Because Lipoparticle suspensions approximate the physical properties of membrane proteins in solution, they are ideal for studying membrane protein interactions in mobile-phase microfluidic systems such as optical biosensors. When proteins such as antibodies are immobilized to a biosensor chip, the binding of molecules that are flowed across the chip can be optically detected and quantified by changes in refractive index.
Nanosized Lipoparticles are readily flowed through the 50 µm channel of an optical biosensor, enabling their use as mobile-phase reagents, as shown here for the binding interaction between chip-immobilized antibodies and Lipoparticles containing the cannabinoid receptor (CB1). Lipoparticle binding to flow cells on which anti-CB1 antibodies were immobilized was readily detectable, while Lipoparticles did not bind to flow cells on which a nonspecific monoclonal antibody was immobilized (Figure 3).
Lipoparticles can also be immobilized directly on optical biosensor surfaces (Figure 4A). While conventional binding assays allow only for measurements of equilibrium binding affinity, immobilization to a biosensor chip permits on and off rates of the binding interaction to be measured in real time.
Lipoparticles incorporating the chemokine receptor CXCR4 were immobilized on a biosensor chip. Sensorgrams measure the concentration-dependent association of the anti-CXCR4 monoclonal antibody 12G5 as it was flowed across the chip, and its slow dissociation when buffer alone was flowed (Figure 4B). Sensorgram analyses from diverse antibodies against CXCR4 show a distinct kinetic profile for each antibody despite similar equilibrium binding affinities in a number of cases (Figure 4C).
Lipoparticles are useful for studying membrane protein interactions, enabling homogeneous suspension of purified, high-concentration receptors. The technology is compatible with numerous assay formats and is especially appropriate for high-throughput screening and lead-optimization studies. The capture of complex proteins on Lipoparticles generates new avenues for investigating membrane protein interactions using novel techniques that were previously difficult to apply.