Phosphorylation of DARPP-32 by Cdk5 modulates dopamine signalling in neurons

The physiological state of the cell is controlled by signal transduction me chanisms which regulate the balance between protein kinase and protein phos phatase activities(1). Here we report that a single protein can, depending on which particular amino-acid residue is phosphorylated, function either a s a kinase or phosphatase inhibitor. DARPP-32 (dopamine and cyclic AMP-regu lated phospho-protein, relative molecular mass 32,000) is converted into an inhibitor of protein phosphatase 1 when it is phosphorylated by protein ki nase A (PKA) at threonine 34 (refs 2, 3), We find that DARPP-32 is converte d into an inhibitor of PKA when phosphorylated at threonine 75 by cyclin-de pendent kinase 5 (Cdk5), Cdk5 phosphorylates DARPP-32 in vitro and in intac t brain cells. Phospho-Thr 75 DARPP-32 inhibits PKA in vitro by a competiti ve mechanism. Decreasing phospho-Thr 75 DARPP-32 in striatal slices, either by a Cdk5-specific inhibitor or by using genetically altered mice, results in increased dopamine-induced phosphorylation of PKA substrates and augmen ted peak voltage-gated calcium currents. Thus DARPP-32 is a bifunctional si gnal transduction molecule which, by distinct mechanisms, controls a serine /threonine kinase and a serine/threonine phosphatase.