Is mTOR signalling decided by where the complex is active, rather than by how much of it is active?

Framing gap · evidence stands at: Mechanism in cells · Atlas ID F3

The gap

The lysosome is normally described as mTOR's parking space. Newer structural work describes it more like a workbench: what the enzyme can reach depends on how it is docked. If that is right, two cells with the same amount of activity can be doing completely different jobs — and grinding them up to measure the total would hide exactly that.

Technical framing: The lysosome is usually described as the place mTORC1 sits. The structural work of 2025 describes something else: a signalling platform where Rag and Ragulator, Rheb, and direct membrane engagement have to be assembled in order, and where substrate access depends on that assembly. If access is positional, then two cells with identical total mTORC1 activity can be running different programmes, and no lysate will tell them apart.

What changed

Structures published in 2025 resolved both the assembly on the lysosomal membrane and the dynamic regulation by amino acids through the GATOR2, Sestrin2 and CASTOR1 interfaces (Nature 2025). The authors are explicit that how nutrient binding is converted into GATOR2 activation remains unresolved — so the mechanism is now detailed enough to generate positional predictions, and those predictions are untested.

What is still open

Whether substrate identity actually depends on location in a living cell, as opposed to in a reconstituted system. The specific unanswered question: does the same amount of active mTORC1 produce a different substrate profile when it is tethered to the lysosome than when it is tethered elsewhere?

How it could be tested

Forced relocalisation. Tether active mTORC1 to the lysosomal membrane, to the trans-Golgi and to the plasma membrane at matched activity, and read the substrate profile at each — S6K1, 4E-BP1, ULK1, TFEB. A differing profile at equal activity establishes that position is an independent variable; an identical profile retires the idea.

Why it matters

It decides whether pathway-activity biomarkers can mean anything. A blood panel reports how much, and if the biology runs on where, the panel is measuring a quantity the cell does not use.

Bears on these open questions

• A sensor/autophagy biomarker panel as a surrogate endpoint for human trials
• Sensor-selective geroprotection without the metabolic penalty

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