Strongly coupled transmembrane mechanisms control MCU-mediated mitochondrial Ca 2+ uptake

Ca uptake by mitochondria regulates bioenergetics, apoptosis, and Ca signaling. The primary pathway for mitochondrial Ca uptake is the mitochondrial calcium uniporter (MCU), a Ca-selective ion channel in the inner mitochondrial membrane. MCU-mediated Ca uptake is driven by the sizable inner-membrane potential generated by the electron-transport chain. Despite the large thermodynamic driving force, mitochondrial Ca uptake is tightly regulated to maintain low matrix [Ca] 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 Ca regulation of intermembrane space-localized MICU1/2 is controlled by strongly-coupled Ca-regulatory mechanisms localized across the membrane in the mitochondrial matrix. Ca that permeates through the channel pore regulates Ca affinities of coupled inhibitory and activating sensors in the matrix. Ca binding to the inhibitory sensor within the MCU amino-terminus closes the channel despite Ca binding to MICU1/2. Conversely, disruption of the interaction of MICU1/2 with the MCU complex abolishes matrix Ca regulation of channel activity. Our results demonstrate how Ca influx into mitochondria is tuned by coupled Ca-regulatory mechanisms on both sides of the inner mitochondrial membrane.