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. Component of the uniplex complex, composed of MCU, MCUB, MICU1, MICU2 and EMRE/SMDT1 (By similarity). Heterooligomer with MCU; this reduces MCU channel activity. Homooligomer. Detected in lung, brain and heart, and at lower levels in white fat, skeletal muscle and spleen. Detected at very low levels in kidney and liver. Belongs to the MCU (TC 1.A.77) family. Truncated N-terminus.