UCSF

The neuropeptide substance P (SP) is important in pain and inflammation. SP activates the neurokinin-1 receptor (NK1R) to signal via G_q and G_s proteins. Neurokinin A also activates NK1R, but leads to selective G_q signaling. How two stimuli yield distinct G protein signaling at the same G protein-coupled receptor remains unclear. We determined cryogenic-electron microscopy structures of active NK1R bound to SP or the G_q-biased peptide SP6-11. Peptide interactions deep within NK1R are critical for receptor activation. Conversely, interactions between SP and NK1R extracellular loops are required for potent G_s signaling but not G_q signaling. Molecular dynamics simulations showed that these superficial contacts restrict SP flexibility. SP6-11, which lacks these interactions, is dynamic while bound to NK1R. Structural dynamics of NK1R agonists therefore depend on interactions with the receptor extracellular loops and regulate G protein signaling selectivity. Similar interactions between other neuropeptides and their cognate receptors may tune intracellular signaling.