Journal Article

Differential requirement of CAAX-mediated posttranslational processing for Rheb localization and signaling

Der, C J, Cox, A D, Hanker, A B, Mitin, N, Wilder, R S, Tamanoi, F, Henske, E P

2010

Overview

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.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: Amino acids / Animals / Apoptosis / Binding Sites - genetics / Biological and medical sciences / Blotting, Western / Cancer / Care and treatment / Cell Biology / Cell Line, Tumor / Cell physiology / Cell transformation and carcinogenesis. Action of oncogenes and antioncogenes / Chlorocebus aethiops / COS Cells / Endoplasmic Reticulum - metabolism / Enzymes / Farnesol - analogs & derivatives / Farnesol - pharmacology / Fundamental and applied biological sciences. Psychology / Genes / Genetics / Golgi Apparatus - metabolism / Green Fluorescent Proteins - genetics / Green Fluorescent Proteins - metabolism / Human Genetics / Humans / Internal Medicine / Intracellular Signaling Peptides and Proteins - genetics / Intracellular Signaling Peptides and Proteins - metabolism / Medicine / Medicine & Public Health / Mice / Microscopy, Fluorescence / Molecular and cellular biology / Monomeric GTP-Binding Proteins - genetics / Monomeric GTP-Binding Proteins - metabolism / Mutation / Neuropeptides - genetics / Neuropeptides - metabolism / NIH 3T3 Cells / Oncology / original-article / Phosphorylation - drug effects / Physiological aspects / Post-translational modification / Prenylation / Protein Processing, Post-Translational / Protein Serine-Threonine Kinases - genetics / Protein Serine-Threonine Kinases - metabolism / Ras Homolog Enriched in Brain Protein / Rats / Ribosomal Protein S6 Kinases, 70-kDa - metabolism / Salicylates - pharmacology / Signal Transduction / TOR Serine-Threonine Kinases / Transfection