Mitochondrial inner membrane calcium uniporter that mediates calcium uptake into mitochondria. Constitutes the pore-forming and calcium-conducting subunit of the uniporter complex (uniplex). Activity is regulated by micu1 and micu2. At low Ca(2+) levels mcu activity is down-regulated by micu1 and micu2; at higher Ca(2+) levels micu1 increases mcu activity. Mitochondrial calcium homeostasis plays key roles in cellular physiology and regulates cell bioenergetics, cytoplasmic calcium signals and activation of cell death pathways. Involved in buffering the amplitude of systolic calcium rises in cardiomyocytes. While dispensable for baseline homeostatic cardiac function, acts as a key regulator of short-term mitochondrial calcium loading underlying a 'fight-or-flight' response during acute stress: acts by mediating a rapid increase of mitochondrial calcium in pacemaker cells. Inhibited by ruthenium red or its derivative Ru360. Component of the uniplex complex (By similarity). Homooligomer; forms a pentamer (By similarity). 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. In absence of SMDT1/EMRE regulator, the calcium ions cannot exit the channel, suggesting that SMDT1/EMRE-binding induces conformational rearrangements to allow calcium to exit. The N-terminal MCU domain is required for efficient Ca(2+) uptake and for interaction with MCUR1. It is not required for targeting to the mitochondria, oligomerization, interaction with MICU1 and MICU2, or assembly of the uniplex complex. Belongs to the MCU (TC 1.A.77) family.