Calcium uptake protein 2, mitochondrial
calcium ion sensor activity
High-affinity cooperative Ca<sup>2+</sup> binding by MICU1-MICU2 serves as an on-off switch for the uniporter.
The mitochondrial calcium uniporter is a Ca<sup>2+</sup>-activated Ca<sup>2+</sup> channel that is essential for dynamic modulation of mitochondrial function in response to cellular Ca<sup>2+</sup> signals. It is regulated by two paralogous EF-hand proteins-MICU1 and MICU2, but the mechanism is unknown. Here, we demonstrate that both MICU1 and MICU2 are stabilized by Ca<sup>2+</sup> We reconstitute the MICU1-MICU2 heterodimer and demonstrate that it binds Ca<sup>2+</sup> cooperatively with high affinity. We discover that both MICU1 and MICU2 exhibit affinity for the mitochondria-specific lipid cardiolipin. We determine the minimum Ca<sup>2+</sup> concentration required for disinhibition of the uniporter in permeabilized cells and report a close match with the Ca<sup>2+</sup>-binding affinity of MICU1-MICU2. We conclude that cooperative, high-affinity interaction of the MICU1-MICU2 complex with Ca<sup>2+</sup> serves as an on-off switch, leading to a tightly controlled channel, capable of responding directly to cytosolic Ca<sup>2+</sup> signals.