Essential regulatory subunit of the mitochondrial calcium uniporter complex (uniplex), a complex that mediates calcium uptake into mitochondria. Plays a central role in regulating the uniplex complex response to intracellular calcium signaling. Component of the uniplex complex. Interacts (via the transmembrane region) with mcu (via the first transmembrane region); the interaction is direct. Belongs to the SMDT1/EMRE family.