J.M.B. produced and characterized proteins, performed crystallization, collected diffraction data, and constructed CLR and RAMP mutants; C.S.W., M.A.J., and D.L.H. designed, performed, and analyzed cell-based assays for the CGRP and AM1 receptors; J.B., G.K., J.S., R.M.B., and D.R.P. designed and constructed CLR mutants and performed and analyzed cell-based assays for the CGRP receptor; J.S. performed RAMP3 homology modeling; M.L.W. performed peptide binding experiments; P.W.H. and M.A.B. synthesized CGRP analog; A.A.P. designed and managed the structural project, solved/refined the structures, and wrote the manuscript with D.R.P. and D.L.H.