Interaction with phospho-eIF4E commits S6 Kinase 1 for mTORC2 and PHLPP1 mediated activation

Eukaryotic initiation factor (eIF4E) phosphorylation is a recognized attribute for enhanced cell growth and proliferation. Our recent data that identifies eIF4E as mTORC1 substrate with ability to influence rapamycin response highlights its prospect as a mediator of mTORC1 signaling. Here, we present evidence that eIF4E phosphorylated at S209 interacts with TOS motif of S6 Kinase1 (S6K1). We show that this interaction is sufficient to overcome rapamycin sensitivity and mTORC1 dependence of S6K1. We present data to demonstrate that eIF4E-S6K1 interaction serves to relieve S6K1 of its carboxy terminal auto inhibition, otherwise rate limiting for its activation. We go on to identify a highly conserved sequence segment that mediates hydrophobic motif phosphorylation of S6K1 in mTORC2 rather than mTORC1 dependent manner. We also identify a novel eight amino-acid PHLPP1 (Pleckstrin Homology Domain Leucine-rich Repeat Protein Phosphatase) binding region in S6K1. Finally, we present evidence that serum/growth factor response of S6K1 is accomplished by the dynamics of its interaction with PHLPP1 over and above its regulation by mTORC1 and 2. We show that deletion of PHLPP1 binding region imparts growth factor resistance to S6K1 without influencing its mTORC1/2 response. Our data identifies eIF4E as a novel TOS binding S6K1 subunit that realigns mTORC1 pathway for crosstalk with other growth promoting signals.