Protein phosphatase regulation by PRIP, a PLC-related catalytically inactive protein-Implications in the phospho-modulation of the GABAA receptor

PRIP, phospholipase C related, but catalytically inactive protein was first identified as a novel inositol 1,4,5-trisphosphate binding protein. It has a number of binding partners including protein phosphatase (PP1 and 2A), GABAA receptor associated protein, and the β subunits of GABAA receptors, in addition to inositol 1,4,5-trisphosphate. The identification of these molecules led us to examine the possible involvement of PRIP in the phospho-regulation of the β subunits of GABAA receptors using hippocampal neurons prepared from PRIP-1 and 2 double knock-out (DKO) mice. Experiments were performed with special reference to the dephosphorylation processes of the β subunits. The phosphorylation of β3 subunits by the activation of protein kinase A in cortical neurons of the control mice continued for up to 5 min, even after washing out of the stimulus, followed by a gradual dephosphorylation. That of DKO mice gradually increased in spite of the lower phosphorylation levels induced by the stimulation. There was little difference in the amount of cellular cyclic AMP and protein kinase A activity between the control and mutant mice, indicating that phosphatases such as PP1 and PP2A are primarily involved in the difference. The time course of PP1 activity changes in the vicinity of the receptors in control mice corresponded to the phosphorylation of PRIP, while that of the mutant mice decreased with the period of the incubation. This is a good agreement with the suggestion that PRIP binds to and inactivates PP1, which is regulated by the phosphorylation of PRIP at threonine 94. These results suggest that PRIP plays an important role in controlling the dynamics of GABAA receptor phosphorylation by through PP1 binding and, therefore, the efficacy of synaptic inhibition mediated by these receptors.