title: XPLN is an endogenous inhibitor of mTORC2.
abstract: Mammalian target of rapamycin complex 2 (mTORC2) controls a wide range of cellular and developmental processes, but its regulation remains incompletely understood. Through a yeast two-hybrid screen, we have 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 overexpression of XPLN suppresses it, suggesting that XPLN inhibits mTORC2 signaling to Akt. Consistent with Akt promoting cell survival and XPLN playing a negative role in this process, XPLN knockdown protects cells from starvation-induced apoptosis. Importantly, this effect of XPLN depletion is abolished by inhibition of Akt or mTOR kinase activity, as well as by rictor knockdown. In vitro, purified XPLN inhibits mTORC2 kinase activity toward Akt without affecting mTORC1 activity. Interestingly, the GEF activity of XPLN is dispensable for its regulation of mTORC2 and Akt in cells and in vitro, 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 negatively regulates myoblast differentiation by suppressing mTORC2 and Akt, and this function is through the XPLN N terminus and independent of GEF activity. Our study identifies XPLN as an endogenous inhibitor of mTORC2 and delineates a noncanonical mechanism of XPLN action.
authors.affiliation: Department of Cell and Developmental Biology, University of Illinois at Urbana-Champaign, Urbana, IL 61801.
authors.forename: Nidhi
authors.initials: N
authors.lastname: Khanna
authors.forename: Yimin
authors.initials: Y
authors.lastname: Fang
authors.forename: Mee-Sup
authors.initials: MS
authors.lastname: Yoon
authors.forename: Jie
authors.initials: J
authors.lastname: Chen
mesh_terms: D000818:Animals; D015153:Blotting, Western; D002352:Carrier Proteins / Q000378:metabolism; D057809:HEK293 Cells; D006367:HeLa Cells; D006801:Humans; D047468:Immunoprecipitation; D016086:Lentivirus; D000076225:Mechanistic Target of Rapamycin Complex 2; D051379:Mice; D046912:Multiprotein Complexes / Q000037:antagonists & inhibitors*; D032446:Myoblasts; D010957:Plasmids / Q000235:genetics; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D034622:RNA Interference; D000076226:Rapamycin-Insensitive Companion of mTOR Protein; D064067:Rho Guanine Nucleotide Exchange Factors / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction / Q000235:genetics / Q000502:physiology*; D058570:TOR Serine-Threonine Kinases / Q000037:antagonists & inhibitors*; D020798:Two-Hybrid System Techniques
chemical_list: C576601:ARHGEF3 protein, human; D002352:Carrier Proteins; D046912:Multiprotein Complexes; C492571:RICTOR protein, human; D000076226:Rapamycin-Insensitive Companion of mTOR Protein; D064067:Rho Guanine Nucleotide Exchange Factors; D000076225:Mechanistic Target of Rapamycin Complex 2; D051057:Proto-Oncogene Proteins c-akt; D058570:TOR Serine-Threonine Kinases
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