mTOR
Meaning of mTOR: The cell's growth switch, wired to read food, energy and hormone signals before deciding whether building is affordable.
Definition of mTOR
mTOR is a protein kinase that acts as the central control point for cell growth. It gathers signals about the supply of nutrients, energy and growth hormones, and when all are favorable it switches the cell toward building proteins and dividing. When they are not, it shuts that program down and the cell begins recycling its own components instead. Its name stands for mechanistic target of rapamycin.
What does it control?
Its outputs run in two directions at once. When active, it drives the machinery that translates genetic instructions into protein, promotes the building of fats and the uptake of sugar, and permits the cell to enlarge and divide. At the same time it suppresses autophagy, the housekeeping process by which a cell digests its own worn components for parts. When mTOR falls quiet, both switch over: building stops and recycling begins. That reciprocal arrangement is what makes it the deciding point between growth and conservation.
Why is it named after a drug?
It was found by working backward from rapamycin, a compound produced by a soil bacterium from Easter Island and originally investigated as an antifungal. The protein it bound had no name, so it became the target of rapamycin. The first letter once stood for mammalian and was later redefined as mechanistic when equivalents were identified across other organisms, so both expansions appear in older and newer writing.
What goes wrong with it?
| Setting | What happens to the pathway |
|---|---|
| Many cancers | Stuck on, through loss of the brakes upstream |
| Tuberous sclerosis | An inherited fault in a direct brake protein |
| Transplanted organs | Deliberately blocked to suppress lymphocyte growth |
| Overfeeding and insulin resistance | Chronically raised signalling |
How is it blocked, and why is that only partly effective?
The kinase works inside two different assemblies, and rapamycin and its relatives inhibit one far more completely than the other. Blocking the first also removes a feedback loop that normally restrains a growth pathway upstream, so that pathway can rebound and partly undo the effect. This explains why the earliest inhibitors slowed rather than stopped tumors, and why later drugs were designed to block the kinase itself rather than tether to it, and why combinations are used against cancer driven by this route.
Also known as
Used in a sentence
The tumor showed loss of an upstream brake, leaving mTOR signalling constitutively active.
Good to know
The m in the name once stood for mammalian and was redefined as mechanistic, and rapamycin inhibits only one of the two assemblies the kinase works within, which limits how completely the pathway can be shut down.