Figure 1.

Snake plot of the human histamine H₄ receptor. The full-length receptor consists of 390 amino acids, which form seven transmembrane helices, three extracellular loops and three intracellular loops, with an extracellular N-terminal and an intracellular C-terminal peptide; the dotted line represents the putative disulfide bridge that links the cysteines in the third transmembrane and second extracellular loop. Residues Asp^3.32 and Glu^5.46 that play important roles in histamine binding are marked with bold border. The H₄R₆₇ isoform only contains the first 67 N-terminal residues (marked in white), while the H₄R₃₀₂ lacks the residues marked in grey. The conserved residues in the family A of G protein-coupled receptors are depicted in black circles, while the putative glycosylation sites (Asn⁵ and Asn⁹) are indicated with arrows. A potential palmitoylation site (Cys³⁷⁴) at the C-terminal tail is suggested to be close to the membrane following membrane insertion of a putative attached palmitic acid.