Probing the salmeterol binding site on the β2-adrenergic receptor using a novel photoaffinity ligand, [125I]iodoazidosalmeterol

Salmeterol is a long-acting beta(2)-adrenergic receptor (beta(2)AR) agonist used clinically to treat asthma. In addition to binding at the active agonist site, it has been proposed that salmeterol also binds with very high affinity at a second site, termed the "exosite", and that this exosite contributes to the long duration of action of salmeterol. To determine the position of the phenyl ring of the aralkyloxyalkyl side chain of salmeterol in the beta(2)AR binding site, we designed and synthesized the agonist photoaffinity label [I-125]iodoazidosalmeterol ([I-125]IAS). In direct adenylyl cyclase activation, in effects on adenylyl cyclase after pretreatment of intact cells, and in guinea pig tracheal relaxation assays, IAS and the parent drug salmeterol behave essentially the same. Significantly, the photoreactive azide of IAS is positioned on the phenyl ring at the end of the molecule which is thought to be involved in exosite binding. Carrier-free radioiodinated [I-125]IAS was used to photolabel epitope-tagged human beta(2)AR in membranes prepared from stably transfected HEK 293 cells. Labeling with [I-125]IAS was blocked by 10 mu M (-)-alprenolol and inhibited by addition of GTP gamma S, and [I-125]IAS migrated at the same position on an SDS-PAGE gel as the beta(2)AR labeled by the antagonist photoaffinity label [I-125]iodoazidobenzyIpindolol ([I-125]IABP). The labeled receptor was purified on a nickel affinity column and cleaved with factor Xa protease at a specific sequence in the large loop between transmembrane segments 5 and 6, yielding two peptides. While the control antagonist photoaffinity label [I-125]IABP labeled both the large N-terminal fragment [containing transmembranes (TMs) 1-5] and the smaller C-terminal fragment (containing TMs 6 and 7), essentially all of the [I-125]IAS labeling was on the smaller C-terminal peptide containing TMs 6 and 7, This direct biochemical evidence demonstrates that when salmeterol binds to the receptor, its hydrophobic aryloxyalkyl tail is positioned near TM 6 and/or TM 7. A model of IAS binding to the beta(2)AR is proposed.