A Faster Route to Active G-protein coupled receptors (GPCRs)

A Faster Route to Active G-protein coupled receptors (GPCRs)

GPCRs account for roughly one-third of approved drug targets¹, yet they remain among the hardest targets to produce in a purified, stable, active form. In this blog, we will explore why timelines slip at the "expression-to-function" step, and how cell-free protein synthesis paired with nanodiscs deliver functional GPCRs faster by enabling rapid, systematic condition screening

Key Takeaways

• The bottleneck is function, not just yield. High expression in cells means little if the receptor aggregates and/or misfolds during detergent extraction.
• Detergent extraction is a risk. "Rescuing" proteins from membranes often destabilizes native conformations.
• Cell-free protein synthesis enables a feedback loop. By decoupling expression from cell viability, you can rapidly and easily screen constructs, nanodiscs, and redox conditions in parallel.
• Nanodiscs are a design variable. Adjusting lipid composition and size can tune ligand affinity and stability.
• Work from Prof. Frank Bernhard demonstrates how this workflow creates cryo-EM-ready complexes in days, not months.

Where time is lost: the expression-to-function gap

For decision-makers, the bottleneck is rarely target identification; it is solubilizing and purifying the receptor itself. Without a purified GPCR that is active and stable, timelines slip across hit-finding and structure-based design.

The detergent extraction penalty

In cell-based membrane protein expression, GPCRs are embedded in the host membrane. To purify them, teams typically perform detergent extraction (solubilization into micelles), which can introduce new problems:

• Loss of key lipid interactions that stabilize native conformations
• Enrichment of non-native or inactive states
• High aggregation risk during solubilization
• Assay interference in sensitive biophysical formats

Iteration compounds the timeline

Conventional workflows front-load weeks of effort. Whether generating baculovirus stocks or selecting stable mammalian cell lines, the clock starts running long before the protein's function is tested. Teams often wait 3-4 weeks for an "assay-ready" preparation⁵, only to find the solubilized receptor is inactive. Long empirical campaigns of construct redesign, host changes, detergent/buffer screening, reconstitution in membrane mimetics, and assay reconfiguration prolong the time before you have suitable protein for downstream assays.

In one high-throughput membrane-protein production pipeline study, only ~23% of targets passed small- and mid-scale selection steps-driving rework and extended timelines⁶.

Cell-free protein synthesis: decoupling from cell constraints

Cell-free protein synthesis (CFPS), removes the need to keep living cells healthy while they attempt to express a difficult membrane protein. In a lysate-based (or purified recombinant-based) biochemical environment, transcription and translation can occur simultaneously (often within ~24 hours), and reaction conditions can be tuned without triggering toxicity or stress responses. This allows you to turn GPCR production into a rapid optimization loop.

Co-translational insertion into nanodiscs

A critical advantage of CFPS is co-translational insertion into MSP nanodiscs. Nanodiscs are soluble nanoscale lipid bilayers wrapped by a membrane scaffold protein (MSP). They are often more native-like than detergent micelles, and they're broadly compatible with binding assays, biophysics, and structural methods.

Instead of extracting receptors with disruptive detergents, CFPS allows receptors to fold directly into defined nanodiscs. That means teams can avoid the "extract-then-rescue" sequence that often destabilizes receptors in cell-based workflows.

Why nanodiscs (and their chemistry) matter

Nanodiscs aren't one fixed condition-they're a design space. Lipid headgroup charge, acyl chain length, saturation, and additives (including cholesterol analogues) can shift conformational equilibria and alter ligand affinity.

This matters because GPCR readouts can be extremely sensitive to the membrane context. In practice, lipid composition and nanodisc concentration can change function even when total yield looks similar. In a screening-first workflow, nanodiscs are not just a stabilizer; they become a tunable parameter that can be optimised to reach an assay-ready, structurally coherent receptor.

