Different roles for the TOS and RAIP motifs of the translational regulator protein 4E-BP1 in the association with raptor and phosphorylation by mTOR in the regulation of cell size

The translational regulator protein 4E-BP1, that binds to eukaryotic initiation factor-4E (eIF4E) to prevent the formation of the active translation complex, dissociates from eIF4E by phosphorylation through the mammalian target of rapamycin (mTOR) in the cells stimulated by amino acids. 4E-BP1 has been shown to associate with the scaffold protein raptor through its TOS and RAIP motifs to be recognized by mTOR. We revealed that the TOS motif mutant was phosphorylated by mTOR only at the priming sites of Thr37/46 but the RAIP motif mutant was phosphorylated not only at the priming sites but also at the subsequent site of Thr70 in vitro and in response to amino acid treatment in HEK293 cells. Analysis using the phosphorylation site mutants indicated that phosphorylation of the priming and subsequent sites of 4E-BP1 was required for dissociation from raptor as well as for the release of eIF4E. The expression of the 4E-BP1 mutants replacing the TOS motif and phosphorylation sites, that are poor substrates for mTOR and have no or little dissociation ability from raptor and eIF4E, respectively, significantly reduced the size of K562 cells. These results indicate that the the TOS motif has an essential function whereas the RAIP motif has an accessory role in the association with raptor and mTOR-mediated phosphorylation of 4E-BP1 to dissociate it from raptor and release eIF4E in response to amino acid stimulation leading to the control of cell size.