Mechanisms of ion selectivity and throughput in the mitochondrial calcium uniporter

The mitochondrial calcium uniporter, which regulates aerobic metabolism by catalyzing mitochondrial Ca²⁺ influx, is arguably the most selective ion channel known. The mechanisms for this exquisite Ca²⁺ selectivity have not been defined. Here, using a reconstituted system, we study the electrical properties of the channel's minimal Ca²⁺-conducting complex, MCU-EMRE, from Tribolium castaneum to probe ion selectivity mechanisms. The wild-type TcMCU-EMRE complex recapitulates hallmark electrophysiological properties of endogenous Uniporter channels. Through interrogation of pore-lining mutants, we find that a ring of glutamate residues, the "E-locus," serves as the channel's selectivity filter. Unexpectedly, a nearby "D-locus" at the mouth of the pore has diminutive influence on selectivity. Anomalous mole fraction effects indicate that multiple Ca²⁺ ions are accommodated within the E-locus. By facilitating ion-ion interactions, the E-locus engenders both exquisite Ca²⁺ selectivity and high ion throughput. Direct comparison with structural information yields the basis for selective Ca²⁺ conduction by the channel.