NUCLEAR-LOCALIZATION AND CELL-CYCLE REGULATION OF A MURINE PROTEIN-TYROSINE-PHOSPHATASE

MPTP is a murine homolog of the human T-cell protein tyrosine phosphat ase (PTPase) and the rat PTP-S enzyme. Enzymatic activity of this ubiq uitously expressed protein was demonstrated in immunoprecipitates from NIH 3T3 cells and in recombinant protein overexpressed in bacteria. E xpression of beta-galactosidase-MPTP chimeric proteins in COS1 cells i dentified a nuclear localization signal at the carboxyl terminus of th e MPTP that was sufficient to direct beta-galactosidase as well as a t agged version of the MPTP to the nucleus. Deletion analysis of amino a cids within the nuclear targeting signal showed that this sequence doe s not conform to the bipartite type of nuclear localization signals. F urthermore, it was shown that the steady-state levels of MPTP RNA fluc tuate in a cell cycle-specific manner. On the basis of these experimen ts, we discuss the possible function of MPTP in the cell cycle and oth er nuclear processes.