Consequences of hydrophobic mismatch between lipids and melibiose permeaseon melibiose transport

The structural and functional consequences of a mismatch between the hydrop hobic thickness rip Of a transmembrane protein and that d(L) of the support ing lipid bilayer were investigated using melibiose permease (MelB) from Es cherichia coli reconstituted in a set of bis saturated and monounsaturated phosphatidylcholine species differing in acyl-chain length. Influence of Me lB on the midpoint gel-to-liquid-phase transition temperature, T-m,, of the saturated lipids was investigated through fluorescence polarization experi ments, with 1,6-diphenyl-1,3,5-hexatriene as the probe, for varying protein /lipid molar ratio. Diagrams in temperature versus MelB concentration showe d positive or negative shifts in T-m with the short-chain lipids DiC12:0-PC and DiC14:0-PC or the long-chain lipids DiC16:0-PC and DiC18:OPC, respecti vely. Theoretical analysis of the data yielded a d(L) value of 3.0 +/- 0.1 nm for the protein, similar to the 3.02 nm estimated from hydropathy profil es. Influence of the acyl chain length on the carrier activity of MelB was investigated in the liquid phase, using the monounsaturated PCs. Binding of the sugar to the transporter showed no dependence on the acyl chain length . In contrast, counterflow and Delta Psi-driven experiments revealed strong dependence of melibiose transport on the lipid acyl chain length. Similar bell-shaped transport versus acyl chain length profiles were obtained, opti mal activity being supported by diC16:l-PC. On account of a d(p) value of 2 .65 nm for the lipid and of various local constraints which would all tend to elongate the acyl chains in contact with the protein, one can conclude t hat maximal activity was obtained when the hydrophobic thickness of the bil ayer matched that of the protein.