Mitochondrial inner membrane calcium uniporter that mediates calcium uptake into mitochondria (PubMed:28726639). Constitutes a pore-forming and calcium-conducting subunit (PubMed:28726639). Mitochondrial calcium homeostasis plays key roles in cellular physiology and regulates cell bioenergetics, cytoplasmic calcium signals and activation of cell death pathways (PubMed:28726639). Together with Itpr, has a role in oxidative stress-induced ER-mitochondria calcium transfer (PubMed:28726639). During pupation, required for memory function in mushroom body neurons (PubMed:27568554). Inhibited by ruthenium red or its derivative Ru360. Expressed throughout development, from embryo to adult stage (at protein level). 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. Mitochondrial calcium uptake defects (PubMed:28726639). RNAi-mediated knockdown in neurons and mushroom body neurons, from third instar larva to pupation stage, results in impaired mitochondrial calcium entry, decreased synaptic vesicle content, increased mushroom body neuron length and field volume which lead to decreased intermediate-term memory after conditioning (PubMed:27568554). Does not affect olfactory learning (PubMed:27568554). Belongs to the MCU (TC 1.A.77) family. It is unclear whether the absence of MCU causes lethality (PubMed:28726639, PubMed:27568554). According to a report based on a genetic knockout, it is viable (PubMed:28726639). According to a report based on RNAi-mediated knockdown, it is lethal (PubMed:27568554).