Protein phosphatase-1 inhibitor-3 is co-localized to the nucleoli and centrosomes with PP1γ1 and PP1α, respectively

In this study, we show that protein phosphatase-1 (PP1) inhibitor-3 (Inh3) is localized to the nucleoli and centrosomes in interphase HEK 293 cells. Inh3 exhibited a specific co-localization to the nucleoli with PP1γ1, and to the centrosomes with PP1α. These findings indicate that Inh3 may act as a modulator of PP1 functions in the processes of cytokinesis, as well as of nucleolar events. The specificity of the interaction of Inh3 with the PP1 isoforms was also demonstrated in vitro, where Inh3 co-immunoprecipitated with PP1α and PP1γ1, but not with PP1β. The nuclear localization signal of Inh3 was identified as a N-terminal basic cluster (33RKRK36), while nucleolar localization was shown to be dependent on a C-terminal basic cluster (94HRKGRRR100). The importance of the individual basic residues was quantitatively assessed by site-directed mutagenesis and a novel use of laser scanning cytometry.