Modeling the AM-bound AM2 receptor ECD complex (Supplemental Experimental Procedures) suggests that RAMP3 augments the binding site as a RAMP1-2 hybrid (Figure 7C). RAMP3 E74, which is equivalent to RAMP2 E101, would favor AM binding by hydrogen bonding with Y52. RAMP3 W84, which is equivalent to RAMP1 W84, could contact the AM Y52 and CGRP F37 phenyl rings, thereby explaining diminished potency of both peptides at the AM2 receptor with RAMP3 W84A and why CGRP is more active at the AM2 receptor than the AM1 receptor (Watkins et al., 2014). The key RAMP residues proposed as selectivity determinants are conserved across species: W84 and a lack of Glu at position 74 in RAMP1, E101 and F or Y at position 111 in RAMP2, and E74 and W84 in RAMP3 (Figure 7D). A small amino acid is conserved at position 70 in RAMP1/3, which would avoid steric clash with W84. Notably, the lack of conservation of RAMP2 R97 and E105 is consistent with the mutagenesis data that indicated that these residues are not critical for AM signaling.