Coupled transmembrane mechanisms control MCU-mediated mitochondrial Ca2+uptake
Ca2+uptake by mitochondria regulates bioenergetics, apoptosis, and Ca2+signaling. The primary pathway for mitochondrial Ca2+uptake is the mitochondrial calcium uniporter (MCU), a Ca2+-selective ion channel in the inner mitochondrial membrane. MCU-mediated Ca2+uptake is driven by the sizable inner-membrane potential generated by the electron-transport chain. Despite the large thermodynamic driving force, mitochondrial Ca2+uptake is tightly regulated to maintain low matrix [Ca2+] and prevent opening of the permeability transition pore and cell death, while meeting dynamic cellular energy demands. How this is accomplished is controversial. Here we define a regulatory mechanism of MCU-channel activity in which cytoplasmic Ca2+regulation of intermembrane space-localized MICU1/2 is controlled by Ca2+-regulatory mechanisms localized across the membrane in the mitochondrial matrix. Ca2+that permeates through the channel pore regulates Ca2+affinities of coupled inhibitory and activating sensors in the matrix. Ca2+binding to the inhibitory sensor within the MCU amino terminus closes the channel despite Ca2+binding to MICU1/2. Conversely, disruption of the interaction of MICU1/2 with the MCU complex disables matrix Ca2+regulation of channel activity. Our results demonstrate how Ca2+influx into mitochondria is tuned by coupled Ca2+-regulatory mechanisms on both sides of the inner mitochondrial membrane.