Imaging Giant Vesicle Membrane Domains with a Luminescent Europium Tetracycline Complex

Microdomains in lipid bilayer membranes are routinelyimaged usingorganic fluorophores that preferentially partition into one of thelipid phases, resulting in fluorescence contrast. Here, we show thatmembrane microdomains in giant unilamellar vesicles (GUVs) can bevisualized with europium luminescence using a complex of europiumIII (Eu3+) and tetracycline (EuTc). EuTc is unlike typicalorganic lipid probes in that it is a coordination complex with a uniqueexcitation/emission wavelength combination (396/617 nm), a very largeStokes shift (221 nm), and a very narrow emission bandwidth (8 nm).The probe preferentially interacts with liquid disordered domainsin GUVs, which results in intensity contrast across the surface ofphase-separated GUVs. Interestingly, EuTc also alters GM1 gangliosidepartitioning. GM1 typically partitions into liquid ordered domains,but after labeling phase-separated GUVs with EuTc, cholera toxin B-subunit(CTxB), which binds GM1, labels liquid disordered domains. We alsodemonstrate that EuTc, but not free Eu3+ or Tc, significantlyreduces lipid diffusion coefficients. Finally, we show that EuTc canbe used to label cellular membranes similar to a traditional membraneprobe. EuTc may find utility as a membrane imaging probe where itslarge Stokes shift and sharp emission band would enable multicolorimaging.