Editing 293E Protein Production (section)

=== <br>METHOD  ===

==== <br>Column Charging  ====

1. Pack a gravity column with Fractogel resin. 

Refer to the manufacturer’s binding capacity specifications in order to use an adequate volume of resin. 

2. Add five column-volumes of 0.5 M NaCl. 

3. Add four column-volumes of 200 mM CoCl2. 

4. Add two column-volumes 0.5 M NaCl (pH 5 ± 0.2). 

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==== Sample Loading and r-Protein Elution  ====

Use SDS-PAGE analysis on reserved 500-μL aliquots of the original sample feed, flowthrough, washes, and pooled protein fractions to determine whether the purification process is sufficient or whether the column resin is overloaded beyond its protein-binding capacity. 

5. Add 10 column-volumes of PBS to equilibrate the column. 

6. Load the feed onto the column: 

For small volumes (&lt; 100 mL) 

i. Allow the sample to pass through the column by gravity. 

For large volumes (&gt; 100 mL) 

ii. Attach a peristaltic pump to the column outlet to pull the sample feed through the column more quickly. 

iii. Use a flow rate up to a maximum of 10 mL/min (190 cm/h) for a 5-mL Qiagen-tip 500 column or 20 mL/min (125 cm/h) for a 10-mL HiSpeed Maxi Tip column. 

iv. Stop and disconnect the pump 1-2 min before the entire sample empties from its reservoir. 

v. Add the last few milliliters of sample to the reservoir with a pipette. Allow it to flow through by gravity. 

This prevents the column from drying out. 

7. Keep the flowthrough and store at 4°C. Retain a 500-μL aliquot for subsequent gel analysis. 

Do not discard the flowthrough or wash fractions until it has been determined that the r-protein has been entirely captured and eluted from the column. 

8. Add 10 column-volumes of IMAC wash buffer 1. Retain a 500-μL aliquot of the flowthrough for subsequent gel analysis. 

9. Add 10 column-volumes of IMAC wash buffer 2 to remove impurities bound to the column. Retain a 500-μL aliquot of the flowthrough for subsequent gel analysis. 

10. Add four column-volumes of IMAC column elution buffer in 1-mL increments. Harvest individual 1-mL fractions in prelabeled 1.5-mL tubes. 

11. Add 5-20 μL of each eluted fraction to a 96-well plate. 

12. Add 200 μL of diluted protein assay reagent to each well. 

Make sure that each sample is well mixed as the reagent is being added. 

13. Wait 5-10 min for color development. 

The wells with the darkest blue coloration correspond to the fractions containing the r-protein. For more quantitative determination, read the absorbance at 595 nm using a microplate reader. 

See Troubleshooting. 

14. Pool the 1-mL fractions containing r-protein. 

15. Desalt the purified r-protein using a desalting column equilibrated in PBS or in your preferred buffer. 

16. Based on the molar extinction coefficient of the protein, determine its concentration after measuring its absorbance at 280 nm (use an appropriate buffer as the blank). 

The molar extinction coefficient of your protein can be obtained by uploading its amino acid sequence at http://ca.expasy.org/tools/protparam.html. 

==== <br>Column Washing and Storage  ====

17. Add two column-volumes of cobalt stripping buffer to the column. 

18. Add two column-volumes of H2O. 

19. Add 15 column-volumes of cleaning buffer. 

20. Add five column-volumes of H2O. 

21. Add five column-volumes of PBS (or more if necessary) to neutralize the pH of the column to 7.0-7.2. 

22. Add five column-volumes of storage buffer for IMAC. 

23. Add an additional 1-2 mL of storage buffer for IMAC on top of the column. Cover the top and bottom. Store at 4°C. 

==== <br>TROUBLESHOOTING  ====

Problem: Purification from IMAC does not work. 

[Step 13] 

Solution: Consider the following: 

1. Perform SDS-PAGE analysis of the feed, flowthrough, washes, and pooled fractions to see whether the r-protein was captured by the resin or not. If the His-tagged protein is in the flowthrough, the tag on the protein itself may be sterically hindered and inaccessible for capture by the column. 

2. The r-protein could be aggregated, thus hampering its capture onto the column. Check this by gel filtration chromatography. 

3. Increasing the pH of the feed to 8.0 or higher can enhance r-protein binding to the column, and charging the IMAC column with nickel instead of cobalt can increase binding affinity. 

4. Sometimes, the r-protein elutes in IMAC wash buffer 2. In this case, decrease the imidazole concentration of the wash buffer. 

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[[Category:293_Expression]] [[Category:Protocols]]