A NOVEL POOL OF PROTEIN PHOSPHATASE 2A IS ASSOCIATED WITH MICROTUBULES AND IS REGULATED DURING THE CELL-CYCLE

Immunofluorescence microscopy revealed the presence of protein phospha tase 2A (PP2A) on microtubules in neuronal and nonneuronal cells. Inte rphase and mitotic spindle microtubules, as well as centrosomes, were all labeled with antibodies against individual PP2A subunits, showing that the AB alpha C holoenzyme is associated with microtubules. Bioche mical analysis showed that PP2A could be reversibly bound to microtubu les in vitro and that similar to 75% of the PP2A in cytosolic extracts could interact with microtubules. The activity of microtubule-associa ted PP2A was differentially regulated during the cell cycle. Enzymatic activity was high during S phase and intermediate during G1, while th e activity in G2 and M was 20-fold lower than during S phase. The amou nt of microtubule-bound PP2A remained constant throughout the cell cyc le, implying that cell cycle regulation of its enzymatic activity invo lves factors other than microtubules. These results raise the possibil ity that PP2A regulates cell cycle-dependent microtubule functions, su ch as karyokinesis and membrane transport.