PerspectiveStructural Biology

Mitoribosome oddities

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Science  17 Apr 2015:
Vol. 348, Issue 6232, pp. 288-289
DOI: 10.1126/science.aab1054

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Eukaryotic cells contain two separate translation machineries for protein synthesis—one in the cytosol and one in mitochondria. This situation is attributable to the evolutionary history of eukaryotic cells, which originate from a merger of two formerly independent cells—the host cell and the bacterial endosymbiont—with each contributing a full-fledged protein synthesis system. However, during the past 1.5 billion years, the two translation machineries have evolved very differently. That of the host cell, which acts in the cytosol, synthesizes almost all cellular proteins, including most mitochondrial proteins. By contrast, in mitochondria—the descendants of the bacterial endosymbionts—ribosomes (mitoribosomes) are now highly specialized for the synthesis of a very small number (13 in humans) of membrane proteins that function in energy production. It has been assumed that mitoribosomes are still similar to those of bacteria. Now, advances in high-resolution cryo–electron microscopy have allowed fascinating insights into the molecular structure of mitoribosomes, as reported by Greber et al. (1) on page 303 of this issue and by Amunts et al. (2). It is clear that the mitoribosome differs dramatically from the “canonical” cytosolic ribosome of bacteria and eukaryotes (see the figure).