Fast Transient Cytokine–Receptor Interactions Monitored in Real Time by Reflectometric Interference Spectroscopy

Abstract

Investigating protein–protein interactions by mutational analysis requires practical techniques for quantifying rate constants and equilibrium constants over several orders of magnitude with reasonably high sample throughput. We have employed spectroscopic interferometry for label-free monitoring of the interaction between the cytokine interferon α2 (IFNα2) and the extracellular domain of its receptor ifnar2 (ifnar2-EC). We implemented a versatile surface chemistry for the glass substrate of this transducer for covalent immobilization of proteins. Affinity capturing with a monoclonal anti-ifnar2-EC antibody (mAb) followed by crosslinking with a second, noncompetitive mAb provided stable, but still reversible, immobilization of ifnar2-EC. We measured kinetics and affinity of numerous of mutants of IFNα2 and ifnar2-EC. Dissociation rate constants up to 0.3 s −1 and association rate constants up to 3 × 10 6 M −1 s −1 were resolved by the system. Dissociation constants down to 200 μM were measured with protein concentrations up to 50 μM without no background signal or nonspecific binding. The instrument detection limit is ∼10 pm without the need for temperature stabilization or referencing channels. The system proved effective for large-scale mutational analysis involving alanine scanning mutagenesis and double mutant cycles.