Probing the binding domain of the Saccharomyces cerevisiae alpha-mating factor receptor with fluorescent ligands

Three analogues of the alpha -mating factor pheromone of Saccharomyces cere visiae containing the 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) group were sy nthesized that had high binding affinity to the receptor and retained biolo gical activity. The fluorescence emission maximum of the NBD group in [K-7( NBD),Nle(12)]-alpha -factor was blue shifted by 35 nm compared to buffer wh en the pheromone bound to its receptor. Fluorescence quenching experiments revealed that the NBD group in [K-7(NBD),Nle(12)]-alpha -factor bound to th e receptor was shielded from collision with iodide anion when in aqueous bu ffer. In contrast, the emission maximum of NBD in [K-7(ahNBD),Nle(12)]-alph a -factor or [Orn(7)(NBD),Nle(12)]-alpha -factor was not significantly shif ted and iodide anion efficiently quenched the fluorescence of these derivat ives when they were bound to receptor. The fluorescence investigation sugge sts that when the alpha -factor is bound to its receptor, K-7 resides in an environment that has both hydrophobic and hydrophilic groups within a few angstroms of each other.