Optimizing the Genetic Incorporation of Chemical Probes into GPCRs for Photo-crosslinking Mapping and Bioorthogonal Chemistry in Live Mammalian Cells

A facile fluorescence assay is presented to evaluate the efficiency of amino-acyl-tRNA-synthetase/tRNA pairs incorporating non-canonical amino-acids (ncAAs) into proteins expressed in mammalian cells. The application of ncAAs to study G-protein coupled receptors (GPCRs) is described, including photo-crosslinking mapping of binding sites and bioorthogonal GPCR labeling on live cells.

The overall goal of this protocol is to enhance the incorporation rate of non-canonical amino acids in mammalian cells to tackle biological questions in live cell settings including mapping of protein-protein interaction surfaces, and bioorthogonal labeling of surface receptors. Using amber codon suppression, different non-canonical amino acids are incorporated with different efficiencies. We established fluorescence assay to easily compare non-canonical amino acids incorporation rates in mammalian cells within a short time.

The systematic incorporation of photo-crosslinking, non-canonical amino acids through our GPCR surfaces allows mapping of ligand binding sites and provides precious information of the intake receptor directly from the life cell. By incorporating last generation, non-canonical amino acids for bioorthogonal chemistry, we achieve site-specific, super fast, catalyst free labeling of GPCRs with small, bright fluorophores for high-end imaging applications in live mammalian cells. First, seed 600, 000 HEK 293 cells in two milliliters of complete growth medium per well in two six-well plates.

Prepare as many wells as the number of samples and two additional wells for the wild type EGFP and a mock transfected sample respectively. One hour prior to transfection, add the appropriate amount of freshly prepared non-canonical amino acid stock solution to all wells for a final amino acid concentration of 0.25 to 0.5 millimolar. To transfect the cells, mix one microgram of plasmid DNA and coding for the synthetase t-RNA pair to be tested with one microgram of reporter plasmid DNA in a microcentrifuge tube.

In separate tubes, prepare an identical transfection using the EGFP wild type reference and a mock transfection. Next, add 100 microliters of lactate buffered saline to each tube containing the DNA. After briefly mixing, add six microliters of one microgram per microliter PEI and lactate buffered saline to each tube.

Vortex immediately, and then incubate at room temperature for 10 to 15 minutes. Transfer 400 microliters of cell medium from each well to the DNA PEI mixture to neutralize the pH. Then, dribble the DNA mixture onto the cells.

Harvest the cells 48 hours post-transfection by aspirating the medium and rinsing the cells once with two milliliters of pre-warmed PBS. Add 800 microliters of PBS supplemented with 0.5 millimolar EDTA, and incubate for 20 minutes at 37 degrees Celsius. After incubation, detach and suspend the cells by pipetting up and down.

Next, transfer the cell suspension into microcentrifuge tubes containing 200 microliters of PBS supplemented with five millimolar magnesium chloride. Centrifuge the samples for two minutes at 800 times gravity. When finished, discard the supernatant.

Now, add 100 microliters of Tris lysis buffer to the cell pellets and incubate on ice for 30 minutes. To facilitate lysis, vortex every five minutes. Spin down the cell debris for 10 minutes at four degrees Celsius and 14, 000 times gravity.

Transfer 90 microliters of the supernatant into black 96-well plates. Then, measure EGFP and mCherry fluorescence using a plate reader equipped with the fluorescence module. Seed 500, 000 293 t-cells in two milliliters of complete growth medium per well in two six-well plates.

For each position to be screened, prepare one well per ligand, plus one well for the binding control. One hour prior to transfection, add the photo-crosslinker, p-azidophenylalanine, or Azi, to all wells to a final concentration of 0.5 millimolar. Transfect a total amount of two micrograms of DNA per well using one microgram of plasmid and coding for the flag tagged GPCR bearing a tag codon at the desired position, and one microgram of the plasmid and coding for the orthogonal pair dedicated to Azi.

At 48 hours post-transfection, prepare a 1, 000x ligand stock solution by dissolving the peptide ligand at a concentration of 100 micromolar in DMSO. Dilute the ligand stock solution one to 1, 000 in binding buffer. Replace the cell medium with one milliliter of the ligand solution and incubate the samples for 10 minutes at room temperature.

Following incubation, irradiate the cells for 20 minutes in a UV cross-linker at 365 nanometers and five centimeter distance to the cells. When finished, detach the cells by pipetting and transfer them into a microcentrifuge tube. Pellet the cells for three minutes at 800 times gravity.

After discarding the supernatant, resuspend the cell pellets in 50 microliters of HEPES dissociation buffer, or HDB, containing a cocktail of protease inhibitors. Then, flash freeze the cells in liquid nitrogen. Next, thaw the cells in a water bath at 37 degrees Celsius for 30 to 45 seconds, and then vortex briefly.

