Visualizing the Main Gate into Mitochondria

Researchers at the Max Planck Institute of Biophysics have determined a high-resolution structure of the main gate into mitochondria using cryoEM.

Mitochondria are essential organelles in eukaryotic cells, but they must import proteins from the cytoplasm in order to function. At the heart of this process, the TOM complex acts as both a selection filter and a channel that allows unfolded mitochondrial proteins reach their destination within the organelle.

Proteins are the main working machines of cells, carrying out nearly all essential functions. Most are produced by ribosomes in the cytoplasm, and travel to other cellular destinations, including the mitochondria, the powerhouses that provide necessary energy to the cell. However, getting proteins into mitochondria depends on a sophisticated import system that receives, filters and sorts them. Now, researchers at the Max Planck Institute of Biophysics have resolved a high-resolution structure of the main gate into mitochondria: the Translocase of the Outer Mitochondrial Membrane, better known as the TOM complex.

Using electron cryo-microscopy (cryoEM), postdoctoral researchers Noor Agip and Pamela Ornelas from the Department of Structural Biology, in collaboration with Melanie McDowells’s group, resolved the TOM complex structure to remarkable detail. Their new publication in the journal PNAS shows incoming proteins caught in transit as they interact with various TOM components.

The Molecular Entry Control

Mitochondria are special. They come with their own DNA and ribosomes, stored in their innermost compartment, the matrix. Nevertheless, their limited machinery constrains them to produce only about 1% of the proteins they need to function. This makes them dependent on proteins synthesized in the cytosol, and this is where the TOM complex comes into play. TOM acts as the main entry gate that scans the incoming proteins for mitochondrial access codes and facilitates their import.

The TOM complex is composed of two barrel-like channels that span the outer mitochondrial membrane and six supporting helical subunits. Among them, Tom20 is a receptor that recognizes specific incoming proteins. Until now, its intrinsic flexibility made it difficult to visualize at high detail through microscopy techniques, but Agip and Ornelas et al. used Chaetomium thermophilum, a fungus that tolerates high temperatures, to capture it in action. This temperature tolerance allows C. Thermophilum to remain stable under harsh conditions, meaning that its proteins are more robust and amenable for high-resolution structural studies.

They found two copies of Tom20 hovering in above the TOM channel, as two hands extending from the sides of the complex, positioned in a handshake. They observed the Tom20 receptor domain in multiple conformations, suggesting that it is flexible and changes position during the import process. Additionally, they found proteins caught mid-import, as they interact with Tom20 and as they enter the translocation channel, providing a snapshot of TOM in action.

Advancing Mitochondrial Research

This work continues a long line of TOM complex studies at the Max Planck Institute of Biophysics. The department of Structural Biology, led by director Werner Kühlbrandt, has been at the forefront of TOM and mitochondrial research, resolving structures of the complex from four different species, S. cerevisiae, N. crassa, D. melanogaster, and now C. thermophilum, building a comprehensive view of mitochondrial protein import.

Understanding the role of the TOM complex as the entry gate into mitochondria is fundamental for health research. Dysfunctional import pores have been linked to neurodegenerative diseases like Parkinson’s and Alzheimer’s. Missing TOM components are associated with congenital disorders. Together with its interaction partners, TOM is also linked to mitochondrial quality control and aging, where malfunction results in serious diseases. Knowing the three-dimensional structures of these amazing machines in detail will help in future to combat such conditions that are at present incurable.