Diffusion of Lipid Nanovesicles Bound to a Lipid Membrane Is Associated with the Partial-Slip Boundary Condition

Curved Membrane Mimics for Quantitative Probing of Protein-Membrane Interactions by Surface Plasmon Resonance

A majority of biomimetic membranes used for current biophysical studies rely on planar structures such as supported lipid bilayer (SLB) and self-assembled monolayers (SAMs). While they have facilitated key information collection, the lack of curvature makes these models less effective for the investigation of curvature-dependent protein binding. Here, we report the development and characterization of curved membrane mimics on a solid substrate with tunable curvature and ease in incorporation of cellular membrane components for the study of protein-membrane interactions. The curved membranes were generated with an underlayer lipid membrane composed of DGS-Ni-NTA and POPC lipids on the substrate, followed by the attachment of histidine-tagged cholera toxin (his-CT) as a capture layer. Lipid vesicles containing different compositions of gangliosides, including GA1, GM1, GT1b, and GQ1b, were anchored to the capture layer, providing fixation of the curved membranes with intact structures. Characterization of the curved membrane was accomplished with surface plasmon resonance (SPR), fluorescence recovery after photobleaching (FRAP), and nano-tracking analysis (NTA). Further optimization of the interface was achieved through principal component analysis (PCA) to understand the effect of ganglioside type, percentage, and vesicle dimensions on their interactions with proteins. In addition, Monte Carlo simulations were employed to predict the distribution of the gangliosides and interaction patterns with single point and multipoint binding models. This work provides a reliable approach to generate robust, component-tuning, and curved membranes for investigating protein interactions more pertinently than what a traditional planar membrane offers.

Recruitment of Receptors and Ligands in a Weakly Multivalent System with Omnipresent Signatures of Superselective Binding

Recruitment of receptors at membrane interfaces is essential in biological recognition and uptake processes. The interactions that induce recruitment are typically weak at the level of individual interaction pairs, but are strong and selective at the level of recruited ensembles. Here, a model system is demonstrated, based on the supported lipid bilayer (SLB) that mimics the recruitment process induced by weakly multivalent interactions. The weak (mm range) histidine-nickel-nitrilotriacetate (His₂ -NiNTA) pair is employed owing to its ease of implementation in both synthetic and biological systems. The recruitment of receptors (and ligands) induced by the binding of His₂ -functionalized vesicles on NiNTA-terminated SLBs is investigated to identify the ligand densities necessary to achieve vesicle binding and receptor recruitment. Threshold values of ligand densities appear to occur in many binding characteristics: density of bound vesicles, size and receptor density of the contact area, and vesicle deformation. Such thresholds contrast the binding of strongly multivalent systems and constitute a clear signature of the superselective binding behavior predicted for weakly multivalent interactions. This model system provides quantitative insight into the binding valency and effects of competing energetic forces, such as deformation, depletion, and entropy cost of recruitment at different length scales.

Label-free nanofluidic scattering microscopy of size and mass of single diffusing molecules and nanoparticles

Label-free characterization of single biomolecules aims to complement fluorescence microscopy in situations where labeling compromises data interpretation, is technically challenging or even impossible. However, existing methods require the investigated species to bind to a surface to be visible, thereby leaving a large fraction of analytes undetected. Here, we present nanofluidic scattering microscopy (NSM), which overcomes these limitations by enabling label-free, real-time imaging of single biomolecules diffusing inside a nanofluidic channel. NSM facilitates accurate determination of molecular weight from the measured optical contrast and of the hydrodynamic radius from the measured diffusivity, from which information about the conformational state can be inferred. Furthermore, we demonstrate its applicability to the analysis of a complex biofluid, using conditioned cell culture medium containing extracellular vesicles as an example. We foresee the application of NSM to monitor conformational changes, aggregation and interactions of single biomolecules, and to analyze single-cell secretomes.

Nanofluidic scattering microscopy enables label-free, quantitative measurements of the molecular weight and hydrodynamic radius of biological molecules and nanoparticles freely diffusing inside a nanofluidic channel.