Effects on membrane lateral pressure suggest permeation mechanisms for bacterial quorum signaling molecules

Quorum sensing is an intricate example of "social" behavior in microbial communities mediated by small secreted molecules (autoinducers). The mechanisms of membrane permeation remain elusive for many of them. Here we present the assessment of membrane permeability for three natural autoinducers and four synthetic analogues based on their polarity, surface activity, affinity for lipid monolayers, and ability to induce lateral pressure changes in the inner E. coli membrane sensed by the bacterial tension-activated channel MscS. AI-1 (N-(3-oxodecanoyl)-l-homoserine lactone) is surface-active, and it robustly inserts into lipid monolayers, indicating strong propensity toward membranes. When presented to membrane patches from the cytoplasmic side, AI-1 transiently shifts MscS's activation curve toward higher tensions due to intercalation into the cytoplasmic leaflet followed by redistribution to the opposite side. Indole showed no detectable surface activity at the air-water interface but produced a moderate increase of lateral pressure in monolayers and was potent at shifting activation curves of MscS, demonstrating transients on sequential additions. AI-2 (4,5-dihydroxy-2,3-pentanedione, DPD) showed little activity at the interfaces, correspondingly with no effect on MscS activation. After chemical modification with isobutyl, hexyl, or heptyl chains, AI-2 displayed strong surface activity. Hexyl and especially heptyl AI-2 induced robust transient shifts of MscS activation curves. The data strongly suggest that both AI-1 and indole are directly permeable through the membrane. AI-2, more hydrophilic, shows low affinity toward lipids and thus requires a transport system, whereas alkyl analogues of AI-2 should permeate the membrane directly.