|Structure of the Mammalian mitochondrial ribosome reveals an expanded functional role for its component proteins.|
Sharma MR, Koc EC, Datta PP, Booth TM, Spremulli LL, Agrawal RK
Category: mitochondria-gene expression ¤ Added: Oct 10, 2003 ¤ Rating: ◊◊
The mitochondrial ribosome is responsible for the biosynthesis of protein components crucial to the generation of ATP in the eukaryotic cell. Because the protein:RNA ratio in the mitochondrial ribosome ( approximately 69: approximately 31) is the inverse of that of its prokaryotic counterpart ( approximately 33: approximately 67), it was thought that the additional and/or larger proteins of the mitochondrial ribosome must compensate for the shortened rRNAs. Here, we present a three-dimensional cryo-electron microscopic map of the mammalian mitochondrial 55S ribosome carrying a tRNA at its P site, and we find that instead, many of the proteins occupy new positions in the ribosome. Furthermore, unlike cytoplasmic ribosomes, the mitochondrial ribosome possesses intersubunit bridges composed largely of proteins; it has a gatelike structure at its mRNA entrance, perhaps involved in recruiting unique mitochondrial mRNAs; and it has a polypeptide exit tunnel that allows access to the solvent before the exit site, suggesting a unique nascent-polypeptide exit mechanism.