The mechanism of MICU-dependent gating of the mitochondrial Ca(2+)uniporter

Ca(2+) entry into mitochondria is through the mitochondrial calcium uniporter complex (MCU(cx)), a Ca(2+)-selective channel composed of five subunit types. Two MCU(cx) subunits (MCU and EMRE) span the inner mitochondrial membrane, while three Ca(2+)-regulatory subunits (MICU1, MICU2, and MICU3) reside in the intermembrane space. Here, we provide rigorous analysis of Ca(2+) and Na(+) fluxes via MCU(cx) in intact isolated mitochondria to understand the function of MICU subunits. We also perform direct patch clamp recordings of macroscopic and single MCU(cx) currents to gain further mechanistic insights. This comprehensive analysis shows that the MCU(cx) pore, composed of the EMRE and MCU subunits, is not occluded nor plugged by MICUs during the absence or presence of extramitochondrial Ca(2+) as has been widely reported. Instead, MICUs potentiate activity of MCU(cx) as extramitochondrial Ca(2+) is elevated. MICUs achieve this by modifying the gating properties of MCU(cx) allowing it to spend more time in the open state.