A nuclear transport signal in mammalian target of rapamycin is critical for its cytoplasmic signaling to S6 kinase 1

The mammalian target of rapamycin ( mTOR) regulates nutrient-dependent cell growth and proliferation through cytoplasmic targets, such as S6 kinase 1 (S6K1). Consistent with its main function in the cytoplasm, mTOR is predominantly cytoplasmic. However, previously we have found that mTOR shuttles between the nucleus and cytoplasm, and we have proposed that the nucleocytoplasmic shuttling of mTOR is required for the maximal activation of S6K1. The intrinsic signals directing mTOR nuclear transport and the underlying mechanisms are unknown. In this study we initially set out to identify nuclear export signals in mTOR. A systematic scan of the mTOR sequence revealed 16 peptides conforming to the canonical leucine-rich nuclear export signal, of which 3 were found by reporter assays to contain leptomycin B-sensitive and leucine-dependent nuclear export activity. Unexpectedly, mTOR proteins with those conserved leucines mutated to alanines were unable to enter the nucleus. Further investigation revealed that the L982A/ L984A and L1287A/L1289A mutations likely induced a global structural change in mTOR, whereas the L545A/L547A mutation directly impaired the nuclear import of the protein, potentially regulated by a nucleocytoplasmic shuttling signal. The loss of nuclear import was accompanied by the significantly reduced ability of the L545A/L547A mutant to activate S6K1 in cells. Most importantly, when nuclear import was restored in the L545A/L547A mutant by the addition of an exogenous nuclear import signal, signaling to S6K1 was rescued. Taken together, our observations suggest the existence of a nuclear shuttling signal in mTOR and provide definitive evidence for the requirement of mTOR nuclear import in its cytoplasmic signaling to S6K1.