Encyclopedia of Experiments: Biological Techniques
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Misfolding-Prone Protein Degradation Assay - ConceptTranscript
To carry out a degradation assay for Atxn1 82Q GFP, plate approximately 3 x 105 HeLa cells in 35-millimeter plates with DMEM medium and 10% FBS.
Incubate the cells overnight so that they reach 40% to 60% confluence at the time of transfection. 4 to 5 hours after transfecting the cells with Atxn1 82Q-GFP/pRK5, under a fluorescence microscope, use an excitation wavelength of 450 to 490 nanometers to examine the live cells for GFP expression in the nuclei, characterized by the presence of diffused as well as small speckles of GFP signals.
Just before cycloheximide treatment, harvest one plate of cells by removing the medium, and using 3 milliliters of ice-cold PBS to wash the cells twice. Then, snap-freeze the plate on dry ice.
For the remaining plates of cells, remove the medium by vacuum aspiration and add 2 milliliters of fresh DMEM containing 50 micrograms per milliliter of cycloheximide. Also, add 10 micromolar of the proteasome inhibitor MG132 to one plate. Incubate treated cells for 4, 8, 12, and 16 hours before freezing on dry ice.
After harvesting the MG132-treated cells at 16 hours, scrape the frozen cells from all the plates into 150 microliters of ice-cold cell lysis buffer, and incubate on ice for 30 minutes.
Centrifuge the lysates in a benchtop centrifuge at 17,000 x g and 4 degrees Celsius for 15 minutes. Then, transfer the supernatant, which contains NP-40-soluble proteins, to another tube.
Rinse the pellets by gently adding approximately 200 microliters of 1x PBS to the tubes without disturbing the pellets. Carefully remove the PBS by aspiration or pipette. Resuspend the pellets in 150 microliters of ice-cold pellet buffer, and then, incubate them on ice for 15 to 30 minutes.
Next, add 75 microliters of 3x boiling buffer into NP-40-soluble fractions and NP-40-insoluble fractions resuspended from the pellets. Then, heat the samples at 95 degrees Celsius on a heat block for 5 minutes.
Add SDS-gel loading buffer to an aliquot of boiled NP-40-soluble and insoluble fractions. Load equal volumes of samples collected from all time points onto an SDS-PAGE gel. Detect NP-40-soluble and SDS-soluble Atxn1 82Q GFP by western blot using anti-GFP antibody and enhanced chemiluminescence.
To examine SDS-resistant Atxn1 82Q from the pellet fraction with a filter retardation assay, set up a dot-blot apparatus holding a 0.2-micron cellulose acetate membrane. Then, load 80 to 120 microliters of boiled NP-40-insoluble samples into each well of the dot-blot apparatus.
After filtering the samples through the membrane by vacuum, detect Atxn1 82Q GFP aggregates stuck on the membrane by anti-GFP immunoblotting.
