Cell-based focal molography enables real-time quantification of specific intracellular protein interactions at membrane receptors in living cells, free from interference by off-target signaling.

The Challenge

Understanding G protein-coupled receptor (GPCR) signaling requires monitoring specific molecular interactions at receptors in living cells. Conventional label-free biosensors such as SPR or BLI depend on laborious receptor preparation in artificial membrane mimetics (detergent micelles, nanodiscs, liposomes), which strips the receptor from its native lipid and protein environment and can distort its pharmacology. Fluorescence-based live-cell assays preserve context but require molecular modifications that can alter the biology being studied, and traditional label-free live-cell readouts cannot separate receptor-specific events from nonspecific cellular changes such as morphological shifts or off-target receptor activation.

The Approach

Using focal molography, the authors created a transmembrane mologram by spatially arranging SNAP-tagged beta-2 adrenergic receptors (β2ARs) in the plasma membrane of HEK293 cells. The receptors self-assembled from the sensor chip surface into the cell membrane, forming a coherent nanopattern. This diffractive arrangement renders only target-receptor interactions visible while nonspecific cellular changes remain undetectable.

Key Results

• Real-time intracellular binding kinetics: Agonist-induced mass recruitment at the receptor was resolved in real time, with signal increasing over 20 min post-stimulation and partially reversing upon addition of a competitive antagonist
• Identification of recruited intracellular partner: Combined molographic and fluorescence readout on the same chip identified beta-arrestin-2 as the protein recruited to the receptor; cells lacking beta-arrestin-2 showed no molographic response to agonist stimulation
• Quantitative agreement with established assays: Dose-response curves from cell-based molography matched classical BRET arrestin-recruitment assays for both isoproterenol and formoterol, validating the method as quantitatively reliable
• Insensitive to off-target signaling: Stimulation of off-target adenosine A2A/A2B receptors with NECA produced no measurable molographic signal, while the same cells responded to beta-2AR agonists, confirming that focal molography reports exclusively on the coherently arranged receptor

Why It Matters

This work extends focal molography from purified protein assays to living cells, enabling label-free, real-time monitoring of intracellular signaling events at specific membrane receptors in their native lipid and protein environment. By avoiding detergents, nanodiscs, and fluorescent labels, the method preserves the receptor pharmacology that reconstituted-system measurements often distort, and resolves binding kinetics that endpoint or bulk readouts cannot. The same coherent-nanopattern strategy generalizes naturally beyond β2AR to other GPCR subfamilies and to membrane protein classes such as ion channels, transporters, and receptor tyrosine kinases. For drug discovery teams investigating biased agonism, allosteric modulation, or partner-protein recruitment at specific receptors, this delivers a unique combination of native-environment context, binding-kinetic resolution, and label-free specificity that classical biochemical assays and bulk live-cell readouts cannot match.

Reichmuth, A. M., Zimmermann, M., Wilhelm, F., Frutiger, A., Blickenstorfer, Y., Fattinger, C., Waldhoer, M., & Voros, J. (2020). Quantification of Molecular Interactions in Living Cells in Real Time using a Membrane Protein Nanopattern. Analytical Chemistry, 92(13), 8983–8991. https://doi.org/10.1021/acs.analchem.0c00987

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