POSTNATAL CHANGES IN SERINE THREONINE PROTEIN PHOSPHATASES AND THEIR ASSOCIATION WITH THE MICROTUBULES

The activities and protein levels of three serine/threonine protein ph osphatases were determined in homogenates and microtubule preparations from rat brain at various ages from postnatal day 1 (P1) through adul thood. The activities and levels of the calcium/calmodulin-dependent p rotein phosphatase, phosphatase 2B increased significantly from P1 to P21 in brain homogenates and remained elevated in the adult. The assoc iation of phosphatase 2B with microtubules was also found to be increa sed in the adult compared to the neonate (P3). In contrast, protein ph osphatase 2A in brain homogenates decreased significantly from P1 to a dult. However, the association of phosphatase 2A with the microtubules was found to increase with age. Finally, the activity and levels of p hosphatase 1 in brain homogenates did not change with postnatal age, a lthough the association of phosphatase 1 with microtubules was signifi cantly decreased in the adult brain compared to P3. These studies clea rly indicate that the activity, levels and association of these serine /threonine phosphatases with microtubules are independently regulated during postnatal development and suggest unique roles of phosphatase 1 , phosphatase 2A and phosphatase 2B in modulating the phosphorylation state and function of microtubule-associated proteins at different pos tnatal ages.