Example case studies: The β1-adrenergic receptor (β1AR) showed a 12-fold improvement in activity in DEPG (dimyristoyl-Phosphatidylglycerol) nanodiscs compared to DMPC (dimyristoyl-Phosphatidylcholine)⁷. For the endothelin B receptor, significant increases in activity were observed when negatively charged headgroups such as PG (phosphatidylglycerol) or PS (phosphatidylserine) were used instead of zwitterionic PC (phosphatidylcholine) headgroups^8,9. Similarly, both proteins displayed a preference for longer chain lengths containing 18 carbon atoms and flexibility in the fatty acid chain with the transconfiguration of the double bond producing higher activity than the cis configuration⁷. Functional nanodisc libraries prepared from mammalian cells using MSP1E3D1 nanodiscs composed of 20% POPS (1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine), 8% cholesterol, and 72% POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) have shown to enrich GPCR activity because they closely mimic plasmas membranes¹⁰.

Fine-tuning for function: stabilizers, chaperones, redox

This is what turns GPCR production into a rapid optimisation loop:

• Screen constructs in parallel (boundaries, tags, variants)
• Screen membrane environments (nanodisc lipid composition, stoichiometry, concentration)
• Tune reaction chemistry (redox, cofactors, additives)
• Read out function early

Example case study: Combining nanodisc lipid screening, redox optimization, chaperone enrichment and receptor engineering, Frank Bernhard's team reported >10³-fold improvements in folding efficiency of human endothelin B receptor and achieved up to ~100 µg of active protein per mL of CFPS reaction¹¹.

Bridge to structural biology-and back to cells

These workflows can feed directly into structure. CFPS/nanodisc approaches have produced cryo-EM structures of GPCR complexes, including a cell-free synthesized full-length human β1AR/Gs complex resolved at 3.46 Å (0.346 nm), preserving challenging intracellular regions without truncations often used historically¹².

Nanodiscs can also help bridge back to biology. In "nanotransfer" concepts, nanodisc-embedded membrane proteins can be transferred into mammalian membranes to support cellular validation after in vitro optimization, tightening the loop between biochemical optimisation, structure, and cell-based testing¹³.

What to do next?

GPCRs will keep dominating pipelines, but programs slip when a functional receptor is slow to obtain. Cell-free protein synthesis paired with nanodiscs provides a rational alternative: co-translational insertion into defined bilayers, systematic screening, and faster convergence on assay-compatible receptors and complexes.

If you'd like to see an end-to-end example of this approach, watch Nuclera's on-demand session featuring Prof. Frank Bernhard.

Talk to our team about GPCR workflows

References

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3. RCSB PDB. PDB Statistics: Overall Growth of Released Structures Per Year. RCSB PDB https://www.rcsb.org/stats/growth/growth-released-structures (2026).
4. GPCRdb. https://gpcrdb.org/ (2025).
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6. Jarvis, D. L. Recombinant protein expression in baculovirus-infected insect cells. Methods Enzymol. 538, 149-163 (2014).
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9. Proverbio, D., Roos, C., Beyermann, M., Orbán, E., Dötsch, V. & Bernhard, F. Functional properties of cellfree expressed human endothelin A and endothelin B receptors in artificial membrane environments.Biochim. Biophys. Acta (BBA) Biomembranes 1828, 2182-2192 (2013).
10. Roy, J., Pondenis, H., Fan, T. M. & Das, A. Direct capture of functional proteins from mammalian plasma membranes into nanodiscs. Biochemistry 54, 6299-6302 (2015).
11. Rues, R.-B., Dong, F., Dötsch, V. & Bernhard, F. Systematic optimization of cell-free synthesized human endothelin B receptor folding. Methods 147, 73-83 (2018).
12. Merino, F. et al. Cryo-EM structure of a cell-free synthesized full-length human β1-adrenergic receptor in complex with Gs. Structure 33, 1867-1877.e5 (2025).
13. Köck, Z. et al. Cryo-EM structure of cell-free synthesized human histamine 2 receptor/Gs complex in nanodisc environment. Nat. Commun. 15, 1831 (2024).

This article was originally published on DDN