title: Regulation of ribosomal S6 kinase 2 by mammalian target of rapamycin.
abstract: Phosphorylation of the ribosomal S6 subunit is tightly correlated with enhanced translation initiation of a subset of mRNAs that encodes components of the protein synthesis machinery, which is an important early event that controls mammalian cell growth and proliferation. The recently identified S6 kinase 2 (S6K2), together with its homologue S6K1, is likely responsible for the mitogen-stimulated phosphorylation of S6. Like S6K1, the activation of S6K2 requires signaling from both the phosphatidylinositol 3-kinase and the mammalian target of rapamycin (mTOR). Here we report the investigation of the mechanisms of S6K2 regulation by mTOR. We demonstrate that similar to S6K1 the serum activation of S6K2 in cells is dependent on mTOR kinase activity, amino acid sufficiency, and phosphatidic acid. Previously we have shown that mTOR is a cytoplasmic-nuclear shuttling protein. As a predominantly nuclear protein, S6K2 activation was facilitated by enhanced mTOR nuclear import with the tagging of an exogenous nuclear localization signal and diminished by enhanced mTOR nuclear export with the tagging of a nuclear export sequence. However, further increase of mTOR nuclear import by the tagging of four copies of nuclear localization signal resulted in its decreased ability to activate S6K2, suggesting that mTOR nuclear export may also be an integral part of the activation process. Consistently, the nuclear export inhibitor leptomycin B inhibited S6K2 activation. Taken together, our observations suggest a novel regulatory mechanism in which an optimal cytoplasmic-nuclear distribution or shuttling rate for mTOR is required for maximal activation of the nuclear S6K2.
authors.affiliation: Department of Cell and Structural Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.
authors.forename: In-Hyun
authors.initials: IH
authors.lastname: Park
authors.forename: Rebecca
authors.initials: R
authors.lastname: Bachmann
authors.forename: Haider
authors.initials: H
authors.lastname: Shirazi
authors.forename: Jie
authors.initials: J
authors.lastname: Chen
mesh_terms: D020001:1-Butanol / Q000494:pharmacology; D000730:Androstadienes / Q000494:pharmacology; D000818:Animals; D002460:Cell Line; D002522:Chlorocebus aethiops; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects; D006801:Humans; D007668:Kidney; D007700:Kinetics; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases; D021122:Protein Subunits; D011505:Protein-Tyrosine Kinases / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt; D011994:Recombinant Proteins / Q000378:metabolism; D019893:Ribosomal Protein S6 Kinases / Q000737:chemistry / Q000235:genetics*; D012270:Ribosomes / Q000201:enzymology; D020123:Sirolimus / Q000494:pharmacology*; D058570:TOR Serine-Threonine Kinases; D014162:Transfection; D000077191:Wortmannin
chemical_list: D000730:Androstadienes; D021122:Protein Subunits; D011518:Proto-Oncogene Proteins; D011994:Recombinant Proteins; D020001:1-Butanol; D011494:Protein Kinases; C546842:MTOR protein, human; D011505:Protein-Tyrosine Kinases; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt; D019893:Ribosomal Protein S6 Kinases; D058570:TOR Serine-Threonine Kinases; D020928:Mitogen-Activated Protein Kinases; D020123:Sirolimus; D000077191:Wortmannin