Nucleocytoplasmic localization of p70 S6K1, but not of its isoforms p85 and p31, is regulated by TSC2/mTOR

The tuberous sclerosis complex gene 2 (TSC2)/mammalian target of rapamycin (mTOR) pathway controls many cellular functions via phosphorylation of ribosomal protein S6 kinases (S6Ks). Alternative splicing and translation generate three S6K1 proteins. Although nuclear and cytoplasmic S6K targets are known, the nucleocytoplasmic localization of the S6K1 proteins has not been comparatively elucidated so far. We show that in primary fibroblasts p85 S6K1 is cytoplasmic, p70 can be found in both compartments and p31 is exclusively nuclear. As already known for p70 and p85, our data suggest that p31 is also a target of mTOR-mediated phosphorylation. Blocking mTOR kinase activity via rapamycin and its activation in TSC2−/− cells and via TSC2 small interfering RNAs revealed that it regulates the localization of p70, but not of p85 and p31. The mTOR-dependent phosphorylation of p70 S6K1 at T389 is essential for its nuclear localization and exclusively hyperphosphorylated p70 S6K1 can be found in the nucleus. We further demonstrate this mTOR-controlled p70 S6K1 localization to be growth factor dependent. During the cell-cycle phosphorylation and nuclear localization of p70 S6K1 occur in mid G1 phase. We report that the different S6K1 proteins exhibit different nucleocytoplasmic localizations and that the TSC2/mTOR cascade not only regulates p70 S6K1 activity, but also its localization. These findings provide new important insights into the temporal and spatial dynamics of TSC2/mTOR/S6K regulation.