Figure 1.

Sequence comparison between the ATRs, ETRs, and bovine rhodopsin (bOPSD) or human β-2 adrenergic receptor (hADRB2). The length of each transmembrane helix (TM1-7) or loops (IL, intracellular loop; EL, extracellular loop) are indicated above the sequence according to an AT₁R structure [PDB ID: 4zud (53)] but can differ slightly in other structures. The overall sequence similarity between ET_AR and ET_BR is approximately 63%, whereas between AT₁R and AT₂R ~47%. Sequence similarities between ATRs and ETRs, respectively, are around 30%. The sequences of prototypical class A GPCRs bOPSD and hADRB2 are provided additionally for comparison. The alignment was visualized using the software BioEdit (54). Specific background colors reflect chemical properties of the amino acid side chains or the type of amino acid: black-proline; blue-positively charged; cyan/green-aromatic and hydrophobic; green- hydrophobic; red-negatively charged; gray-hydrophilic; dark red-cysteines; and magenta-histidine.