DOPAMINE-REGULATED AND CAMP-REGULATED PHOSPHOPROTEIN DARPP-32 - PHOSPHORYLATION OF SER-137 BY CASEIN KINASE-I INHIBITS DEPHOSPHORYLATION OFTHR-34 BY CALCINEURIN

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 phosphopro tein of apparent M(r)32,000, is phosphorylated in vitro by casein kina se I, casein kinase II, and cAMP-dependent protein kinase on sites pho sphorylated in vivo. DARPP-32 phosphorylated on Thr-34 by cAMP-depende nt protein kinase is a potent inhibitor of protein phosphatase 1 and a n excellent substrate for calcineurin, a Ca2+/calmodulin-dependent pro tein phosphatase. Here we provide evidence, using both purified protei ns and brain slices, that phosphorylation of DARPP-32 on Ser-137 by ca sein kinase I inhibits the dephosphorylation of Thr-34 by calcineurin. This inhibition occurs only when phospho-Ser-137 and phospho-Thr-34 a re located on the same DARPP-32 molecule and is not dependent on the m ode of activation of calcineurin. The results demonstrate that the inh ibition is due to a modification in the properties of the substrate wh ich alters its dephosphorylation rate. Thus, casein kinase I may play a physiological role in striatonigral neurons as a modulator of the re gulation of protein phosphatase 1 via DARPP-32.