Inhibitors of protein phosphatase 1 and 2A decrease the level of tubulin carboxypeptidase activity associated with microtubules.

The association of tubulin carboxypeptidase with microtubules may be involved in the determination of the tyrosination state of the microtubules, i.e. their proportion of tyrosinated vs. nontyrosinated tubulin. We investigated the role of protein phosphatases in the association of carboxypeptidase with microtubules in COS cells. Okadaic acid and other PP1/PP2A inhibitors, when added to culture medium before isolation of the cytoskeletal fraction, produced near depletion of the carboxypeptidase activity associated with microtubules. Isolation of the native assembled and nonassembled tubulin fractions from cells treated and not treated with okadaic acid, and subsequent in vitro assay of the carboxypeptidase activity, revealed that the enzyme was dissociated from microtubules by okadaic acid treatment and recovered in the soluble fraction. There was no effect by nor-okadaone (an inactive okadaic acid analogue) or inhibitors of PP2B and of tyrosine phosphatases which do not affect PP1/PP2A activity. When tested in an in vitro system, okadaic acid neither dissociated the enzyme from microtubules nor inactivated it. In living cells, prior stabilization of microtubules with taxol prevented the dissociation of carboxypeptidase by okadaic acid indicating that dynamic microtubules are needed for okadaic acid to exert its effect. On the other hand, stabilization of microtubules subsequent to okadaic acid treatment did not reverse the dissociating effect of okadaic acid. These results suggest that dephosphorylation (and presumably also phosphorylation) of the carboxypeptidase or an intermediate compound occurs while it is not associated with microtubules, and that the phosphate content determines whether or not the carboxypeptidase is able to associate with microtubules.