Mitoregulin: A lncRNA-Encoded Microprotein that Supports Mitochondrial Supercomplexes and Respiratory Efficiency

Mitochondria are composed of many small proteins that control protein synthesis, complex assembly, metabolism, and ion and reactive oxygen species (ROS) handling. We show that a skeletal muscle- and heart-enriched long non-coding RNA, LINC00116, encodes a highly conserved 56-aminoacid microprotein that we named mitoregulin (Mtln). Mtln localizes to the inner mitochondria! membrane, where it binds cardiolipin and influences protein complex assembly. In cultured cells, Mtln overexpression increases mitochondria! membrane potential, respiration rates, and Ca2+ retention capacity while decreasing mitochondria! ROS and matrix-free Ca2+. Mtln-knockout mice display perturbations in mitochondrial respiratory (super)complex formation and activity, fatty acid oxidation, tricarboxylic acid (TCA) cycle enzymes, and Ca2+ retention capacity. Blue-native gel electrophoresis revealed that Mtln co-migrates alongside several complexes, including the complex I assembly module, complex V, and supercomplexes. Under denaturing conditions, Mtln remains in high-molecular-weight complexes, supporting its role as a sticky molecular tether that enhances respiratory efficiency by bolstering protein complex assembly and/or stability.