The regulation of 4E-BP1 phosphorylation involves a two-step mechanism. 4E-BP1 is a repressor of eIF4E, which is a key component of the eIF4F complex that promotes translation initiation. 4E-BP1 is hypophosphorylated in quiescent cells and hyperphosphorylated in response to extracellular stimuli. Phosphorylation of 4E-BP1 is regulated by the PI3-kinase/Akt pathway and the FRAP/mTOR pathway. FRAP/mTOR is responsible for phosphorylating 4E-BP1 on Thr-37 and Thr-46. These phosphorylations are not sufficient to release 4E-BP1 from eIF4E, but they serve as a priming event for subsequent phosphorylation of other sites on 4E-BP1. The phosphorylation of Thr-37 and Thr-46 is required for the phosphorylation of serum-sensitive sites on 4E-BP1. These sites are located in the carboxy-terminal region of 4E-BP1 and are phosphorylated by an unidentified kinase that acts downstream of Akt. The second phosphorylation event results in the release of 4E-BP1 from eIF4E and the stimulation of translation. The two-step mechanism of 4E-BP1 phosphorylation is essential for the regulation of translation in response to extracellular stimuli.The regulation of 4E-BP1 phosphorylation involves a two-step mechanism. 4E-BP1 is a repressor of eIF4E, which is a key component of the eIF4F complex that promotes translation initiation. 4E-BP1 is hypophosphorylated in quiescent cells and hyperphosphorylated in response to extracellular stimuli. Phosphorylation of 4E-BP1 is regulated by the PI3-kinase/Akt pathway and the FRAP/mTOR pathway. FRAP/mTOR is responsible for phosphorylating 4E-BP1 on Thr-37 and Thr-46. These phosphorylations are not sufficient to release 4E-BP1 from eIF4E, but they serve as a priming event for subsequent phosphorylation of other sites on 4E-BP1. The phosphorylation of Thr-37 and Thr-46 is required for the phosphorylation of serum-sensitive sites on 4E-BP1. These sites are located in the carboxy-terminal region of 4E-BP1 and are phosphorylated by an unidentified kinase that acts downstream of Akt. The second phosphorylation event results in the release of 4E-BP1 from eIF4E and the stimulation of translation. The two-step mechanism of 4E-BP1 phosphorylation is essential for the regulation of translation in response to extracellular stimuli.