MCU.

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