The Role of Protein Interaction Motifs in Regulating the Polarity and Clustering of the Metabotropic Glutamate Receptor mGluR1a

When expressed in cultured hippocampal neurons, the metabotropic glutamate receptor mGluR1a is polarized to dendrites and concentrated at postsynaptic sites. We used a mutational analysis to determine how previously identified protein interaction motifs in the C terminus of mGluR1a contribute to its localization. Our results show that the polyproline motif that mediates interaction with Homer family proteins is critical for the synaptic clustering of mGluR1a. A single point mutation in this motif, which prevents the binding of Homer with mGluR1a, reduced its colocalization with a postsynaptic marker to near-chance levels but did not affect its dendritic polarity. In contrast, deleting the PDZ (postsynaptic density-95/Discs large/zona occludens-1) binding domain, which interacts with Tamalin and Shank, had no effect on synaptic localization. Neither of these protein interaction motifs is important for trafficking to the plasma membrane or for polarization to dendrites. Although deleting the entire C terminus of mGluR1a only modestly reduced its dendritic polarity, this domain was sufficient to redirect an unpolarized reporter protein to dendrites. These observations suggest that mGluR1a contains redundant dendritic targeting signals. Together, our results indicate that the localization of mGluR1a involves two distinct steps, one that targets the protein to dendrites and a second that sequesters it at postsynaptic sites; different protein interactions motifs mediate each step.