Dissociation of the eukaryotic initiation factor-4E/4E-BP1 complex involves phosphorylation of 4E-BP1 by an mTOR-associated kinase

mTOR immunoprecipitates contain two 4E-BP1 protein kinase activities. One appears to be due to mTOR itself and results in the phosphorylation of 4E-BP1 on residues T 36 and T 45, as shown previously by others. The other is a kinase which can be separated from mTOR and which phosphorylates 4E-BP1 within a peptide(s) containing residues S 64 and T 69. This phosphorylation, which occurs predominantly on S 64, results in the dissociation of 4E-BP1 from eIF-4E.