LAUSR

Histamine exerts critical physiological roles by activating four receptor subtypes, each exhibiting a specific G protein preference. Among these, the histamine H4 receptor (H4R) modulates chemotaxis and interferon production through Gi protein activation, suggesting its therapeutic potential. Despite its physiological significance, the mechanisms underlying H4R signalling and G protein preference across histamine receptors remain poorly understood. Here, we present the cryo-electron microscopy structure of the H4R-Gi complex, revealing unique mechanisms of histamine recognition and receptor activation. We further solved the structures of the histamine H1 receptor (H1R) bound to the non-canonical G proteins Gi and Gs. Through a combination of functional and computational analyses, we identified the intracellular loop 2 as a critical determinant of G protein preference in H1R and H4R. Collectively, our comprehensive study revealed the structural basis for distinct mechanisms of ligand recognition and receptor activation, offering a profound insight into G protein preference across receptor subtypes. This study presents cryo-EM structures of H4R–Gi and H1R–Gi/Gs complexes, revealing distinct histamine recognition, activation mechanisms, and a critical determinant of G protein selectivity in H1R and H4R.