The CDC25B dual specificity phosphatase is involved in the control of the G
2/M transition of the cell cycle. Subcellular localization might represent
an important aspect of the regulation of its activity, We have examined in
transiently transfected asynchronous HeLa cells the localization of HA-tagg
ed CDC25B proteins and found that they are nuclear or cytoplasmic suggestin
g the existence of an active shuttling. Accordingly, localization analysis
of deletion and truncation proteins indicates that CDC25B contains a putati
ve nuclear localization signal located between residues 335 and 353. We als
o demonstrated that a short 58 residues deletion of the amino-terminus end
of CDC25B is sufficient to retain it to the nucleus, Mutational analysis in
dicates that a nuclear export sequence is located between residues 28 and 4
0. In addition, treatment of the cells with the exportin inhibitor, Leptomy
cin B, has the same effect, The mutation of Ser-323, a residue that is esse
ntial for the interaction with 14-3-3 proteins, also abolishes cytoplasmic
staining. The subcellular localization of CDC25B is therefore dependent on
the combined effects of a nuclear localization signal, a nuclear export sig
nal and on the interaction with 14-3-3 proteins.