Phosphorylation of protein phosphatase inhibitor-1 by Cdk5

Protein phosphatase inhibitor-1 is a prototypical mediator of cross-talk be tween protein kinases and protein phosphatases. Activation of cAMP-dependen t protein kinase results in phosphorylation of inhibitor-1 at Thr-35, conve rting it into a potent inhibitor of protein phosphatase-1. Here we report t hat inhibitor-1 is phosphorylated in vitro at Ser-67 by the proline-directe d kinases, Cdk1, Cdk5, and mitogen-activated protein kinase, By using phosp horylation state-specific antibodies and selective protein kinase inhibitor s, Cdk5 was found to be the only kinase that phosphorylates inhibitor-1 at Ser-67 in intact striatal brain tissue. In vitro and in vivo studies indica ted that phospho-Ser-67 inhibitor-1 was dephosphorylated by protein phospha tases-2A and -2B, The state of phosphorylation of inhibitor-1 at Ser-67 was dynamically regulated in striatal tissue by glutamate- dependent regulatio n of N-methyl-D-aspartic acid-type channels. Phosphorylation of Ser-67 did not convert inhibitor-1 into an inhibitor of protein phosphatase-1. However , inhibitor-1 phosphorylated at Ser-67 was a less efficient substrate for c AMP-dependent protein kinase, These results demonstrate regulation of a Cdk 5-dependent phosphorylation site in inhibitor-1 and suggest a role for this site in modulating the amplitude of signal transduction events that involv e cAMP-dependent protein kinase activation.