A Regulatory Complex of the Mitochondrial Uniporter Ca2+ Channel

Ca2+ flux across the mitochondrial inner membrane regulates bioenergetics, cytoplasmic Ca2+ signals and cell death pathways. Mitochondrial Ca2+ uptake occurs at regions of close apposition with intracellular Ca2+ release sites, driven by the inner membrane voltage generated by oxidative phosphorylation (OXPHOS) and mediated by a Ca2+ selective ion channel (MiCa) called the uniporter whose complete molecular identity remains unknown. Mitochondrial calcium uniporter (MCU) was identified as the likely ion-conducting pore. Mitochondrial matrix Ca2+ concentration is maintained 5-6 orders of magnitude lower than its thermodynamic equilibrium level, but the molecular mechanisms for how this is achieved are not clear. We found that MICU1, a mitochondrial protein previously suggested to be required for uniporter-mediated Ca2+ uptake, is instead required to preserve normal [Ca2+]m under basal conditions. In its absence, mitochondria become constitutively loaded with Ca2+, triggering excessive reactive oxygen species generation and sensitivity to apoptotic stress. MICU1 interacts with MCU and sets a Ca2+ threshold for Ca2+m uptake without affecting kinetic properties of MCU-mediated Ca2+ uptake, a regulation that requires both functional Ca2+ binding EF hands in MICU1. Thus, MICU1 is a gatekeeper of MCU-mediated Ca2+m uptake that is essential to prevent [Ca2+]m overload and associated stress. Additionally, we have also identified CCDC90A, hereafter referred to as MCUR1 (Mitochondrial Calcium Uniporter Regulator 1), as an inner mitochondrial membrane protein required for MCU-dependent mitochondrial Ca2+ uptake. MCUR1 binds to MCU and regulates ruthenium red-sensitive MCU-dependent Ca2+ uptake. MCUR1 knockdown does not alter MCU localization, but abrogates Ca2+ uptake by energized mitochondria in intact and permeabilized cells. Ablation of MCUR1 disrupts OXPHOS, lowers cellular ATP, and activates AMP kinase-dependent pro-survival autophagy. Thus, MCUR1 and MICU1 are critical components of a mitochondrial uniporter channel complex required for mitochondrial Ca2+ uptake and maintenance of normal cellular bioenergetics.