The four cysteine residues in the second extracellular loop of the human adenosine A2B receptor: Role in ligand binding and receptor function

The adenosine A2B receptor is of considerable interest as a new drug target for the treatment of asthma, inflammatory diseases, pain, and cancer. In the present study we investigated the role of the cysteine residues in the extracellular loop 2 (ECL2) of the receptor, which is particularly cysteine-rich, by a combination of mutagenesis, molecular modeling, chemical and pharmacological experiments. Pretreatment of CHO cells recombinantly expressing the human A2B receptor with dithiothreitol led to a 74-fold increase in the EC50 value of the agonist NECA in cyclic AMP accumulation. In the C783.25S and the C17145.50S mutant high-affinity binding of the A2B antagonist radioligand [3H]PSB-603 was abolished and agonists were virtually inactive in cAMP assays. This indicates that the C3.25–C45.50 disulfide bond, which is highly conserved in GPCRs, is also important for binding and function of A2B receptors. In contrast, the C16645.45S and the C16745.46S mutant as well as the C16645.45S–C16745.46S double mutant behaved like the wild-type receptor, while in the C15445.33S mutant significant, although more subtle effects on cAMP accumulation were observed – decrease (BAY60-6583) or increase (NECA) – depending on the structure of the investigated agonist. In contrast to the X-ray structure of the closely related A2A receptor, which showed four disulfide bonds, the present data indicate that in the A2B receptor only the C3.25–C45.50 disulfide bond is essential for ligand binding and receptor activation. Thus, the cysteine residues in the ECL2 of the A2B receptor not involved in stabilization of the receptor structure may have other functions.

Graphical abstractAdenosine A2B receptors form only one disulfide bond between the conserved cysteine residues C783.25 and C17145.50; the other three cysteines in the loop are not involved in disulfide bond formation.Figure optionsDownload full-size imageDownload as PowerPoint slide