PNUTS forms a trimeric protein complex with GABAC receptors and protein phosphatase 1

Phosphorylation and dephosphorylation of neurotransmitter receptors represent an important mechanism to regulate synaptic signal transduction. Here, we identified PNUTS, a targeting subunit of protein phosphatase 1 (PP1) as a new binding partner of GABAC receptors. In the mammalian retina, PNUTS is co-expressed with GABAC receptors and PP1 in bipolar cells. PNUTS and PP1 were detected in membrane protein preparations of the retina and precipitate with GABAC receptor specific antibodies. Furthermore, PNUTS shuttles from the nucleus to the membrane in cells co-expressing GABAC receptors. We show simultaneous binding of PP1 and GABAC receptors to different domains of PNUTS, demonstrating that PNUTS cross-links PP1 and GABAC receptors. Finally, modeling studies showed that the PP1 docking motif of PNUTS fits into the binding pocket on the enzyme surface, despite a C-terminal adjacent proline. We suggest that PNUTS targets PP1 to synaptic sites, acting as a temporary bridge between the phosphatase and GABAC receptors.