The phosphorylation of ionotropic glutamate receptors (iGluRs) by protein kinases plays a crucial role in synaptic plasticity. In the cerebellum, protein kinase C (PKC) activation is required for the induction of long-term depression (LTD) at parallel fibre-Purkinje cell synapses. Although δ2 glutamate receptors (GluRδ2), expressed predominantly in Purkinje cells, are essential for cerebellar LTD, little is known about the mechanism by which GluRδ2 participates in LTD or its relationship with PKC activation pathways. We found that a PKC activator, phorbol ester, induced postsynaptic LTD in Purkinje cells in mouse cerebellar slice preparations without significantly changing the presynaptic properties. Under this condition, the GluRδ2 prepared from the cerebellar slices was significantly phosphorylated. Indeed, the C-terminus of the GluRδ2 fused with glutathione-S-transferase (GST) was directly phosphorylated by purified PKC at a specific serine residue. In addition, two-dimensional phosphopeptide mapping analysis indicated that the major phosphorylation site of the GST-fusion protein containing the C-terminus of GluRδ2 was identical to that of GluRδ2 prepared from cerebellar slices. Therefore, GluRδ2 is phosphorylated by PKC in vitro and by an LTD-inducing stimulus in slice preparations. Because this region of GluRδ2 is known to associate with certain intracellular molecules, the PKC phosphorylation status of the C-terminus of GluRδ2 may be involved in new signaling pathways during LTD.
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