Enables protein heterodimerization activity. Involved in calcium import into the mitochondrion and negative regulation of mitochondrial calcium ion concentration. Located in mitochondrial inner membrane and mitochondrial intermembrane space. Part of uniplex complex. Key regulator of mitochondrial calcium uniporter (MCU) required to limit calcium uptake by MCU when cytoplasmic calcium is low. MICU1 and MICU2 form a disulfide-linked heterodimer that stimulate and inhibit MCU activity, depending on the concentration of calcium. MICU2 acts as a gatekeeper of MCU that senses calcium level via its EF-hand domains: prevents channel opening at resting calcium, avoiding energy dissipation and cell-death triggering.