MICU proteins facilitate calcium-dependent mitochondrial metabolon formation to regulate cellular energetics independently of MCU

Mitochondrial matrix Ca²⁺ concentration ([Ca²⁺]_m) is theorized to be an essential regulator of mitochondrial metabolism by positively regulating key mitochondrial dehydrogenases. However, ablation or functional inhibition of the mitochondrial calcium uniporter channel (mtCU) fails to significantly perturb basal metabolism and is largely phenotypically silent in the absence of stress. Here we demonstrate that MICU proteins, the reported gatekeepers of mtCU, function in coordination to impart calcium-dependent regulation to FADH₂-dependent mitochondrial dehydrogenases through metabolon formation independently of the mtCU and [Ca²⁺]_m. Our results demonstrate that MICU proteins differentially localize to mitochondrial microdomains and form heterodimers and interactomes in response to intermembrane space Ca²⁺ binding their respective EF-hand domains. Using an equimolar expression platform coupled with unbiased proteomics, we reveal unique interactomes for MICU1/MICU2 versus MICU1/MICU3 heterodimers and demonstrate that MICU proteins control coupling of mitochondrial glycerol-3-phosphate dehydrogenase and succinate dehydrogenase/complex II and impart calcium-dependent changes in activity. We propose that MICU-mediated mitochondrial metabolons are a fundamental system facilitating matching of mitochondrial energy production with cellular demand and is the primary physiological calcium signaling mechanism regulating homeostatic energetics, not mtCU-dependent changes in [Ca²⁺]_m.