Current Views of the Structure of the Mammalian Mitochondrial Ribosome

Mammalian mitochondria synthesize polypeptides crucial for energy generation using ribosomes with a number of unique features. These ribosomes are very protein rich and have very truncated ribosomal RNAs. The bulk of the mammalian mitochondrial ribosome is composed of proteins, only about half of which are homologs of ribosomal proteins found in other translational systems. A number of distinctive features are found in these ribosomes. Among these is a gate-like structure that allows entrance of the primarily leaderless mRNAs that characterize this system. The exit tunnel of the large subunit is also quite unusual and includes a site in which the nascent peptide is visible to solvent prior to the normal exit site. Further, this region of the mitochondrial ribosome is dominated by ribosomal proteins rather than rRNA and is involved in the interaction of the ribosome with the inner membrane where all of the translation products are ultimately located. The proteins of the mitochondrial ribosome appear to play a number of important roles in the cell in addition to their function in protein biosynthesis, including roles in apoptosis and in cell cycle control.