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RHEB (Ras homolog enriched in brain) senses amino acid levels in order to recruit mTOR. See this review describing the process of how RHEB recruits mTOR. So does RHEB have differential selectivity towards different amino acids.

We also discussed this in Matt Kaeberlein's class yesterday and I posed this question but no one could answer it (that said, Matt Kaeberlein called it a good question).

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Might be a good question in the context of your class, but that doesn't make it a good question for this forum since it does not provide any of the necessary context to readers. –  Abe Mar 11 '12 at 6:46

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RHEB does not sense amino acids. See the abstract of this paper.

Activation of this pathway requires inhibition of the tumor suppressor complex TSC1/2. TSC2 is a GTPase-activating protein for the small GTPase Ras homologue enriched in brain (Rheb), GTP loading of which activates mTOR by a yet unidentified mechanism. The level at which this pathway senses the availability of amino acids is unknown but is suggested to be at the level of TSC2. Here, we show that amino-acid depletion completely blocks insulin- and TPA-induced Rheb activation. This indicates that amino-acid sensing occurs upstream of Rheb. Despite this, amino-acid depletion can still inhibit mTOR/S6 kinase signaling in TSC2-/- fibroblasts. Since under these conditions Rheb-GTP levels remain high, a second level of amino-acid sensing exists, affecting mTOR activity in a Rheb-independent fashion.

RHEB also doesn't have any amino-acid binding (or any small molecule binding) domain.

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