The Mechanism of MICU-Dependent Gating of the Mitochondrial Ca 2+ Uniporter

Mitochondrial Ca 2+ uniporter (MCU) mediates mitochondrial Ca 2+ uptake, regulating ATP production and cell death. According to the existing paradigm, MCU is occluded at the resting cytosolic [Ca 2+ ] and only opens above an ∼400 nM threshold. This Ca 2+ -dependent gating is putatively conferred by MICUs, EF hand-containing auxiliary subunits that block/unblock the MCU pore depending on cytosolic [Ca 2+ ]. Here we provide the first direct, patch-clamp based analysis of the Ca 2+ -dependent MCU gating and the role played by MICUs. Surprisingly, MICUs do not occlude the MCU pore, and MCU is a constitutively active channel without cytosolic [Ca 2+ ] activation threshold. Instead, MICUs potentiate MCU activity when cytosolic Ca 2+ binds to their EF hands. MICUs cause this potentiation by increasing the probability of open state of the MCU channel. One Sentence Summary Auxiliary MICU subunits do not occlude the mitochondrial Ca 2+ uniporter (MCU) but increase its activity as cytosolic Ca 2+ is elevated.