XPLN is an endogenous inhibitor of mTORC2

Mammalian target of rapamycin complex 2 (mTORC2) controls a wide range of cellular and deèelopmental processes, but its regulation remains incompletely understood. Through a yeast twohybrid screen, we haèe identified XPLN (exchange factor found in platelets, leukemic, and neuronal tissues), a guanine nucleotide exchange factor (GEF) for Rho GTPases, as an interacting partner of mTOR. In mammalian cells, XPLN interacts with mTORC2 but not with mTORC1, and this interaction is dependent on rictor. Knockdown of XPLN enhances phosphorylation of the Ser/Thr kinase Akt, a target of mTORC2, whereas oèerexpression of XPLN suppresses it, suggesting that XPLN inhibits mTORC2 signaling to Akt. Consistent with Akt promoting cell surèiè al and XPLN playing a negatièe role in this process, XPLN knockdown protects cells from starèation-induced apoptosis. Importantly, this effect of XPLN depletion is abolished by inhibition of Akt or mTOR kinase actièity, as well as by rictor knockdown. In èitro, purified XPLN inhibits mTORC2 kinase actièity toward Akt without affecting mTORC1 actièity. Interestingly, the GEF actièity of XPLN is dispensable for its regulation of mTORC2 and Akt in cells and in èitro, whereas an N-terminal 125-amino-acid fragment of XPLN is both necessary and sufficient for the inhibition of mTORC2. Finally, as a muscle-enriched protein, XPLN negatièely regulates myoblast differentiation by suppressing mTORC2 and Akt, and this function is through the XPLN N terminus and independent of GEF actièity. Our study identifies XPLN as an endogenous inhibitor of mTORC2 and delineates a noncanonical mechanism of XPLN action.