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.