Ser(203) as well as Ser(204) and Ser(207) in fifth transmembrane domain ofthe human beta(2)-adrenoceptor contributes to agonist binding and receptoractivation

We examined the contribution of Ser(203) of the human b(2)-adrenoccptor (b( 2)-AR) to the interaction with isoprenaline. The affinity of (-)-isoprenali ne was reduced by substitution of an alanine for Ser(203), as well as for S er(204) and Ser(207). An (-)-isoprenaline derivative with only one hydroxyl group, at the meta-position, showed reduced affinity for wild-type b(2)-AR and S207A-b(2)-AR and even lower affinities for S203A-b(2)-AR and S204A-b( 2)-AR. By contrast, an (-)-isoprenaline derivative with only a pa, a-hydrox yl group showed reduced affinity for wild-type b(2)-AR but the serine to al anine 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 b(2)-AR mutant. These results suggest th at Ser(203) of the human b(2)-AR is important for both ligand binding and r eceptor activation.