Positively regulates mitochondrial complex assembly and/or stability (PubMed:29949756). Increases mitochondrial membrane potential while decreasing mitochondrial reactive oxygen species (By similarity). Increases mitochondrial respiration rate (PubMed:31296841). Increased mitochondrial respiratory activity promotes myogenic differentiation which facilitates muscle growth and regeneration (PubMed:31296841, PubMed:32393776). Increases mitochondrial calcium retention capacity (PubMed:29949756). Plays a role in maintenance of cellular lipid composition through its interaction with cytochrome b5 reductase CYB5R3 which is required for mitochondrial respiratory complex I activity (PubMed:30796188). Interacts with the mitochondrial trifunctional enzyme complex (MTE) and enhances fatty acid beta-oxidation (PubMed:29949755, PubMed:32243843). Not required for MTE formation or stability (PubMed:29949755). Modulates triglyceride clearance in adipocytes through its role in regulating fatty acid beta-oxidation and lipolysis (PubMed:32243843). Interacts with mitochondrial trifunctional enzyme, a heterotetrameric complex composed of 2 HADHA subunits and 2 HADHB subunits (PubMed:29949755). Interacts with cytochrome b5 reductase CYB5R3; the interaction is required to maintain cellular lipid composition and leads to stimulation of mitochondrial respiratory complex I activity (PubMed:31296841). Interacts with ATP synthase subunit ATP5F1B/ATP5B (By similarity). Preferentially binds to cardiolipin relative to other common cell membrane lipids. Enriched in heart and skeletal muscle (at protein level) (PubMed:29949755, PubMed:29949756, PubMed:31296841). Also enriched in adipose tissue with lower levels detected in liver, pancreas and brain (at protein level) (PubMed:29949756). Higher levels in differentiated myotubes than in satellite cells (PubMed:32393776). Levels increase after birth and remain high at postnatal days 14 and 21. No visible phenotype (PubMed:29949756). Mutants are viable and develop normally with no overt phenotypes later in life (PubMed:29949756). In the fed state, no effect on mitochondrial respiratory activities in cardiac or skeletal muscle fibers but, in fasted mutants, skeletal muscle fibers and left ventricular fibers show significantly lower rates of maximal fatty acid oxidation relative to wild-type controls (PubMed:29949756). Left ventricular fibers also show reduced mitochondrial calcium retention capacity (PubMed:29949756). Aberrant distribution of respiratory chain complex I (CI) with distinct loss of a high-molecular weight supercomplex and concomitant increase in free or disassembled CI-associated complexes (PubMed:29949756). Severely decreased presence of dimers of the mitochondrial 2-oxoglutarate dehydrogenase OGDH and the mitochondrial acyl-CoA dehydrogenase ACAD9 (PubMed:29949756). Significantly altered distribution of the mitochondrial acyl-CoA dehydrogenase ACADVL/VLCAD (PubMed:29949756). Impaired muscle formation, muscle weakness and impaired muscle regeneration due to defects in satellite cell differention (PubMed:32393776). Belongs to the mitoregulin family.