Negative allosteric modulation of the glucagon receptor by RAMP2

Receptor activity-modifying proteins (RAMPs) modulate the activity of many Family B heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs). The glucagon receptor (GCGR), a Family B GPCR responsible for maintenance of proper blood sugar levels, interacts with RAMP2, though the purpose and consequence of this interaction is poorly understood. Using a series of biochemical and cell-based assays, we show that RAMP2 interacts with and broadly inhibits GCGR-induced downstream signaling. Hydrogen-deuterium exchange monitored by mass spectrometry (HDX-MS) demonstrates that RAMP2 enhances local flexibility in select locations in and near the receptor extracellular domain (ECD) as well as at a key region in the 6th transmembrane helix, while single-molecule fluorescence resonance energy transfer (smFRET) experiments show that this RAMP2-induced ECD disorder results in inhibition of active and intermediate states of the intracellular face of the receptor. Using cryo-electron microscopy (cryoEM), we determined the structure of the GCGR-Gs complex at 2.9 [A] resolution in the presence of RAMP2. RAMP2 apparently does not interact with GCGR in an ordered manner, yet the ECD of GCGR is indeed largely disordered in the presence of RAMP2. This disorder is accompanied by rearrangements of several key areas of the receptor, resulting in the formation of a likely unproductive complex. Together, our studies suggest that RAMP2 acts as a negative allosteric modulator of GCGR by enhancing conformational sampling of the ECD.