Exploration of interactions between membrane proteins embedded in supported lipid bilayers and their antibodies by reflectometric interference spectroscopy-based sensing.

A microfluidic reflectometric interference spectroscopy (RIfS)-based sensor was fabricated to investigate the activity of multidrug resistance-associated protein 1 (MRP1), applied as a model membrane protein. Vesicles containing MRP1 were immobilized simply by injecting a vesicle solution (50 μg mL(-1)) onto a zirconium oxide (ZrO2) chip under constant flow conditions. Monitoring the shift of the minimum reflectance wavelength (Δλ) of the RIfS demonstrated that the vesicles were adsorbed onto the ZrO2 chip in a Langmuir-like fashion and suggested that the lipid bilayer structure was preserved on the ZrO2 chip. The theoretical maximum physical thickness of the layer was 4.97 nm, which was close to the values previously reported for supported lipid bilayers (4.2 to 5.2 nm). When a model protein, the anti-MRP1 antibody (1-50 μg mL(-1)), was injected onto the MRP1-immobilizing ZrO2 chip a concentration-dependent increase in Δλ was observed. In contrast, a ZrO2 chip on which the supported lipid bilayers did not contain MRP1 exhibited no response. Moreover, an anti-human IgG antibody generated no change in Δλ, confirming that anti-MRP1 antibodies were selectively bound to the MRP1 immobilized on the chip. These results show that the RIfS sensor can follow specific binding events of biologically active membrane proteins and represents a simple, label-free system capable of facilitating biomedical investigations.