Pellet the membranes for 10 minutes at four degrees Celsius and 2500 times gravity. When finished, discard the supernatant which contains the bulk of cytosolic proteins. Thoroughly resuspend the pellets in 50 microliters of HEPES lysis buffer using a pipet.

Then, lyse the cells for 30 minutes on ice, vortexing every five minutes. Following this, spin down the cell debris for 10 minutes at four degrees Celsius and 14, 000 times gravity. Immediately transfer each supernatant to a fresh reaction tube for western blot analysis.

When the GPCR is glycosylated and faint or smeared bands are a problem in western blot analysis, deglycosylate the samples with PNGase F to increase signal intensity and sharpen the bands. Then, resolve the samples on standard SDS page and blot transfer proteins to a PVDF membrane. After blocking the membrane, probe it with an anti-ligand antibody to detect the occurrence of cross-linking.

To detect the expression level of the Azi-GPCR, probe the membrane with a commercial anti-flag antibody. After washing the membrane, soak it in enhanced chemiluminescence solution, and visualize the signals in the dark. Seed 100, 000 HEK 293 t-cells in 600 microliters of dye-free, complete DMEM per well of a microscopy slide equipped with four wells.

One hour prior to transfection, prepare a fresh 100 millimolar stock solution of TCOK and 0.2 molar sodium hydroxide and 15%DMSO. Mix three microliters of the TCOK stock solution with 12 microliters of one molar, pH 7.4 HEPES buffer. Gently add the solution to each well for a final TCOK concentration of 0.5 millimolar.

In a microcentrifuge tube, dilute 200 nanograms of the plasmid and coding for the receptor containing the amber codon and 200 nanograms of the plasmid and coding for the orthogonal pair in 50 microliters of medium. Dilute 1.25 microliters of lipofectamine 2, 000 in 50 microliters of dye-free, serum-free, and antibiotic-free medium, and add the solution to the DNA solution. Vortex immediately and incubate for five to 10 minutes at room temperature.

Then, add the DNA lipid complexes to the cells. At 24 hours post-transfection, transfer 100 microliters of medium from each well into a 1.5 milliliter reaction tube. Add 1.8 microliters of an orange fluorescent dye tetrazine stock solution, and 0.3 microliters of a DNA staining dye stock solution.

Transfer the medium containing the dyes back to the well and incubate for five minutes at 37 degrees Celsius. Aspirate the medium, and gently rinse the cells twice with PBS to remove the excess dye. Then, add 600 microliters of pre-heated, complete dye-free growth medium to the cells.

Now, visualize the labeled receptors under 63x magnification using filters appropriate for GFP orange fluorescent dye and DNA-staining dye. After adding peptide agonist stock solution to the cells, observe the internalization under the microscope using the aforementioned filters and take pictures after the clearly detectable occurrence of internalization. Different tRNA's give different suppression efficiency, which is clearly reflected in the amount of measured fluorescence.

Azi incorporation into CRF1R was highly enhanced when using the codon-optimized Azi-tRNA synthetase gene compared to the native gene, showing that even a moderate improvement for a soluble protein can have a greater impact on the expression level of more challenging membrane proteins. The optimized system for Azi incorporation, including the humanized E2 Azi RS gene was deployed to map the binding pocket of the CRF1R and to find binding paths of five ligands. A band at the correct size of the crosslinking product in the western blot reveals that the position lies in the proximity of the ligand within the ligand receptor complex.

Multiple crosslinking hits with all ligands tested revealed distinct binding patterns for the peptide Agonists and Antagonists. Both fluorescence and western blot data suggests that TCOK is the non-canonical amino acids for bioorthogonal chemistry that gets incorporated with the highest efficiency of the AF variant of the paralysine tRNA synthetase. TCOK was incorporated into a CRF1R EGFP fusion protein and enabled installing a small, bright fluorophore on the receptor.

After adding a peptide Agaonist, fluorescent compartments were observed throughout the cytosol, revealing the physiological process of GPCR internalization. To do a fluorescence assay, we presented here is a simple and effective tool to evaluate and optimize the efficiency of a synthetase TI and API in mammalian cells. High incorporation rates are crucial for applying non-canonical amino acids to investigate challenging biological questions.

Highly efficient incorporation of crosslinking non-canonical amino acids enables extensive surface mapping of virtually any protein, including membrane receptors or low-banded proteins. The method reveals the topology of interaction interfaces which can be further analyzed to obtain accurate molecular models. High incorporation efficiency of last generation, non-canonical amino acids for bioorthogonal chemistry allows us to equip GPCRs with small, bright fluorophores, thus leading the way towards the study of GPCRs structural dynamics in live cells via advanced microscopy techniques.