Protein phosphatase 2A is associated in an inactive state with microtubules through 2A1-specific interaction with tubulin

Protein phosphatase (PP) 2A1, a trimer composed of A-, B- and C-subunits in the PP2A family, has been regarded as a principal form localizing at micro tubules (MT), but PP2A2, the dimer of A- and C-subunits, has not. Substanti ating the claim, the present work shows that the PP2A1 but not PP2A2, both isolated from bovine extract, largely associated with the purified preparat ion of MT. Furthermore, PP2A1 was found to bind purified tubulin polymerize d by taxol. The presence of MT associated proteins with purified tubulin ha rdly affected the binding of PP2A1 to the tubulin. In addition, PP2A1 activ ity towards glycogen phosphorylase, a probably unphysiological but good sub strate, was similarly inhibited by MT proteins and purified tubulin, which accounts for greater than or equal to 85 % of MT proteins, with their IC50 of about 0.15 mg/ml. In contrast, the inhibition of PP2A2 was about 40 %, w ith 1 mg/ml MT proteins and 20 % with 0.8 mg/ml tubulin, consistent with it s weak association with MT. Therefore, the association with and resultant i nhibition by MT proteins of PP2A1 is largely effected by the binding of PP2 A1 to tubulin molecule. Moreover, PP2A1 isolated from MT has higher affinit y for polymerized MT proteins than has PP2A1 from the postmicrotubule super natant. The MT PP2A1 has also higher sensitivity to the inhibition by tubul in and MT proteins than has the supernatant PP2A1 (IC50: 0.1-0.2 mg/ml vs. 0.3-0.6 mg/ml), demonstrating the importance of its association with polyme rized tubulin.