While mitochondria are the chemical energy generators in our cells, mitochondrial ribosomes (mitoribosomes) are vital workers behind the scenes, assembling amino acids into important protein complexes essential for energy production. However, the composition and function of these mitoribosomes have remained a mystery for biologists.
Now, researchers at the Swiss Federal Institute of Technology in Zurich (ETHZ) have obtained a partial 3-D reconstruction of the mammalian mitochondrial ribosome at a near-atomic resolution of 4.9 Angstroms. The group says their structure reveals an important first look into the unique architecture and structural evolution of the mitoribosome. The findings are published in the journal Nature.
“It’s very exciting to see how different from other ribosomes these mitochondrial ribosomes are and to reveal how these differences are actually important for their specialized function,” explained Nenad Ban, professor at the Institute of Molecular Biology and Biophysics at ETHZ. “It’s also very exciting for a structural biologist to obtain such high quality reconstructions that reveal so much detail.”
Ban and colleagues reconstructed the larger of two subunits that make up the mitoribosome’s structure. In their analysis, the team was able to identify a ribosomal protein serving as a membrane anchor at the ribosomal exit site for newly synthesized proteins.
Ban said that the mitoribosome may have integrated this protein during the course of evolution in order to permanently attach itself to the mitochondrial inner membrane. The unique ribosomal arrangement could have evolved solely to streamline trafficking of newly synthesised proteins directly to their destination in the membrane.
The machinery for anchoring ribosomes on membranes doesn’t exist in the cytosol said Ban. The mitoribosomes “just sit there and are highly specialized for synthesis of membrane proteins.”
Unlike bacterial or eukaryotic cytoplasmic ribosomes, mitoribosomes are comparatively much fewer in number. This makes them extremely hard to analyze with methods that require large amounts of sample,such as X-ray crystallography—a technique Ban’s lab had used in the past to obtain structural information.
To move beyond these technical limitations, the researchers turned to cryo-electron microscopy, a high-resolution microscopy method requiring much smaller sample sizes. With that technique, the lab generated millions of transmission images of the ribosomal subunit to reconstruct their 3-D structure.
The team then positioned individual RNAs and proteins within the subunit and later interpreted this structural information using chemical cross-linking and mass spectrometry.
“The method kind of puts a ruler to distances between the proteins and individual amino acids in these proteins,” said Ban. “It allowed us to identify a number of proteins that we have placed into electron microscopy maps of the mitochondrial ribosome.”
Along with identifying the ribosome’s membrane anchor protein, Ban’s team also found that the mitoribosome’s proteins had replaced large portions of ribosomal RNA during evolution.
“We concluded that there is a step-wise elimination of the RNA because many proteins are still kept in the same place, but the contacts that are usually mediated by RNA are now mediated by additional proteins that have been added to the mitochondrial ribosome,” explained Ban.
Ban’s lab will continue to build on this study with the goal of elucidating the full mitoribosome structure at atomic resolution. But for now, “It’s nice to have another fascinating system under control that might pave the way for future studies,” said Ban.
Greber BJ, Boehringer D, Leitner A, Bieri P, Voigts-Hoffmann F, Erzberger JP, Leibundgut M, Aebersold R, Ban N. Architecture of the large subunit of the mammalian mitochondrial ribosome. Nature. 2014 Jan 23;505(7484):515-9.