Translation initiation is one of the key events regulated in response to mitogenic stimulation and nutrient availability, tightly coupled to mammalian cell cycle progression and growth. FKBP12-rapamycin-associated protein (FRAP; also named mTOR or RAFT1), a member of the ataxia telangiectasia mutated (ATM)-related kinase family, governs a rapamycin-sensitive membrane-to-cytoplasm signaling cascade that modulates translation initiation via p70 S6 kinase (p70^s6k) and eIF-4E binding protein 1 (4E-BP1). Our studies reported here reveal a surprising regulatory mechanism of this signaling, which involves cytoplasmic–nuclear shuttling of FRAP. By using leptomycin B (LMB), a specific inhibitor of nuclear export receptor Crm1, we show that FRAP is a cytoplasmic–nuclear shuttling protein. Inhibition of FRAP nuclear export by LMB coincides with diminished p70^s6k activation and 4E-BP1 phosphorylation. Further investigation by altering FRAP's nuclear shuttling activity with exogenous nuclear import and export signals has yielded results that are consistent with a direct link between nuclear shuttling of FRAP and mitogenic stimulation of p70^s6k activation and 4E-BP1 phosphorylation. Furthermore, by using a reporter system, we provide evidence suggesting that nuclear shuttling of FRAP regulates mitogen-stimulated rapamycin-sensitive translation initiation. These findings uncover a function for the nucleus in the direct regulation of the protein synthesis machinery via extracellular signals.