ATR-FTIR study of the structure and orientation of transmembrane domains of the Saccharomyces cerevisiae alpha-mating factor receptor in phospholipids

The structures of seven synthetic transmembrane domains (TMDs) of the alpha -factor receptor (Ste2p) from Saccharomyces cerevisiae were studied in pho spholipid multilayers by transmission Fourier transform infrared (FTIR) and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectros copies. 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 sh owed that peptides corresponding to TMDs one, two, and seven were mostly al pha -helical in DMPC multilayers. Peptides corresponding to TMDs three and six assumed predominantly beta -sheet structures, whereas those correspondi ng to TMDs four and five were a mixture of alpha -helices and beta -sheets. ATR-FTIR showed that in DMPC the alpha -helices of TMDs two and five orien ted with tilt angles of 34 degrees and 32 degrees, respectively, with respe ct to the multilayer normal. Similar results were obtained for six of the t ransmembrane domains in DMPC/DMPG (4: 1) multilayers. In a mixture [POPC/PO PE/POPS/PI/ergosterol (30:20:5:20:25)] which mimicked the lipid compositio alphan of the S. cerevisiae cell membrane, the percentage of -helical struc tures found for TMDs one and five increased compared to those in DMPC and D MPC/ DMPG (4: 1) multilayers, and TMD six exhibited a mixture of beta -shee t (similar to 60%) and alpha -helical (similar to 40%) structure. These exp eriments provide biophysical evidence that peptides representing the seven transmembrane domains in Ste2p assume different structures and tilt angles within a membrane multilayer.