The C‐terminal truncated splicing variant of NK1R negatively modulates substance P‐stimulated NK1R signaling

Mammalian neurokinin 1 receptor (NK1R) exists as full‐length (NK1L) and truncated (NK1S) splice variants. However, the functional role of NK1S remains controversial; therefore, we investigated their functional interplay. Using NanoBiT and co‐immunoprecipitation, we demonstrate that NK1L and NK1S form heterodimeric complexes. Through this interaction, NK1S negatively modulates NK1L‐mediated G protein signaling, specifically impairing Gαq coupling and Ca 2+ mobilization. Conversely, NK1S enhances β‐arrestin1 recruitment to NK1L, altering receptor trafficking. In A549 cells, NK1S suppressed NK1L‐mediated cell migration despite sustaining ERK phosphorylation. These findings provide mechanistic evidence that NK1S can regulate NK1L through dimerization and may shift signaling bias from G proteins toward β‐arrestin‐linked pathways to influence cellular outcomes.