Modulation of the conformation of a membrane glycosyltransferase by specific lipids

Fluorescence techniques were used to study conformational changes in UDPgalactose:lipopolysaccharide alpha,3-galactosyltransferase (EC 2.4.1.44). Intramolecular energy transfer was measured from the single tryptophan residue in the peptide to a pyridoxal phosphate group linked to the same peptide via a reduced Schiff's base. Significant differences in energy transfer were seen when the enzyme was studied in aqueous solution and after entry into a phospholipid-lipopolysaccharide matrix, paralleling the restoration of its catalytic activity. Further differences were seen when the structures of the phospholipids and lipopolysaccharides were changed. Application of the Förster theory indicated that the changes in energy transfer resulted from changes in distances between the chromophores and/or changes in their relative orientations. The results suggest that entry of this membrane protein into a lipid matrix induces a change in conformation of the protein and that other alternative conformations can be induced by further changes in its lipid environment.