The binding of PRAS40 to 14-3-3 proteins is not required for activation of mTORC1 signalling by phorbol esters/ERK.
PRAS40 binds to the mTORC1 (mammalian target of rapamycin complex 1) and is released in response to insulin. It has been suggested that this effect is due to 14-3-3 binding and leads to activation of mTORC1 signalling. In a similar manner to insulin, phorbol esters also activate mTORC1 signalling, in this case via PKC (protein kinase C) and ERK (extracellular-signal-regulated kinase). However, phorbol esters do not induce phosphorylation of PRAS40 at Thr(246), binding of 14-3-3 proteins to PRAS40 or its release from mTORC1. Mutation of Thr(246) to a serine residue permits phorbol esters to induce phosphorylation and binding to 14-3-3 proteins. Such phosphorylation is apparently mediated by RSKs (ribosomal S6 kinases), which lie downstream of ERK. However, although the PRAS40(T246S) mutant binds to 14-3-3 better than wild-type PRAS40, each inhibits mTORC1 signalling to a similar extent. Our results show that activation of mTORC1 signalling by phorbol esters does not require PRAS40 to be phosphorylated at Thr(246), bind to 14-3-3 or be released from mTORC1. It is conceivable that phorbol esters activate mTORC1 by a distinct mechanism not involving PRAS40. Indeed, our results suggest that PRAS40 may not actually be involved in controlling mTORC1, but rather be a downstream target of mTORC1 that is regulated in response only to specific stimuli, such as insulin.
Fonseca
Bruno D
BD
Department of Biochemistry and Molecular Biology, University of British Columbia, Vancouver, BC V6T1Z3, Canada.
Lee
Vivian H-Y
VH
Proud
Christopher G
CG
D048948:14-3-3 Proteins / Q000378:metabolism*; D048868:Adaptor Proteins, Signal Transducing; D002460:Cell Line; D048049:Extracellular Signal-Regulated MAP Kinases; D006801:Humans; D007328:Insulin; D000076222:Mechanistic Target of Rapamycin Complex 1; D046912:Multiprotein Complexes; D010703:Phorbol Esters / Q000494:pharmacology*; D010750:Phosphoproteins / Q000378:metabolism*; D010766:Phosphorylation; D011485:Protein Binding; D011506:Proteins; D015398:Signal Transduction / Q000187:drug effects*; D058570:TOR Serine-Threonine Kinases; D014157:Transcription Factors / Q000378:metabolism*
D048948:14-3-3 Proteins; C484523:AKT1S1 protein, human; D048868:Adaptor Proteins, Signal Transducing; D007328:Insulin; D046912:Multiprotein Complexes; D010703:Phorbol Esters; D010750:Phosphoproteins; D011506:Proteins; D014157:Transcription Factors; D000076222:Mechanistic Target of Rapamycin Complex 1; D058570:TOR Serine-Threonine Kinases; D048049:Extracellular Signal-Regulated MAP Kinases