Eukaryotic initiation factor 4E is a novel effector of mTORC1 signaling pathway in cross talk with Mnk1

mTORC1 induces eukaryotic translation initiation factor 4E interaction with TOS-S6 kinase 1 and its activation

“…Addition of rapamycin, 2-DG to HEK293 cells or amino-acid withdrawal caused significant reduction in eIF4E phosphorylation (Fig 2A). The observed dependence of eIF4E phosphorylation on mTORC1 conformed with the ability of GST-4E to act as mTORC1 substrate in vitro, with faithful adherence to rapamycin inhibition (described by us previously 17 , see also Supplementary Fig S2). To test this observation further, we generated cells wherein mTORC1 influence was mitigated using shRNA knockdown of mTOR kinase or its adaptor protein raptor and observed complete loss of 4E phosphorylation in both cases ( Fig 2B).…”

“…While the prevailing view implicates TOS motif in raptor binding 5 , our recent data describes eIF4E as a raptor binding protein, instead 17 . Hence, we set out to investigate if raptor could be an intermediate in eIF4E-TOS/S6K1 interaction.…”

“…Conversely, immunoprecipitates of ectopically expressed eIF4E also coprecipitated endogenous raptor ( Fig 2D). We have previously identified TPTPNPP motif in eIf4E as a potential binding site for raptor 17 . Since this sequence appeared to be conserved among several mTORC1 substrates ( Fig 2D), we conducted flexible peptide propensity modelling of these sequence motifs 27 and evaluated their binding potential with raptor.…”

“…We have recently shown that phosphorylation of eIF4E at Ser209 site governs its binding with amino terminal region of S6K1 17 . Here we examined if S6K1 activation, governed by the phosphorylation at Thr412 site, extends any influence on its interaction with eIF4E.…”

“…Our recent observation that identifies eIF4E rather than raptor as the cellular factor regulating rapamycin response further deepens the disconnect between mTORC1 regulation and S6K1 phosphorylation to uphold eIF4E as an emerging link in regulating rapamycin response 16 . We have also identified eIF4E as a novel member of the mTORC1 pathway whose phosphorylation by mTORC1, and feedback phosphorylation by map kinase interacting kinase (MNK) kinase, puts forth a fresh perspective of signaling downstream of mTORC1, especially in the context of its possible interaction with S6K1 17 . We therefore assessed whether eIF4E influences the state of S6K1 regulation.…”