 Leucine is a dietary amino acid with the capacity to directly stimulate myofibrillar muscle protein synthesis.This effect of leucine arises results from its role as an activator of the mechanistic target of rapamycin (mTOR), a serine-threonine protein kinase that regulates protein biosynthesis and cell growth.The activation of mTOR by leucine is mediated through Rag GTPases, leucine binding to leucyl-tRNA synthetase, leucine binding to sestrin 2, and possibly other mechanisms.