Kinetics and Mechanism of Mammalian Mitochondrial Ribosome Assembly

Mammalian mtDNA encodes only 13 proteins, all essential components of respiratory complexes, synthesized by mitochondrial ribosomes. Mitoribosomes contain greatly truncated RNAs transcribed from mtDNA, including a structural tRNA in place of 5S RNA as a scaffold for binding 82 nucleus-encoded proteins, mitoribosomal proteins (MRPs). Cryoelectron microscopy (cryo-EM) studies have determined the structure of the mitoribosome, but its mechanism of assembly is unknown. Our SILAC pulse-labeling experiments determine the rates of mitochondrial import of MRPs and their assembly into intact mitoribosomes, providing a basis for distinguishing MRPs that bind at early and late stages in mitoribosome assembly to generate a working model for mitoribosome assembly. Mitoribosome assembly is a slow process initiated at the mtDNA nucleoid driven by excess synthesis of individual MRPs. MRPs that are tightly associated in the structure frequently join the complex in a coordinated manner. Clinically significant MRP mutations reported to date affect proteins that bind early on during assembly.