The mitochondrial calcium uniporter is a multisubunit Ca2+ channel that mediates mitochondrial Ca2+ uptake, a cellular process crucial for the regulation of oxidative phosphorylation, intracellular Ca2+ signaling, and apoptosis. In the last few years, genes encoding uniporter proteins have been identified, but a lack of efficient tools for electrophysiological recordings has hindered quantitative analysis required to determine functional mechanisms of this channel complex. Here, we redirected Ca2+-conducting subunits (MCU and EMRE) of the human uniporter to the plasma membrane of Xenopus oocytes. Two-electrode voltage clamp reveals inwardly rectifying Ca2+ currents blocked by a potent inhibitor, Ru360 (half maximal inhibitory concentration, ~4 nM), with a divalent cation conductivity of Ca2+ > Sr2+ > Ba2+, Mn2+, and Mg2+. Patch clamp recordings further reveal macroscopic and single-channel Ca2+ currents sensitive to Ru360. These electrical phenomena were abolished by mutations that perturb MCU-EMRE interactions or disrupt a Ca2+-binding site in the pore. Altogether, this work establishes a robust method that enables deep mechanistic scrutiny of the uniporter using classical strategies in ion channel electrophysiology.

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