The Mechanism of MICU-Dependent Gating of theMitochondrial Ca2+ Uniporter

Mitochondrial Ca2+ uniporter (MCU) mediates mitochondrial Ca2+ uptake, regulating ATP production and cell death. According to the existing paradigm, MCU is occluded at the resting cytosolic [Ca2+] and only opens above an [~]400 nM threshold. This Ca2+-dependent gating is putatively conferred by MICUs, EF hand-containing auxiliary subunits that block/unblock the MCU pore depending on cytosolic [Ca2+]. Here we provide the first direct, patch-clamp based analysis of the Ca2+-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 [Ca2+] activation threshold. Instead, MICUs potentiate MCU activity when cytosolic Ca2+ binds to their EF hands. MICUs cause this potentiation by increasing the probability of open state of the MCU channel. One Sentence SummaryAuxiliary MICU subunits do not occlude the mitochondrial Ca2+ uniporter (MCU) but increase its activity as cytosolic Ca2+ is elevated.