Negatively regulates the activity of MCU, the mitochondrial inner membrane calcium uniporter, and thereby modulates calcium uptake into the mitochondrion. Does not form functional calcium channels by itself. Mitochondrial calcium homeostasis plays key roles in cellular physiology and regulates cell bioenergetics, cytoplasmic calcium signals and activation of cell death pathways. Heterooligomer with MCU; this reduces MCU channel activity. Homooligomer (By similarity). Component of the uniplex complex, composed of MCU, MCUB, MICU1, MICU2 and EMRE/SMDT1. Belongs to the MCU (TC 1.A.77) family.