Sauer:Expression/purification of 35S-Met proteins/Native purification

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Expression

1. Inoculate 4 mL of LB+appropriate antibiotic with strain containing the plasmid that expresses the protein to be labeled. Grow overnight @ 37 ˚C.

2. Inoculate 50mL of LB/antibiotic with 0.5mL of overnight culture. Shake @ 37˚C to OD600 = 1.0-1.5.

3. During the growth for step 2, prepare M9+MAM media about 1 hr before step 4:

4. Pellet cells.

5. Discard LB supernatent, and resuspend cell pellet in 50 mL M9 + MAM media.

6. Incubate 37˚C for 20 minutes.

7. Induce cells with 1 mM IPTG, incubate 20 minutes.

8. Add 2 mCi Expre³⁵S³⁵S label mix (catalog #NEG 772 from Perkin Elmer) to culture. Shake for an additional 2 hrs.

9. Harvest cells by spinning in J6 20 minutes @ 4000 rpm. Discard supernatent appropriately.

• Note: You have a lot of radiation at this step and it tends to mysteriously escape from tubes onto centrifuges, even if the tube is closed tightly. Filling tubes no more than half full seems to reduce/eliminate this problem.

10. Resuspend pellet in 5 mL 10 mM NTA buffer.

11. Freeze at –80˚C, thaw at room temperature, freeze at –80˚C again until ready to purify.

• If desired, add lysozyme (~0.3 mg/mL final concentration), incubate on ice 20 min before refreezing. This may help with the efficiency of cell lysis.

Purification

12. Thaw resuspended, frozen cells at RT. Make 8 x 750 µl aliquots in 1.5 mL microfuge tubes.

• Again, it is best not to fill the tubes more than half full at this step.

13. Spin 15 min at full speed in a benchtop microcentrifuge @ 4˚C.

14. During step 13, prepare NTA column: Apply 250µl of Qiagen NTA resin to a BIO-RAD poly-prep disposable column, and wash with 10 mL 10 mM NTA.

15. Remove supernate from step 13, and apply to NTA column. Collect flow-through.

16. Wash column with 50 mL 20 mM NTA buffer. Collect washes.

17. Elute material from the column with 4 x 1 mL 250 mM NTA buffer. Collect each 1 mL elution separately.

18. Run all fractions on an SDS PAGE gel. Use coomassie staining and phosphorimager detection/film to determine the purity of your protein..

19. Pool all elution fractions containing your protein. If necessary, concentrate to 250-500µl using the appropriate Millipore centrifugal filtration device.

20. Buffer exchange by column or dialysis into desired buffer.

Buffers and Solutions

Expression buffers and solutions

For 50 mL M9 + MAM Media:

0.5 mL 40% Glucose

50 µL 2 mg/mL Thiamine

50 µL 10 mM Trace Metals Mix

50 µL 100 mM FeCl₃•6H2O

50 µL 500 mM CaCl₂•2H₂O

50 µL 1.0M MgCl₂

1.25 mL MAM

25 mL 2xM9

23 mL H₂O

appropriate antibiotic

MAM (Methionine Assay Medium)

105 g/L DIFCO MAM (dissolved in H₂O)

10 mM Trace Metals Mix

12.4 mg/mL (NH₄₎(Mo0₇)24•4H₂O

2.4 mg/mL CoCl₂•6H₂O

2.5 mg/mL CuSO₄•5H₂O

0.62 mg/mL H₃BO₃

2 mg/mL MnCl₂•4H₂O

1.4 mg/mL ZnCl₂

2xM9 media - from kitchen (at MIT!), but here’s the recipe:

4.4 g/L Na₂HPO₄•7H₂O

6g/L KH₂PO₄ 1g/L NaCl 2g/L NH₄Cl

Purification buffers

10 mM NTA buffer

50 mM NaH₂PO₄

500 mM NaCl

10 mM Imidazole

(20 mM and 250 mM NTA buffer are identical to the 10 mM NTA buffer except they have 20 mM and 250 mM, respectively)

Sauer:Expression/purification of 35S-Met proteins

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