DETERMINING THE SECONDARY STRUCTURE AND ORIENTATION OF EMRE, A MULTIDRUG TRANSPORTER, INDICATES A TRANSMEMBRANE 4-HELIX BUNDLE

EmrE is a member of a newly emerging family of MiniTEXANS, a family of multi-drug antiporters from bacteria characterized by their small siz e of roughly 100 amino acids. In this report we have obtained transmis sion FTIR spectra of EmrE in CHCl3:MeOH, DMPC vesicles, and Escherichi a coli lipid vesicles. Secondary structure analysis has shown that bot h in DMPC vesicles and in CHCl3: MeOH the protein adopts a highly heli cal secondary structure that correlates remarkably well with that pred icted by hydropathy analysis. The protein was shown to be resistant to amide proton H/D exchange, providing evidence that most of the protei n is embedded in the lipid bilayer. Polarized ATR-FTIR spectra of the protein in DMPC vesicles have shown that the helices are oriented with an average tilt angle of 27 degrees from the bilayer normal. The prot ein was found to be less oriented in E. coli lipid vesicles, most like ly as a result of the poor orientation of the bilayer lipids themselve s. Thus, the protein is identified as a transmembrane four-helix bundl e providing valuable structural data for this family of multi-drug tra nsporters. The results set the stage for further studies aimed at deri ving a detailed model for this protein.