Nanodiscs are small-sized and flat model membranes that provide a close to native environment for reconstitution of integral membrane proteins. Incorporation of membrane proteins into nanodiscs results in water-soluble proteolipid particles making the membrane proteins amenable to a multitude of bioanalytical techniques originally developed for soluble proteins. The transmembrane domain of the human CD4 receptor was fused to ubiquitin with a preceding N-terminal decahistidine tag. The resulting integral membrane protein was incorporated into nanodiscs. Binding of the nanodisc-inserted histidine-tagged protein to a monoclonal anti-pentahistidine antibody was quantified using surface plasmon resonance (SPR) experiments. For the first time, a membrane-inserted transmembrane protein was employed as analyte while the antibody served as ligand immobilized on the sensor chip surface. SPR experiments were conducted in single-cycle mode. We demonstrate that the nanodisc-incorporated membrane protein showed nearly identical affinity toward the antibody as did the soluble decahistidine-tagged ubiquitin studied in a comparative experiment. Advantages of the new experimental setup and potential applications are discussed.
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