MAPPING THE LIPID-EXPOSED SURFACES OF MEMBRANE-PROTEINS

Phospholamban forms a stable complex of five long transmembrane helice s. We show that the relative rotational orientation of the helices in the pentameric complex can be distinguished by S-H to S-D exchange of cysteine sulphydryl groups located in the transmembrane segment of the protein and exposed to the lipid environment, Of the three cysteine r esidues in phospholamban, two residues (Cys 36 and Cys 46) are oriente d towards the helix interface and protected from exchange, while the t hird cysteine (Cys 41) is oriented towards the lipid interface and und ergoes exchange with water diffused into the bilayer, Distinguishing t he external and internal faces of a membrane protein by sulphydryl exc hange provides a general approach for determining the three-dimensiona l fold of membrane proteins and enhances model building efforts to gen erate high-resolution structures.