Regulation of subcellular localization of the antiproliferative protein Tob by its nuclear export signal and bipartite nuclear localization signal sequences.

Tob, a member of the Tob and BTG antiproliferative protein family, plays an important role in many cellular processes including cell proliferation. In this study, we have addressed molecular mechanisms regulating subcellular localization of Tob. Treatment with leptomycin B, an inhibitor of nuclear export signal (NES) receptor, resulted in a change in subcellular distribution of Tob from its pan-cellular distribution to nuclear accumulation, indicating the existence of NES in Tob. Our results have then identified an N-terminal region (residues 2-14) of Tob as a functional NES. They have also shown that Tob has a functional, bipartite nuclear localization signal (NLS) in residues 18-40. Thus, Tob is shuttling between the nucleus and the cytoplasm by its NES and NLS. To examine a possible relationship between subcellular distribution of Tob and its function, we exogenously added a strong NLS sequence or a strong NES sequence or both to Tob. The obtained results have demonstrated that the strong NLS-added Tob has a much weaker activity to inhibit cell cycle progression from G0/G1 to S phase. These results suggest that cytoplasmic localization or nucleocytoplasmic shuttling is important for the antiproliferative function of Tob.