Phospholipase C? (PLC?) enzymes are activated by G protein-coupled receptors through receptor-catalyzed guanine nucleotide exchange on G??? heterotrimers containing Gq family G proteins. Here we report evidence for a direct interaction between M3 muscarinic receptor (M3R) and PLC?3. Both expressed and endogenous M3R interacted with PLC? in coimmunoprecipitation experiments. Stimulation of M3R with carbachol significantly increased this association. Expression of M3R in CHO cells promoted plasma membrane localization of YFP-PLC?3. Deletion of the PLC?3 C terminus or deletion of the PLC?3 PDZ ligand inhibited coimmunoprecipitation with M3R and M3R-dependent PLC?3 plasma membrane localization. Purified PLC?3 bound directly to glutathione S-transferase (GST)-fused M3R intracellular loops 2 and 3 (M3Ri2 and M3Ri3) as well as M3R C terminus (M3R/H8-CT). PLC?3 binding to M3Ri3 was inhibited when the PDZ ligand was removed. In assays using reconstituted purified components in vitro, M3Ri2, M3Ri3, and M3R/H8-CT potentiated G?q-dependent but not G??-dependent PLC?3 activation. Disruption of key residues in M3Ri3N and of the PDZ ligand in PLC?3 inhibited M3Ri3-mediated potentiation. We propose that the M3 muscarinic receptor maximizes the efficiency of PLC?3 signaling beyond its canonical role as a guanine nucleotide exchange factor for G?.
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