The mechanism of MICU-dependent gating of the mitochondrial Ca²⁺uniporter.

Abstract

Ca²⁺ entry into mitochondria is through the mitochondrial calcium uniporter complex (MCU_cx), a Ca²⁺-selective channel composed of five subunit types. Two MCU_cx subunits (MCU and EMRE) span the inner mitochondrial membrane, while three Ca²⁺-regulatory subunits (MICU1, MICU2, and MICU3) reside in the intermembrane space. Here, we provide rigorous analysis of Ca²⁺ 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 Ca²⁺ as has been widely reported. Instead, MICUs potentiate activity of MCU_cx as extramitochondrial Ca²⁺ is elevated. MICUs achieve this by modifying the gating properties of MCU_cx allowing it to spend more time in the open state.