Postnatal changes in serine/threonine protein phosphatases and their association with the microtubules

The activities and protein levels of three serine/threonine protein phosphatases were determined in homogenates and microtubule preparations from rat brain at various ages from postnatal day 1 (P1) through adulthood. The activities and levels of the calcium/calmodulin-dependent protein phosphatase, phosphatase 2B increased significantly from P1 to P21 in brain homogenates and remained elevated in the adult. The association of phosphatase 2B with microtubules was also found to be increased in the adult compared to the neonate (P3). In contrast, protein phosphatase 2A in brain homogenates decreased significantly from P1 to adult. However, the association of phosphatase 2A with the microtubules was found to increase with age. Finally, the activity and levels of phosphatase 1 in brain homogenates did not change with postnatal age, although the association of phosphatase 1 with microtubules was significantly decreased in the adult brain compared to P3. These studies clearly 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 postnatal ages.