TITLE: SARS-CoV-2 nsp3 macrodomain His-tagged expression and purification protocol for assay
AUTHORS: Korvus Wang, michael fairhead, Eleanor Williams

[DESCRIPTION]
This protocol details the expression and purification of SARS-CoV-2 nsp3 macrodomain assay construct bearing a N-terminal His tag at small scale (<6L).


[GUIDELINES]
Construct / plasmid resource-name: SARS-CoV-2 nsp3 macrodomain assayconstruct bearing a N-terminal His tag.

[STEPS]
SECTION: Plasmid Transformation
2. CVNSP3mac1 N-terminal His-tagged assay construct was inoculated from its BL21(DE3)-RR glycerol stock.

 

SECTION: Protein expression
5. When the OD600 approximately 4.0, lower the temperature and shaker speed to  . Add IPTG to final concentration of 1 millimolar (mM) . Incubate overnight.

SECTION: Protein Purifcation
8. Perform IMAC to extract target protein from the lysed cell mixture    
SECTION: Protein Purifcation
8.4. Wash the column with 10 CV of base buffer, followed by 10 CV of wash buffer twice. Allow wash buffer to pass through completely between washes. This is to remove non-specific, weak binding of contaminant proteins from the resin for a cleaner elution.
Collect washes separately for SDS-PAGE analysis. 
SECTION: Protein Purifcation
11. Using the AKTA Pure system:

Inject the sample onto a 5mL sample loop.

Run the sample down HiLoad 16/60 Superdex 75 pg gel filtration column at 1mL/min in gel filtration buffer, collecting 1mL aliquots.
SECTION: Protein Purifcation
12.2. Aliquot into appropriate volumes for future usage to minimise freeze/thaw cycles. Flash-freeze in liquid nitrogen, and store at -80 °C until required.


SECTION: Protein expression
4. Use the 10 mL starter culture to inoculate 1 L  autoinduction TB media (see Materials) supplemented with 50 ug/mL carbenicillin in a baffled flask.   
 

SECTION: Protein expression
6. Harvest the cell by centrifugation  at 4000 x g, 30 min, 4 °C. Discard supernatant and store pellet by freezing at -80 °C .

SECTION: Protein Purifcation
7. Lyse cell pellet
SECTION: Protein Purifcation
7.2. Lyse by sonication 2 s 4 s for a total 'on' time of 15 min at 35% amplitude to fully rupture the cells. Ensure pellet is °C  during sonication to prevent overheating.
SECTION: Protein Purifcation
8.2. Resuspend the equilibrated resin with some binding buffer and add to the supernatant bottle. Incubate the resin with the supernatant for 30 min while rotating or otherwise mixing gently at 4 °C 
SECTION: Protein Purifcation
8.3. Load the resin/supernatant mix back onto the gravity flow column, retaining the FT separately for SDS-PAGE analysis. 

 

SECTION: Protein Purifcation
8.5. Elute the protein with 15 mL of elution buffer.
SECTION: Protein Purifcation
8.6. Repeat step 8.5 one more time, collecting a total of 2 separate elution fractions. This is to ensure maximum retrieval of protein from the resin.

Measure the total protein concentration of the elutions by Nanodrop. Although still a mixture, A280 value can give an estimate of the protein content, which will determine how much protease need to be added to remove the affinity tag.
SECTION: Protein expression
3. Scrape off some of the glycerol stock with a sterile loop and use this to inoculate a 50 mL falcon tube containing 10 mL of LB supplemented with 50 ug/mL carbenicillin. Grow the starter culture at  37 °C  240 min with 200 rpm shaking.   
SECTION: Abbreviations
1. CV - column volume, total volume of resin in a column
IMAC - immobilised metal affinity chromatography
FT - flow through
CVNSP3mac1 - SARS-CoV-2 nsp3 macrodomain 
SECTION: Protein Purifcation
9. Run SDS-PAGE of all samples from total lysis supernatant to final elution. Stain gel with protein staining solution Coomasssie Blue and determine which fractions contain the target protein by finding the band corresponding to the target molecular weight.

 

 

SECTION: Protein Purifcation
10. Purify sample further by size exclusion chromatography.
SECTION: Protein Purifcation
8.1. Dispense 2 mL Nickle affinity resin Ni Sepharose 6 FF - Cytiva into a gravity flow column. Equilibrate resin by first rinsing with  ~ 10 CV  distilled water, then ~ 10 CV  binding buffer to remove the storage solution.
SECTION: Protein Purifcation
7.3. Centrifuge the lysed cells for 38000 x g, 60 min, 4 °C to remove insoluble cell debris, and collect supernatant in a bottle 4 °C  
SECTION: Protein Purifcation
7.1. 
 


 


Thaw and resuspend the pellet in ~7mL of lysis buffer per g of pellet. Stir gently with magnetic stir bar at Room temperature  for 30 min to allow lysozyme and bezonase to start breaking down
 cell components.
SECTION: Protein Purifcation
10.1. Using 10,000 MWCO spin concentrators, concentrate the rIMAC step containing fractions of the target protein to a final volume of under 5 mL . 

SECTION: Protein Purifcation
10.2. Remove any solid aggregates from the sample by centrifugation at 17200 x g, 10 min, 4 °C , then immediately draw up the supernatant with a 5mL syringe and a blunt-tip fill needle, taking care not to disturb the pellet. 

 


SECTION: Protein Purifcation
12. From the chromatogram, fraction F9-H8 analyse by SDS-PAGE.

 

 

SECTION: Protein Purifcation
12.1. Take the fractions that contain the target protein, which in this case are fraction D4-D11. Concentrate the final sample in Vivaspin 500 10kda MWCO centrifugal concentrator until the concentration reaches >6 mg/mL.

Take 1 µL of the final sample for SDS-PAGE. Intact MS can also be carried out to confirm sample purity.