A Luminescent Complex of Europium and Tetracycline Labels Liquid Disordered Membrane Domains and Causes GM1 Redistribution

Microdomains in lipid bilayer membranes are routinely imaged using organic fluorophores that preferentially partition into one of the lipid phases, resulting in fluorescence contrast. Here we show that membrane microdomains in giant unilamellar vesicles (GUVs) can be visualized with europium luminescence using a complex of europium (III) and tetracycline (EuTc). EuTc is unlike typical organic lipid probes in that it has a unique excitation/emission wavelength combination (396/617 nm), a very large Stokes shift (221 nm), and a very narrow emission bandwidth (8 nm). The probe preferentially interacts with liquid disordered domains in GUVs, which results in intensity contrast across the surface of phase-separated GUVs. Interestingly, EuTc also disrupts GM1 partitioning. After labeling phase-separated GUVs with EuTc, CTxB, which binds GM1, labels disordered domains. We hypothesize that this is due to the ability of EuTc to increase lipid membrane order, which we demonstrate by showing that EuTc significantly reduces lipid diffusion coefficients. We conclude that EuTc, and potentially other tetracycline derivatives complexed with metal ions, disrupt membrane organization of lipid bilayer membranes, which may have implications on the antibacterial activity of the tetracycline family of antibiotics. ### Competing Interest Statement The authors have declared no competing interest.