LAUSR

MICU1 is a Ca 2+ -sensor regulating the mitochondrial Ca 2+ uniporter channel (mtCU) and mitochondrial Ca 2+ uptake. MICU1 contains two Ca 2+ - binding EF-hand domains and directly interacts with the mtCU pore-forming subunit, MCU. MICU1-null mouse and fly models display perinatal lethality with disorganized mitochondrial architecture. Interestingly, these phenotypes are distinct from other mtCU loss-of-function models ( MCU, MICU2, EMRE, MCUR1 ) and thus are likely not explained solely by changes in matrix Ca 2+ content, suggesting MICU1 has cellular functions independent of the mtCU. To discern novel MICU1 molecular interactions we utilized a proximity-based biotinylation proteomic approach, BioID2, and expressed a MICU1-BioID2 fusion construct in wild-type and MCU -/- cells. LC-MS analysis of purified biotinylated proteins identified Mitochondrial Contact Site and Cristae Organizing System (MICOS) components MIC60, CHCHD2, and CHCHD3 as MICU1 interacting partners. Loss of MCU did not affect the MICU1-MICOS interaction, suggesting that MICU1 could be regulating the MICOS independent of the mtCU. Fast protein liquid chromatography (FPLC), blue native-PAGE, co-immunoprecipitation, live-cell Ca 2+ imaging, electron microscopy, confocal and super-resolution imaging confirmed that MICU1 (independent of matrix Ca 2+ ) is regulating mitochondrial membrane dynamics, membrane potential, and cell death signaling. Further, the deletion of Chchd2, a core component of the MICOS, resulted in disruption of cristae structure without any observable effect on mitochondrial Ca 2+ uptake. In addition, RNA sequencing of murine models of heart failure revealed a correlation between the expression of MICU1 and MICOS components and disease progression. These results are the first to identify how Ca 2+ regulates cristae structure and function and suggests MICU1, at the MICOS, regulates mitochondrial membrane remodeling in the context of cardiac stress and disease independent of the mtCU.