ATR-FTIR Study of the Structure and Orientation of Transmembrane Domains of the Saccharomyces cerevisiae α-Mating Factor Receptor in Phospholipids^†

The structures of seven synthetic transmembrane domains (TMDs) of the α-factor receptor (Ste2p) from Saccharomyces cerevisiae were studied in phospholipid multilayers by transmission Fourier transform infrared (FTIR) and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopies. Peptide conformation assumed in multilayers depended on the method of sample preparation. Amide proton H/D exchange experiments showed that 60−80% of the NH bonds in these TMDs did not exchange with bulk water in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) multilayers. FTIR results showed that peptides corresponding to TMDs one, two, and seven were mostly α-helical in DMPC multilayers. Peptides corresponding to TMDs three and six assumed predominantly β-sheet structures, whereas those corresponding to TMDs four and five were a mixture of α-helices and β-sheets. ATR-FTIR showed that in DMPC the α-helices of TMDs two and five oriented with tilt angles of 34° and 32°, respectively, with respect to the multilayer normal. Similar results were obtained for six of the transmembrane domains in DMPC/DMPG (4:1) multilayers. In a mixture [POPC/POPE/POPS/PI/ergosterol (30:20:5:20:25)] which mimicked the lipid composition of the S. cerevisiae cell membrane, the percentage of α-helical structures found for TMDs one and five increased compared to those in DMPC and DMPC/DMPG (4:1) multilayers, and TMD six exhibited a mixture of β-sheet (60%) and α-helical (40%) structure. These experiments provide biophysical evidence that peptides representing the seven transmembrane domains in Ste2p assume different structures and tilt angles within a membrane multilayer.