Mitochondrial inner membrane calcium uniporter that mediates calcium uptake into mitochondria (PubMed:24889638). Constitutes a pore-forming and calcium-conducting subunit (PubMed:24889638). Mitochondrial calcium homeostasis plays key roles in cellular physiology and regulates cell bioenergetics, cytoplasmic calcium signals and activation of cell death pathways. Inhibited by ruthenium red or its derivative Ru360. 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: 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. The inner core 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). The critical DXXE motif connecting the transmembrane regions forms a pentameric barrel that constitutes the mouth of the pore. Inside the barrel, two acidic residues are in position to form two carboxylate rings. Belongs to the MCU (TC 1.A.77) family.