Key regulator of mitochondrial calcium uniporter (MCU) that senses calcium level via its EF-hand domains. MICU1 and MICU2 form a disulfide-linked heterodimer that stimulates and inhibits MCU activity, depending on the concentration of calcium. MICU1 acts both as an activator or inhibitor of mitochondrial calcium uptake. Acts as a gatekeeper of MCU at low concentration of calcium, preventing channel opening. Enhances MCU opening at high calcium concentration, allowing a rapid response of mitochondria to calcium signals generated in the cytoplasm. Regulates glucose-dependent insulin secretion in pancreatic beta-cells by regulating mitochondrial calcium uptake. Induces T-helper 1-mediated autoreactivity, which is accompanied by the release of IFNG. Homohexamer; in absence of calcium. Forms a homohexamer in absence of calcium and rearranges into a heterodimer in presence of calcium. Heterodimer; disulfide-linked; heterodimerizes with MICU2. The heterodimer formed with MICU2 associates with MCU at low calcium concentration and dissociates from MCU at high calcium level. Component of the uniplex complex, composed of MCU, MCUB, MICU1, MICU2 and EMRE/SMDT1. Interacts (via polybasic region) with EMRE/SMDT1; the interaction is direct. Interacts (via polybasic region) with MCU (via coiled coil domains); the interaction is direct and precedes formation of the heterodimer with MICU2. Interacts with SLC25A23. Interacts with CHCHD4/MIA40; which introduces the interchain disulfide bond with MICU2. The topology is subject to debate. The C-helix is required for assembling the Ca(2+)-free homohexamer. It also plays a key role in mitochondrial calcium uptake, probably by mediating interaction with MICU2. The EF-hand domains have high affinity for calcium and act as sensors of calcium levels. Belongs to the MICU1 family. MICU1 subfamily.