Electron Microscopy Alone Reveals Protein Machine’s Structure

Structural Biology: Technique gives first glimpse of mitochondrial ribosome at near-atomic resolution

Sarah Everts

The first near-atomic-level glimpse of a massive 3-megadalton biological machine was obtained this year using only cryogenic electron microscopy (cryoEM), without the need for protein crystallization or extensive purification protocols. The analytical milestone was achieved on the yeast mitochondrial ribosome’s large subunit, a 39-protein complex crucial for making mitochondrial membrane proteins in the energy-producing organelle in yeast cells. The 3.2-Å structure was determined by Venkatraman Ramakrishnan, Sjors H. W. Scheres, and their colleagues at the MRC Laboratory of Molecular Biology, in Cambridge, England (Science 2014, DOI:10.1126/science.1249410) “This paper is one of the most exciting results in structural biology in recent years,” commented Nenad Ban, a structural biologist at the Swiss Federal Institute of Technology (ETH), Zurich. It’s a methodological breakthrough to be able to use electron microscopy to structurally analyze such a large protein complex at atomic-level resolution, Ban explained. Being able to do so eliminates the need for crystallography to solve the structures of massive molecular complexes, he added. The achievement was made possible by a new generation of electron detectors of unprecedented speed and sensitivity. These detectors have extremely high resolution, aren’t destroyed by the high-energy electrons used in cryoEM, and are fast enough to compensate for movements of protein complexes that occur during analysis. The development paves the way for using cryoEM to determine the atomic-resolution structure of the human mitochondrial ribosome, which is expected to be structurally different from the yeast version analyzed by the Ramakrishnan-Scheres team.

The high-resolution cryoEM structure of the yeast mitochondrial ribosome (shown) reveals that it shares some features with the bacterial ribosome (blue) and with mammalian mitochondrial ribosomes (red). Some features are present only in yeast (yellow). Credit: Alan Brown

The high-resolution cryoEM structure of the yeast mitochondrial ribosome (shown) reveals that it shares some features with the bacterial ribosome (blue) and with mammalian mitochondrial ribosomes (red). Some features are present only in yeast (yellow).
Credit: Alan Brown

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