Biophysical studies on fragments of the alpha-factor receptor protein

The receptor for the alpha-factor mating pheromone of the yeast Saccharomyces cerevisiae consists of 431 amino acid residues and is a member of a family of membrane proteins predicted to have seven transmembrane helices. Fragments of the receptor corresponding to two of the transmembrane helices [residues 246-269 (M6) and 273-302 (M7)], two of the interhelical loops [residues 107-125 (E2) and 191-206 (E3)], and to a portion of the carboxyl terminus [residues 350-372 (CT)] were synthesized using solid-phase methodologies and purified to near homogeneity. CD was used to characterize the secondary structure of these peptides in trifluoroethanol (TFE), in TFE/water mixtures, in sodium dodecyl sulfate (SDS), and in the presence of dimyristoyl phosphatidylcholine (DMPC) liposomes. In TFE, M6 and M7 exhibited CD spectra consistent with highly helical peptides, whereas CT was partially helical. In contrast, E2 and E3 were either disordered or aggregated in this solvent. M6 did not partition well into DMPC vesicles whereas M7 remained helical. Both M6 and M7 assumed helical conformations in 25 mM SDS. The loop peptides and the carboxyl terminus peptide were either in a beta-structure or disordered in the presence of lipid. These findings represent the first biophysical evidence for conformations assumed by specific segments of the STE2 receptor protein.