Leucine protects mitochondrial proteins, doing more than build muscle

Leucine, the amino acid sold in tubs and bars for building muscle, also stands guard over the machinery inside mitochondria. A team at the University of Cologne led by Professor Thorsten Hoppe traced how high leucine levels stop proteins on the outer mitochondrial membrane from being tagged for destruction, and cells that kept those proteins burned more oxygen. First author Dr. Qiaochu Li and colleagues reported the work in Nature Cell Biology on October 31, 2025.

The result sits in the cell, not in a gym. Nothing in it is a dosing recommendation, and the experiments used roundworms and cultured human cells rather than people eating protein shakes. The paper, titled as a study of leucine inhibiting degradation of outer mitochondrial membrane proteins to adapt respiration, carries the November 2025 issue date of volume 27, pages 1889 to 1901, and the story has been circulating in science news since its release.

The leucine, GCN2 and SEL1L chain

The route runs through three proteins. Leucine inhibits GCN2, a sensor that tracks amino acid supply. With GCN2 dampened, the ubiquitin ligase cofactor SEL1L drops in abundance specifically at mitochondria, so fewer outer membrane proteins receive the ubiquitin tags that send them to the cell’s disposal system.

According to the Nature Cell Biology paper, the proteins that accumulate include TOMM40, a core component of the machinery that imports other proteins into mitochondria. More import machinery means the organelle can assemble a larger proteome, and that larger proteome raises respiratory capacity. In Li’s words, the finding shows that “a cell’s nutrient status, especially its leucine levels, directly impacts energy production.”

The Cologne team describes the targets as mitochondrial surface proteins that import metabolic molecules into the organelle. Stabilizing them lets cells adapt quickly when nutrients are plentiful, which is the setting in which a high leucine signal would normally arise. When leucine is scarce, the same disposal route keeps running and the import hardware turns over at its usual pace.

Worms, HEK293 cells and three lung cancer lines

To watch the process live, the group built a fluorescent reporter in Caenorhabditis elegans that glows according to how fast outer membrane proteins are turned over through the ubiquitin-proteasome system. The same regulation then appeared in human HEK293 cells and in three lung cancer lines named H2030, H1437 and H1666, which the authors took as evidence that the mechanism is conserved from worms to humans.

The full text in PubMed Central reports that oxygen consumption rose in both worms and human cells when the leucine pathway was engaged.

Two side findings carry practical weight. In worms with defective leucine breakdown, outer membrane proteins were stabilized, but reproduction suffered under stress once GCN2 was also disrupted. And lung cancer lines with high intracellular branched-chain amino acids showed less ubiquitylation of those proteins and resisted a block on mitochondrial import, which the authors flag as a possible vulnerability to study in tumors. The University of Cologne’s release states it plainly: certain cancer mutations that affect leucine metabolism enhance survival of tumor cells.

Muscle biology and mTORC1

Leucine’s reputation comes from a different pathway. A 2025 review in the Journal of Animal Science and Biotechnology, by Shuyong Xu, Mark Hanigan and colleagues, describes leucine as a key activator of mTORC1: it binds the sensor Sestrin2, which releases GATOR2 from inhibition and switches on mTORC1 at the lysosome, driving phosphorylation of S6K1 and 4E-BP1 and, with them, protein synthesis in skeletal muscle. The same review reports that leucine alone can activate mTORC1 about as effectively as a full amino acid mixture.

The Cologne work adds a second, separate readout of the same nutrient. The mTORC1 story is about building protein; the GCN2-SEL1L story is about keeping the energy-import hardware from being scrapped. Both respond to leucine abundance, but the paper does not claim that either one explains the other.

The SEL1L trade-off

More respiration is not automatically better, and the team says so. SEL1L also works as quality control, clearing damaged proteins, and Li cautioned that it “plays a crucial role in preventing the accumulation of damaged proteins, which is essential for long-term cellular health.” A cell that permanently turns SEL1L down to boost its power plants could pay for it later.

The group’s account, distributed through the Informationsdienst Wissenschaft, lists leucine-rich foods such as dairy, meat, beans and lentils, but frames the mechanism as a way cells adapt to periods of nutrient abundance and not as advice to eat more of them.

The unresolved piece is dose and tissue. The experiments do not say how much dietary leucine reaches mitochondria in a human muscle fibre or a tumor, or how long SEL1L can stay suppressed before damaged proteins start to build up. The work was funded by Germany’s Excellence Strategy, the German Research Foundation, the European Research Council and the Alexander von Humboldt Foundation, and Hoppe’s laboratory at the CECAD Excellence Cluster is where those questions now sit.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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