β1-Adrenergic receptor association with the synaptic scaffolding protein membrane-associated guanylate kinase inverted-2 (MAGI-2) -: Differential regulation of receptor internalization by MAGI-2 and PSD-95.

The beta (1)-adrenergic receptor (beta (1)AR) is known to be localized to synapses and to modulate synaptic plasticity in many brain regions, but the molecular mechanisms determining beta (1)AR subcellular localization are not fully understood. Using overlay and pull-down techniques, we found that the beta (1)AR 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 beta (1)AR carboxyl terminus binds with high affinity to the first PDZ domain of MAGI-2, with the last few amino acids of the beta (1)AR carboxyl terminus being the key determinants of the interaction. In cells, the association of full-length beta (1)AR 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 beta (1)AR 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 beta (1)AR. Further cellular experiments revealed that MAGI-2 has no effect on beta (1)AR oligomerization but does promote association of beta (1)AR with the cytoplasmic signaling protein beta -catenin, a known MAGI-2 binding partner. These data reveal that MAGI-2 is a specific beta (1)AR binding partner that modulates beta (1)AR function and facilitates the physical association of the beta (1)AR with intracellular proteins involved in signal transduction and synaptic regulation.