The lipid-protein interplay in efflux plump

Abstract

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In this work lactose permease (LacY) of Escherichia coli has been taken as model efflux pump to investigate the interactions between the protein and the main lipid components (POPE and POPG) of the inner membrane. Two main approaches have been followed: (i) measuring the fluorescence energy transfer between a single tryptophan mutant of the protein (W151/C154G LacY) and pyrene labeled phospholipids (Pyr-PE and Pyr-PG); and (ii) pulling the protein from the supported lipid bilayers where it is embedded by using the tip of the atomic force microscope (AFM). On one hand, fluorescence measurements at different pHs indicate that LacY present selectivity for PE. On the other hand the observations of the reconstituted protein in lipid bilayers by AFM show a preference of LacY for the fluid phase (Lα) rather than for the gel phase (Lβ). To get an estimation of the proportion of each lipid in each phase we have constructed a phase diagram for the system POPE:POPG. The diagram shows that at the temperature of the experiments (24 ºC) there is an almost equimolar proportion of each lipid. The results suggest the existence of a boundary region around LacY formed mainly by POPE laterally segregated from a bulk with a random distribution of POPE and POPG. Force spectroscopy allows to establish the force required and the mechanism to unspecifically unfold the protein.