Cell surface receptor membrane localization is strongly dependent on protein-protein interactions often involving regulation by phosphorylation͞dephosphorylation of the intracellular domains of membrane proteins. The present study was carried out to identify metabotropic glutamate receptor (mGluR) 3 regulatory binding proteins. Using the yeast two-hybrid technique, we found that the 50-aa C-terminal cytoplasmic tail of mGluR3 interacts specifically with protein phosphatase 2C␣ (PP2C␣). This interaction was confirmed by GST pull-down and coimmunoprecipitation assays. mGluR3 interacts with PP2C␣, , ␥, and ␦ isoforms; however, among the mGluR family only mGluR3 interacted with PP2C. The minimal interacting domain of mGluR3 comprised residues 836 -855. Alignment between mGluR3 and mGluR2, a closely related group II receptor, indicated that this domain is not conserved between the two receptors. The mGluR3 cytoplasmic C-terminal tail contains one phosphorylation site for protein kinase A (Ser-845), but the phosphatase that dephosphorylates this site has not been previously identified. We find that PP2C, but not PP1, PP2A, or PP2B, dephosphorylates the mGluR3 cytoplasmic tail in vitro. The dephosphorylated form of the mGluR3 cytoplasmic tail, but not the equivalent region of mGluR2, inhibited PP2C assayed by using [ 32 P]casein as a substrate. However, phosphorylation of the mGluR3 cytoplasmic tail at Ser-845 inhibits the interaction with PP2C. These results indicate distinct functions for mGluR2 and mGluR3 and suggest a dynamic regulation of mGluR3 by PP2C.T he vast majority of excitatory synapses in the mammalian central nervous system use the neurotransmitter glutamate to activate two different receptor groups, namely the ''ionotropic'' and the ''metabotropic'' glutamate receptor families. Although ionotropic stimulation is commonly associated with fast neurotransmission, metabotropic glutamate receptor (mGluR) activation drives slower and longer-lasting events. The metabotropic receptor family is divided into three groups based on pharmacological properties, sequence homology, and coupling to intracellular messengers. Group I receptors (mGluR1 and -5) are positively coupled to phospholipase C, whereas group II (mGluR2 and -3) and group III (mGluR4, receptors are negatively coupled to adenylyl cyclase. mGluRs are expressed in most areas of the brain, and extensive data show their importance in major CNS functions and especially neuroplasticity (1, 2). In addition, mGluRs are emerging as new therapeutic targets for neurodegenerative diseases and addiction (3). A variety of drugs acting on group II mGluRs, for instance, have therapeutic potential in the treatment of anxiety, Parkinson disease, schizophrenia, and drug addiction (for reviews, see refs. 4-6).The important role of group II mGluRs in synaptic plasticity, especially in long-term depression, has been extensively characterized (7-9). However, basic and specific properties of group II mGluRs, such as the modulation of glutamate release, origin (vesicular or ...