Biochemical characterization of G protein coupling to calcitonin gene?related peptide and adrenomedullin receptors using a native PAGE assay

Calcitonin gene-related peptide (CGRP), adrenomedullin (AM), and adrenomedullin 2/intermedin (AM2/IMD) have overlapping and unique functions in the nervous and circulatory systems including vasodilation, cardioprotection, and pain transmission. Their actions are mediated by the class B calcitonin-like G protein?coupled receptor (CLR), which heterodimerizes with three receptor activity?modifying proteins (RAMP1?3) that determine its peptide ligand selectivity. How the three agonists and RAMPs modulate CLR binding to transducer proteins remains poorly understood. Here, we biochemically characterized agonist-promoted G protein coupling to each CLR?RAMP complex. We adapted a native PAGE method to assess the formation and thermostabilities of detergent-solubilized fluorescent protein?tagged CLR?RAMP complexes expressed in mammalian cells. Addition of agonist and the purified G(s)protein surrogate mini-G(s)(mG(s)) yielded a mobility-shifted agonist?CLR?RAMP?mG(s)quaternary complex gel band that was sensitive to antagonists. Measuring the apparent affinities of the agonists for the mG(s)-coupled receptors and of mG(s)for the agonist-occupied receptors revealed that both ligand and RAMP control mG(s)coupling and defined how agonist engagement of the CLR extracellular and transmembrane domains affects transducer recruitment. Using mini-G(sq)and -G(si)chimeras, we observed a coupling rank order of mG(s)> mG(sq)> mG(si)for each receptor. Last, we demonstrated the physiological relevance of the native gel assays by showing that they can predict the cAMP-signaling potencies of AM and AM2/IMD chimeras. These results highlight the power of the native PAGE assay for membrane protein biochemistry and provide a biochemical foundation for understanding the molecular basis of shared and distinct signaling properties of CGRP, AM, and AM2/IMD.