Label-Free Analysis of Multivalent Protein Binding Using Bioresponsive Nanogels and Surface Plasmon Resonance (SPR)

Precise identification of protein-protein interactions is required to improve our understanding of biochemical pathways for biology and medicine. In physiology, how proteins interact with other proteins or small molecules is crucial for maintaining biological functions. For instance, multivalent protein binding (MPB), in which a ligand concurrently interacts with two or more receptors, plays a key role in regulating complex but accurate biological functions, and its interference is related to many diseases. Therefore, determining MPB and its kinetics has long been sought, which currently requires complicated procedures and instruments to distinguish multivalent binding from monovalent binding. Here, we show a method for quickly evaluating the MPB over monovalent binding and its kinetic parameters in a label-free manner. Engaging pNIPAm-co-AAc nanogels with MPB-capable moieties (e.g., PD-1 antigen and biocytin) permits a surface plasmon resonance (SPR) instrument to evaluate the MPB events by amplifying signals from the specific target molecules. Using our MPB-based method, PD-1 antibody that forms a type of MPB by complexing with two PD-1 proteins, which are currently used for cancer immunotherapy, is detectable down to a level of 10 nM. In addition, small multivalent cations (e.g., Ca²⁺, Fe²⁺, and Fe³⁺) are distinguishably measurable over monovalent cations (e.g., Na⁺ and K⁺) with the pNIPAm-co-AAc nanogels.