Desdouits F, Siciliano J C, Greengard P, Girault J A
Institut National de la Santé et de la Recherche Médicale U114, Collège de France, Paris.
Proc Natl Acad Sci U S A. 1995 Mar 28;92(7):2682-5. doi: 10.1073/pnas.92.7.2682.
Although protein phosphatases appear to be highly controlled in intact cells, relatively little is known about the physiological regulation of their activity. DARPP-32, a dopamine- and cAMP-regulated phosphoprotein of apparent M(r) 32,000, is phosphorylated in vitro by casein kinase I, casein kinase II, and cAMP-dependent protein kinase on sites phosphorylated in vivo. DARPP-32 phosphorylated on Thr-34 by cAMP-dependent protein kinase is a potent inhibitor of protein phosphatase 1 and an excellent substrate for calcineurin, a Ca2+/calmodulin-dependent protein phosphatase. Here we provide evidence, using both purified proteins and brain slices, that phosphorylation of DARPP-32 on Ser-137 by casein kinase I inhibits the dephosphorylation of Thr-34 by calcineurin. This inhibition occurs only when phospho-Ser-137 and phospho-Thr-34 are located on the same DARPP-32 molecule and is not dependent on the mode of activation of calcineurin. The results demonstrate that the inhibition is due to a modification in the properties of the substrate which alters its dephosphorylation rate. Thus, casein kinase I may play a physiological role in striatonigral neurons as a modulator of the regulation of protein phosphatase 1 via DARPP-32.
尽管蛋白磷酸酶在完整细胞中似乎受到高度调控，但关于其活性的生理调节却知之甚少。DARPP - 32是一种表观分子量为32,000的多巴胺和cAMP调节的磷蛋白，在体外可被酪蛋白激酶I、酪蛋白激酶II和cAMP依赖性蛋白激酶磷酸化，这些位点在体内也会被磷酸化。由cAMP依赖性蛋白激酶在苏氨酸 - 34位点磷酸化的DARPP - 32是蛋白磷酸酶1的有效抑制剂，也是钙调神经磷酸酶（一种Ca2 + /钙调蛋白依赖性蛋白磷酸酶）的优良底物。在这里，我们使用纯化蛋白和脑片提供证据表明，酪蛋白激酶I使DARPP - 32的丝氨酸 - 137位点磷酸化会抑制钙调神经磷酸酶对苏氨酸 - 34位点的去磷酸化作用。这种抑制作用仅在磷酸化的丝氨酸 - 137和磷酸化的苏氨酸 - 34位于同一DARPP - 32分子上时才会发生，并且不依赖于钙调神经磷酸酶的激活方式。结果表明，这种抑制作用是由于底物性质的改变，从而改变了其去磷酸化速率。因此，酪蛋白激酶I可能在纹状体黑质神经元中作为通过DARPP - 32调节蛋白磷酸酶1的调节剂发挥生理作用。
