Differential requirement of CAAX-mediated posttranslational processing for Rheb localization and signaling.
The Rheb1 and Rheb2 small GTPases and their effector mTOR are aberrantly activated in human cancer and are attractive targets for anti-cancer drug discovery. Rheb is targeted to endomembranes via its C-terminal CAAX (C=cysteine, A=aliphatic, X=terminal amino acid) motif, a substrate for posttranslational modification by a farnesyl isoprenoid. After farnesylation, Rheb undergoes two additional CAAX-signaled processing steps, Ras converting enzyme 1 (Rce1)-catalyzed cleavage of the AAX residues and isoprenylcysteine carboxyl methyltransferase (Icmt)-mediated carboxylmethylation of the farnesylated cysteine. However, whether these postprenylation processing steps are required for Rheb signaling through mTOR is not known. We found that Rheb1 and Rheb2 localize primarily to the endoplasmic reticulum and Golgi apparatus. We determined that Icmt and Rce1 processing is required for Rheb localization, but is dispensable for Rheb-induced activation of the mTOR substrate p70 S6 kinase (S6K). Finally, we evaluated whether farnesylthiosalicylic acid (FTS) blocks Rheb localization and function. Surprisingly, FTS prevented S6K activation induced by a constitutively active mTOR mutant, indicating that FTS inhibits mTOR at a level downstream of Rheb. We conclude that inhibitors of Icmt and Rce1 will not block Rheb function, but FTS could be a promising treatment for Rheb- and mTOR-dependent cancers.
Curriculum in Genetics and Molecular Biology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7295, USA.
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Hanker
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Mitin
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Wilder
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Henske
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Tamanoi
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Cox
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D000818:Animals; D001665:Binding Sites / Q000235:genetics; D015153:Blotting, Western; D019556:COS Cells; D045744:Cell Line, Tumor; D002522:Chlorocebus aethiops; D004721:Endoplasmic Reticulum / Q000378:metabolism; D005204:Farnesol / Q000031:analogs & derivatives / Q000494:pharmacology; D006056:Golgi Apparatus / Q000378:metabolism; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000235:genetics / Q000378:metabolism; D051379:Mice; D008856:Microscopy, Fluorescence; D020559:Monomeric GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D009154:Mutation; D041681:NIH 3T3 Cells; D009479:Neuropeptides / Q000235:genetics / Q000378:metabolism*; D010766:Phosphorylation / Q000187:drug effects; D054876:Prenylation; D011499:Protein Processing, Post-Translational; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism; D000076205:Ras Homolog Enriched in Brain Protein; D051381:Rats; D038762:Ribosomal Protein S6 Kinases, 70-kDa / Q000378:metabolism; D012459:Salicylates / Q000494:pharmacology; D015398:Signal Transduction; D058570:TOR Serine-Threonine Kinases; D014162:Transfection
D047908:Intracellular Signaling Peptides and Proteins; D009479:Neuropeptides; C490211:RHEB protein, human; D000076205:Ras Homolog Enriched in Brain Protein; C087693:Rheb protein, rat; D012459:Salicylates; C093323:farnesylthiosalicylic acid; D049452:Green Fluorescent Proteins; D005204:Farnesol; C546842:MTOR protein, human; C546843:mTOR protein, mouse; C546845:mTOR protein, rat; D017346:Protein Serine-Threonine Kinases; D038762:Ribosomal Protein S6 Kinases, 70-kDa; D058570:TOR Serine-Threonine Kinases; D020559:Monomeric GTP-Binding Proteins