DEPHOSPHORYLATION OF SER-137 IN DARPP-32 BY PROTEIN PHOSPHATASES 2A AND 2C - DIFFERENT ROLES IN-VITRO AND IN STRIATONIGRAL NEURONS

DARPP-32 (dopamine-and cAMP-regulated phosphoprotein, M-r = 32000) is highly expressed in striatonigral neurons in which its phosphorylation is regulated by several neurotransmitters including dopamine and glut amate. DARPP-32 becomes a potent inhibitor of protein phosphatase 1 wh en it is phosphorylated on Thr-34 by cAMP- or cGMP-dependent protein k inases. DARPP-32 is also phosphorylated on Ser-137 by protein kinase C K1 (CK1), in vitro and in vivo. This phosphorylation has an important regulatory role since it inhibits the dephos phorylation of Thr-34 by calcineurin in vitro and in striatonigral neurons. Here, we show that DARPP-32 phosphorylated by CK1 is a substrate in vitro for protein pho sphatases 2A and 2C, but not protein phosphatase 1 or calcineurin, How ever, in substantia nigra slices, dephosphorylation of Ser-137 was mar kedly sensitive to decreased temperature, and not detectably affected by the presence of okadaic acid under conditions in which dephosphoryl ation of Thr-34 by protein phosphatase 2A was inhibited. These results suggest that, in neurons, phospho-Ser-137-DARPP-32 is dephosphorylate d by protein phosphatase 2C, but not 2A, Thus, DARPP-32 appears to be a component of a regulatory cascade of phosphatases in which dephospho rylation of Ser-136 by protein phosphatase 2C facilitates dephosphoryl ation of Thr-34 by calcineurin, removing the cyclic nucleotide-induced inhibition of protein phosphatase 1.