Activated CaMKIIα Binds to the mGlu₅ Metabotropic Glutamate Receptor and Modulates Calcium Mobilization

Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) and metabotropic glutamate receptor 5 (mGlu₅) are critical signaling molecules in synaptic plasticity and learning/memory. Here, we demonstrate that mGlu₅ is present in CaMKIIα complexes isolated from mouse forebrain. Further in vitro characterization showed that the membrane-proximal region of the C-terminal domain (CTD) of mGlu_5a directly interacts with purified Thr286-autophosphorylated (activated) CaMKIIα However, the binding of CaMKIIα to this CTD fragment is reduced by the addition of excess Ca²⁺/calmodulin or by additional CaMKIIα autophosphorylation at non-Thr286 sites. Furthermore, in vitro binding of CaMKIIα is dependent on a tribasic residue motif Lys-Arg-Arg (KRR) at residues 866-868 of the mGlu_5a-CTD, and mutation of this motif decreases the coimmunoprecipitation of CaMKIIα with full-length mGlu_5a expressed in heterologous cells by about 50%. The KRR motif is required for two novel functional effects of coexpressing constitutively active CaMKIIα with mGlu_5a in heterologous cells. First, cell-surface biotinylation studies showed that CaMKIIα increases the surface expression of mGlu_5a Second, using Ca²⁺ fluorimetry and single-cell Ca²⁺ imaging, we found that CaMKIIα reduces the initial peak of mGlu_5a-mediated Ca²⁺ mobilization by about 25% while doubling the relative duration of the Ca²⁺ signal. These findings provide new insights into the physical and functional coupling of these key regulators of postsynaptic signaling.