Biochemists discover new rules for importing mitochondrial proteins

Mitochondria are cellular organelles that play an important role in the manufacture of ATP (adenosine triphosphate), molecular fuel which feeds most cellular functions. These organelles are originally more than a billion years ago when a primitive archaeal cell entered a symbiotic relationship with an ancestral bacteria. Over time, mitochondria has become essential for metabolism and energy production, while transferring most of their genes to the host. Consequently, they are now counting on the host cell to provide most of their proteins, which are synthesized by ribosomes outside the ordeal and must be properly delivered to the mitochondria.

Now, Caltech scientists have discovered new details on how mitochondrial proteins are delivered from ribosomes in cytosol, liquid around the nucleus, with mitochondria. In a surprising turn, the process is largely shaped by the technical details of protein folding.

“It turns out that the location of mitochondria proteins implies a complex multilayer route which is wired around the biophysical principles of protein folding”, explains Shu-Ou Shan, professor of Altair chemistry in Caltech.

For decades, the dominant model of biochemistry argued that mitochondrial proteins are imported only after protein synthesis, or translation, ends completely. (This process focused on ribosomes consists in adding amino acids one by one to a growing chain, after the sequence presented by the genetic code of the cell.) In a new article which appears in the newspaper CellShan and his colleagues offer a revision to this model, showing that up to 20% of the mitochondrial proteins can be imported cotranslation, which means that they enter the mitochondria during translation when proteins are still synthesized by the ribosome.

“Once we have identified these mitochondrial proteins which are imported cotranslationally, we asked:” What is the particularity of this protein subset? “” Said Zikun Zhu (Ph.D. ’24), a former Shan graduate student and the main article.

It turns out that the most important characteristic of these proteins is that they are large molecules that fold in a complex way. These topologically complex proteins are rich in residues – the amino acids in the chain which constitute the protein – which, although distant from the other in linear sequence, must be binded together for the protein to fold in the appropriate three -dimensional structure. “It becomes a much more difficult process than simply folding the interactions between neighboring residues,” said Shan.

Consequently, the cotranslational import system in mitochondria hierartis these proteins really difficult to fold. This is logical if you consider that large structures must possibly go through narrow channels on the mitochondrial membrane during import. “There will be a problem if you let these large very complex proteins complete the translation in cytosol,” says Shan. “They will remain stuck in irreversible structures, then you will not only block importation, you will get all the channels.”

But how does the cell know what proteins should be imported during translation?

The team found that almost all of these proteins carry a mitochondrial targeting sequence, which is a signal that directs proteins to the mitochondria. However, surprisingly, this is not enough alone to indicate that this subset of protein is delivered during the translation. Zhu conducted experiences that have shown that the system is expecting a second molecular signal to move a protein early to mitochondria. This signal is in the form of the first large protein area, or foldable structural unit in the sequence, which emerges from the ribosome.

“It’s like having your boarding pass locked up,” said Zhu. “The targeting sequence is the boarding card, but to access it, you need the code to open the suitcase. In this case, the large area is this code.”

Scientists have even been able to transplant examples of protein are also large towards other mitochondrial proteins which are normally imported after translation and have shown that the domains served as transferable signals capable of realloring proteins to import during translation.

“Cotranslational targeting to mitochondria turns out to be completely different from targeting to other organelles,” explains Zhu. “In the future, it will be exciting to discover more mechanistic details and, ultimately, to manipulate the moment of importing mitochondriat protein.

The document is entitled “Principles of the importation of cotransulation mitochondrial proteins”. The additional Caltech authors are Taylor A. Stevens (PH.D. ’24), a postdoctoral research partner in biology and biological engineering, and Rimming Huang, a student graduated in biochemistry and molecular biophysics. Saurav Mallik from the Weizmann Institute of Science in Israel and Emmanuel D. Levy of the University of Geneva in Switzerland are also authors.