Site-specific Effector Protein Functionalization to Create Bead-based Avidity Model Systems

The cooperative effect of multiple affinity binding interactions creating a stable bond, known as avidity, is a universal biological phenomenon seen in diverse systems. For example, avidity based biomolecular interactions are particularly important in assessing the potency of potential drugs such as monoclonal antibodies, chimeric antigen receptor (CAR) T-cell, or Natural Killer, cells to treat cancer or engineering microbes with cell surface immobilized enzyme complexes for consolidated bioprocessing (CBP) of cellulosic biomass to fuels and chemicals. However, predicting or measuring avidity based on in vitro single affinity interactions with non-complexed protein-ligand binding model systems has limitations and often fails to describe the avidity effects observed in vivo with cell surface complexed proteins interacting with multivalent ligands at solid interfaces. Acoustic force spectroscopy (AFS) based assays have recently emerged as a reliable method for direct avidity measurements, expressed as adhesion or rupture forces, which positively correlate with in vivo avidity interactions. However, to better understand and model avidity, in particular for cell-cell interactions and to correlate it with classical binding affinity, a cell mimetic model system with controlled avidity-related properties is needed. Here, we present a method for producing such a cell mimetic model system using "effector beads" that can be used in AFS-based avidity assays or any other bead-based avidity assay. The protein of interest is heterologously expressed and biotinylated in vivo in E. coli, purified, and subsequently tethered with streptavidin coated micron-sized beads to create effector beads. Our experimental results, combined with simulations of the multivalent binding phenomena, demonstrate the dependency of bead rupture force on its receptor protein surface density and force loading rate as well as the intrinsic kinetic binding parameters of the protein-ligand system of interest. These insights provide valuable information for designing future effector bead assays and cell avidity measurements for screening and characterization purposes for diverse applications.