The mitochondrial ribosome, or mitoribosome, is a protein complex that is active in mitochondria and functions as a riboprotein for translating mitochondrial mRNAs encoded in mtDNA. Mitoribosomes, like cytoplasmic ribosomes, consist of two subunits — large (mtLSU) and small (mt-SSU). However, the ratio of rRNA/protein is different from cytoplasmic ribosomes. Mitoribosomes consist of several specific proteins and less rRNAs.
Mitochondria contain around 1000 proteins in yeast and 1500 proteins in humans. However, only 8 and 13 proteins are encoded in mitochondrial DNA in yeast and humans respectively. Most mitochondrial proteins are synthesized via cytoplasmic ribosomes. Proteins that are key components in the electron transport chain are translated in mitochondria.
Mammalian mitoribosomes have small 28S and large 39S subunits, together forming a 55S mitoribosome. Plant mitoribosomes have small 33S and large 50S subunits, together forming a 78S mitoribosome.
Animal mitoribosomes only have two rRNAs, 12S (SSU) and 16S (LSU), both highly minimizeed compared to their larger homologues. Most eukaryotoes use 5S mitoribosomal RNA, animals, fungi, alveolates and euglenozoans being the exceptions. A variety of methods have evolved to fill in the gap left by a missing 5S, with animals co-opting a Mt-tRNA (Val in vertebrates).
The mitochondrial ribosomal protein nomenclature generally follows that of bacteria, with extra numbers used for mitochondrion-specific proteins. (For more information on the nomenclature, see Ribosomal protein § Table of ribosomal proteins.)