Mitochondrial inner membrane calcium uniporter that mediates calcium uptake into mitochondria (PubMed:25313960, PubMed:31983639). Constitutes a pore-forming and calcium-conducting subunit (By similarity). Mitochondrial calcium homeostasis plays key roles in cellular physiology and regulates cell bioenergetics, cytoplasmic calcium signals and activation of cell death pathways (By similarity). Required for rapid mitochondrial calcium uptake and mitochondrial reactive oxygen species (mtROS) production after wounding (PubMed:25313960). In addition, together with mitochondrial calcium regulator micu-1, required for mitochondrial calcium uptake following axon injury in PLM touch receptor neurons (PubMed:31983639). Inhibited by ruthenium red or its derivative Ru360; possibly by obstructing the pore. Forms a well-packed pentamer with an overall cylindrical shape. The inner core of the pentamer is formed with the second transmembrane region and the second coiled-coil region (residues 293-316): while the transmembrane regions pack into a five-helix bundle having a largely polar pore across the membrane, the coiled-coil outside the membrane forms a pentamer with a hydrophobic core, which may contribute to stabilizing the transmembrane pore structure. The transmembrane pore is wrapped by the first transmembrane region through contacts between the first and the second transmembrane regions. The second transmembrane is followed by the inner juxtamembrane region (IJMH) that orients at a wide angle relative to the second transmembrane. The two core domains are held together on the periphery by the outer juxtamembrane helix (OJMH), which contains a kink around Glu-204 and interacts with the IJMH. The critical DXXE motif connecting the transmembrane regions forms a pentameric barrel that constitutes the mouth of the pore. Inside the barrel, both acidic residues are in position to form two carboxylate rings: the Asp-240 ring is solvent exposed, and the Glu-243 ring is located deeper, guarding the entrance of the second transmembrane pore. In absence of emre-1 regulator, the calcium ions cannot exit the channel, suggesting that emre-1-binding induces conformational rearrangements to allow calcium to exit. Null mutants are viable and fertile, but have strongly impaired mitochondrial calcium uptake following laser or needle wounding (PubMed:25313960). Reduces the wound-induced transient elevations in mitochondrial reactive oxygen species superoxide (which are known as mitochondrial flashes or mitoflashes) (PubMed:25313960). Double knockout with the srb-13 xm1 mutant suppresses the sperm navigation defect in the srb-13 single mutant (PubMed:28662030). Belongs to the MCU (TC 1.A.77) family.