All eukaryotic cells contain small "energy factories" called mitochondria, which produce the universal energy molecule ATP. To be able to do this, mitochondria have to maintain a spatial separation of the membrane proteins that carry out different steps in the formation of ATP.

When cells break down sugars, energy is extracted, which is used in the mitochondria to produce ATP. Central to this process are four membrane protein complexes, called complex I, II, III and IV, which together build an energetic gradient that is used in complex V for ATP synthesis. These ATP molecules can then be used to maintain a myriad of reactions throughout the cells, which is essential for life.

It is commonly known that the respiratory complexes I, III and IV interact with each other and form so-called respiratory supercomplexes, which optimises the interaction between the complexes. Until now, researchers have not observed that complex II is part of the supercomplexes. In mammalian mitochondria, supercomplexes are spatially separated in the membrane from complex V, where supercomplexes reside only in membrane regions without curvature. However, there are unicellular eukaryotic organisms such as Tetrahymena thermophila whose mitochondria contain only membranes with curvature, and therefore it has been a major question where supercomplexes reside in these membrane systems.

Now an international team of researchers, with the participation of postdoc Rasmus Kock Flygaard from the Department of Molecular Biology and Genetics at Aarhus University, has answered a number of key questions regarding supercomplexes from Tetrahymena.

"For the first time ever, we have shown that complex II can also form part of a super complex, which shows an incredible optimisation of the process for ATP formation", says Rasmus Kock Flygaard. "Furthermore, with our structure, we can see that supercomplexes do not follow a simple plan for construction, but on the contrary, there is a surprising variety, which was not previously thought possible".

This variation in the structure of the supercomplex is also central to the question of its existence in curved membranes, and Rasmus Kock Flygaard continues:

"The supercomplex from Tetrahymena has been rebuilt and expanded with countless proteins and extra domains, which overall give the supercomplex a curved architecture, so that it is completely adapted and developed to exist in curved membranes. This is an incredible example of how nature is able to adapt otherwise conserved protein complexes to new environments to maintain function. Now, we have investigated the membrane protein structure of a single organism and made completely new discoveries. There are so many more single-celled eukaryotic organisms that are also just waiting to be described, so that we can provide a more nuanced picture of how life has evolved and adapted.”

The results were published in the international journal Nature with Rasmus Kock Flygaard as shared first author.

For further information, please contact

Postdoc Rasmus Kock Flygaard
Department of Molecular Biology and Genetics
Aarhus University, Denmark
rkf@mbg.au.dk

SUPPLEMENTARY INFORMATION

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ITEMS	CONTENT AND PURPOSE
Study type	Experiment
External funding	This work was supported by SciLifeLab EM facility (funded by the KAW, EPS and Kempe foundations), the Center for Scientific Computing, Finland for high-performance supercomputing resources, the EMBO Young Investigator Program, the Swedish Foundation for Strategic Research (FFL15:0325), the Ragnar Söderberg Foundation (M44/16), Cancerfonden (2017/1041), the European Research Council (ERC-2018-StG-805230) and the Knut and Alice Wallenberg Foundation (2018.0080). V.S. was supported by the Academy of Finland, the Sigrid Jusélius Foundation, the Jane and Aatos Erkko Foundation, the Magnus Ehrnrooth Foundation and the University of Helsinki. A.Maréchal was supported by the Medical Research Council (CDA MR/M00936X/1, transition support MR/T032154/1).
Conflict of interest	None
Link to the scientific paper	Alexander Mühleip, Rasmus Kock Flygaard, Rozbeh Baradaran, Outi Haapanen, Thomas Gruhl, Victor Tobiasson, Amandine Maréchal, Vivek Sharma & Alexey Amunts Structural basis of mitochondrial membrane bending by the I–II–III2–IV2 supercomplex Nature doi: 10.1038/s41586-023-05817-y