Negative regulator of the mitochondrial calcium uniporter (MCU), a channel that mediates calcium uptake into the mitochondrial matrix. MCUB is required to limit mitochondrial calcium overload during stress. Acts as a dominant-negative regulator that displaces MCU from the functional uniplex complex and thereby decreases the association of calcium sensors MICU1 and MICU2, preventing channel gating. Mitochondrial calcium homeostasis plays key roles in mitochondrial metabolism. Acts as an important regulator of mitochondrial metabolism in response to stress in muscle cells: induced in response to fasting, leading to restrict mitochondrial calcium uptake, resulting in reprogramming of mitochondria toward fatty acid oxidation preference. Acts as a regulator of macrophage polarization during skeletal muscle regeneration: inhibition of mitochondrial calcium uptake drives differentiation of macrophages with anti-inflammatory profile, promoting the differentiation and fusion of satellite cells