Fig. 4. Subtype selectivity in H₄R and H₁R.

a, b Structural superposition of H₄R_his (pink) and H₁R_his (orange, PDB 7DFL). (a) top view, (b) the agonist-binding pocket. The helices are shown as cylinders. The histamines of H₄R and H₁R are indicated by yellow and cyan sticks. c The aromatic slot on H₄R_ime (blue) and the corresponding region for H₁R_his (orange). Imetit and histamine are shown as magenta and cyan sticks. Residues comprising the pocket for H₄R_ime are indicated by sticks and for H₁R_his are shown as sticks and transparent spheres. d Sequence alignment of residues important for agonist binding in the histamine receptor family. Red triangles indicate residues corresponding to the aromatic slots in H₄R. e, f TGFα shedding assay results of WT H₄R and H₄R mutants activated by histamine and imetit. The activities of the agonists are identified as pEC50 (e). Average Emax values were determined from the “log (agonist) vs. response -variable slope (three or four parameters)” function in GraphPad Prism 9 software (GraphPad Software Inc., San Diego, CA) (f). Data represent the mean ± SEM from n = 3 biologically independent experiments performed in triplicate. ND, not detected.