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The cooperative effect of multiple affinity binding interactions creating a stable bond, known as avidity, is particularly important in assessing the potency of potential drugs such as monoclonal antibodies, CAR T, or NK cells to treat cancer. However, predicting avidity based on in vitro single affinity interactions has limitations and often fails to describe the avidity effects observed in vivo. Acoustic force- based assays have recently emerged as a reliable method for direct avidity measurements, expressed as adhesion forces, which positively correlate with drug efficacy. However, to better understand avidity, in particular for cell-cell interactions and correlate it with affinity, a cell model system with controlled avidity-related properties is needed. This study presents a method for producing a cell model system using effector beads that can be used in acoustic force spectroscopy-based avidity assays or any other bead-based avidity assay. The protein of interest is biotinylated in vivo in E.coli, purified and subsequently mixed with streptavidin coated beads to create effector beads. The results demonstrate the dependency of rupture force on the receptor surface density and force loading rate, thus providing valuable information for designing future effector bead assays as well as cell avidity measurements for screening and characterization purposes.

Protein adsorption to solid carbohydrate interfaces is critical to many biological processes, particularly in biomass deconstruction. To engineer more-efficient enzymes for biomass deconstruction into sugars, it is necessary to characterize the complex protein–carbohydrate interfacial interactions. A carbohydrate-binding module (CBM) is often associated with microbial surface-tethered cellulosomes or secreted cellulase enzymes to enhance substrate accessibility. However, it is not well known how CBMs recognize, bind, and dissociate from polysaccharides to facilitate efficient cellulolytic activity, due to the lack of mechanistic understanding and a suitable toolkit to study CBM–substrate interactions. Our work outlines a general approach to study the unbinding behavior of CBMs from polysaccharide surfaces using a highly multiplexed single-molecule force spectroscopy assay. Here, we apply acoustic force spectroscopy (AFS) to probe a Clostridium thermocellum cellulosomal scaffoldin protein (CBM3a) and measure its dissociation from nanocellulose surfaces at physiologically relevant, low force loading rates. An automated microfluidic setup and method for uniform deposition of insoluble polysaccharides on the AFS chip surfaces are demonstrated. The rupture forces of wild-type CBM3a, and its Y67A mutant, unbinding from nanocellulose surfaces suggests distinct multimodal CBM binding conformations, with structural mechanisms further explored using molecular dynamics simulations. Applying classical dynamic force spectroscopy theory, the single-molecule unbinding rate at zero force is extrapolated and found to agree with bulk equilibrium unbinding rates estimated independently using quartz crystal microbalance with dissipation monitoring. However, our results also highlight critical limitations of applying classical theory to explain the highly multivalent binding interactions for cellulose–CBM bond rupture forces exceeding 15 pN.

No abstract

To rationally engineer more efficient cellulolytic enzymes for cellulosic biomass deconstruction into sugars for biofuels production, it is necessary to better understand the complex enzyme-substrate interfacial interactions. Carbohydrate binding modules (CBM) are often associated with microbial surface-tethered cellulosomal or freely secreted cellulase enzymes to increase substrate accessibility. However, it is not well known how CBM recognize, bind, and dissociate from polysaccharide surfaces to facilitate efficient cellulolytic activity due to the lack of mechanistic understanding of CBM-substrate interactions. Our work outlines a general approach to methodically study the unbinding behavior of CBMs from model polysaccharide surfaces using single-molecule force spectroscopy. Here, we apply acoustic force spectroscopy (AFS) to probe a Clostridium thermocellum cellulosomal scaffoldin protein (CBM3a) and measure its dissociation from nanocellulose surfaces at physiologically relevant, low force loading rates. An automated microfluidic setup and methodology for uniform deposition of insoluble polysaccharides on the AFS chip surfaces is demonstrated. The rupture forces of wild-type CBM3a, and its Y67A mutant, unbinding from nanocellulose surface suggests distinct CBM binding conformations that can also explain the improved cellulolytic activity of cellulase tethered to CBM. Applying established dynamic force spectroscopy theory, the single-molecule unbinding rate at zero force is extrapolated and found to agree well with bulk equilibrium unbinding rates estimated independently using quartz crystal microbalance with dissipation monitoring. However, our results highlight the limitations of applying classical theory to explain the highly multivalent CBM-cellulose interactions seen at higher cellulose-CBM bond rupture forces (>15pN).