Ser203 as well as Ser204 and Ser207 in fifth transmembrane domain of the human β2‐adrenoceptor contributes to agonist binding and receptor activation

We examined the contribution of Ser203 of the human β2‐adrenoceptor (β2‐AR) to the interaction with isoprenaline. The affinity of (−)‐isoprenaline was reduced by substitution of an alanine for Ser203, as well as for Ser204 and Ser207. An (−)‐isoprenaline derivative with only one hydroxyl group, at the meta‐position, showed reduced affinity for wild‐type β2‐AR and S207A‐β2‐AR and even lower affinities for S203A‐β2‐AR and S204A‐β2‐AR. By contrast, an (−)‐isoprenaline derivative with only a para‐hydroxyl group showed reduced affinity for wild‐type β2‐AR but the serine to alanine mutations did not cause further decreases. The EC50 value for cyclic AMP generation in response to (−)‐isoprenaline was increased, by about 120 fold, for each alanine‐substituted β2‐AR mutant. These results suggest that Ser203 of the human β2‐AR is important for both ligand binding and receptor activation.

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