| Summary: | Summary: Calcium is a key regulator of mitochondrial function under both normal and pathological conditions. The mechanisms linking metabolic activity to mitochondrial Ca2+ signaling remain elusive, however. Here, by monitoring mitochondrial Ca2+ transients while manipulating mitochondrial membrane potential (ΔΨm), we found that mild fluctuations in ΔΨm, which do not affect Ca2+ influx, are sufficient to strongly regulate NCLX, the major efflux pathway of Ca2+ from the mitochondria. Phosphorylation of NCLX or expression of phosphomimicking mutant (S258D) rescued NCLX activity from ΔΨm-driven allosteric inhibition. By screening ΔΨm sensitivity of NCLX mutants, we also identified amino acid residues that, through functional interaction with Ser258, control NCLX regulation. Finally, we find that glucose-driven ΔΨm changes in pancreatic β-cells control mitochondrial Ca2+ signaling primarily via NCLX regulation. Our results identify a feedback control between metabolic activity and mitochondrial Ca2+ signaling and the “safety valve” NCLX phosphorylation that can rescue Ca2+ efflux in depolarized mitochondria. : Kostic et al. report that mild mitochondrial depolarization is sufficient to allosterically regulate [Ca2+]mito efflux by NCLX. Downregulation of NCLX is mediated by a cluster of positively charged residues and NCLX activity is rescued by phosphorylation of Ser258. Regulation of NCLX by ΔΨm links metabolic activity and [Ca2+]mito signaling. Keywords: NCLX, mitochondrial Ca2+ signaling, mitochondrial membrane potential
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