Förster resonance energy transfer (FRET) measurements were performed in preceding works to study the selectivity between a single-tryptophan mutant of lactose permease (LacY) of Escherichia coli (used as the donor) and phospholipid probes labeled with pyrene at the acyl chain moiety (used as the acceptor). In the present work, we report the results obtained by using the same LacY mutant (W151/C154G) and binary lipid mixtures of phosphatidylethanolamine (PE) differing in the acyl chain composition and 1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (POPG) (3:1 mol/mol) doped with a phospholipid probe labeled with pyrene at the headgroup. The use of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(1-pyrenesulfonyl) ammonium salt (HPyr-PE), which bears two unsaturated acyl chains, enabled the investigation of the specific interaction between LacY and HPyr-PE. The main conclusions raised from our results suggest that (i) for phase-separated systems, LacY would be located in fluid domains nominally enriched in POPG, and if a given proportion of PE is present in this phase, it will be mainly located around LacY; and (ii) in the absence of phase separation, LacY is preferentially surrounded by PE and, in particular, seems to be sensitive to the lipid spontaneous curvature.
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