b 1 -Adrenergic Receptor Association with the Synaptic Scaffolding Protein Membrane-associated Guanylate Kinase Inverted-2 (MAGI-2)

The b1-adrenergic receptor (b1AR) is known to be localized to synapses and to modulate synaptic plasticity in many brain regions, but the molecular mechanisms determining b1AR subcellular localization are not fully understood. Using overlay and pull-down techniques, we found that the b1AR carboxyl terminus associates with MAGI-2 (membrane-associated guanylate kinase inverted-2), a protein also known as S-SCAM (synaptic scaffolding molecule). MAGI-2 is a multidomain scaffolding protein that contains nine potential protein-protein interaction modules, including 6 PDZ domains, 2 WW domains, and a guanylate kinase-like domain. The b1AR carboxyl terminus binds with high affinity to the first PDZ domain of MAGI-2, with the last few amino acids of the b1AR carboxyl terminus being the key determinants of the interaction. In cells, the association of full-length b1AR with MAGI-2 occurs constitutively and is enhanced by agonist stimulation of the receptor, as assessed by both co-immunoprecipitation experiments and immunofluorescence co-localization studies. Agonist-induced internalization of the b1AR is markedly increased by co-expression with MAGI-2. Strikingly, this result is the opposite of the effect of co-expression with PSD-95, a previously reported binding partner of the b1AR. Further cellular experiments revealed that MAGI-2 has no effect on b1AR oligomerization but does promote association of b1AR with the cytoplasmic signaling protein b-catenin, a known MAGI-2 binding partner. These data reveal that MAGI-2 is a specific b1AR binding partner that modulates b1AR function and facilitates the physical association of the b1AR with intracellular proteins involved in signal transduction and synaptic regulation.