Purity

Protein purity is determined by scanning densitometry of Coomassie blue stained protein on a 4-20% polyacrylamide gradient gel. The Vav2 protein was determined to be c. 80% pure. (see Figure 1).

Storage

Before reconstitution, briefly centrifuge to collect the product at the bottom of the tube. The protein should be reconstituted to 5 mg/ml with ice cold nanopure water (20 µl water per 100 µg protein). When reconstituted, the protein will be in the following buffer: 20 mM Tris pH 7.5, 50 mM NaCl, 1 mM MgCl2, 5% (v/v) sucrose and 1% (v/v) dextran. In order to maintain high biological activity of the protein it is strongly recommended that the protein solution be supplemented with DTT to 1 mM final concentration, aliquoted into "experiment sized" amounts, snap frozen in liquid nitrogen and stored at -70°C. The protein is stable for six months if stored at -70°C. The protein must not be exposed to repeated freeze-thaw cycles. The lyophilized protein is stable at 4°C desiccated (<10% humidity) for one year. Further Vav2 dilutions should be made in 20 mM Tris pH 7.5, 50 mM NaCl, 1 mM MgCl₂ (not supplied).

Biological Activity Assay

The biological activity of Vav2 DH can be determined from its ability to catalyze nucleotide exchange on Rac1 using the nucleotide exchange assay of  Bodipy-GDP for excess GDP or GTP. Rac1 protein is pre-loaded with Bodipy-FL-GDP by adding excess EDTA e.g. 0.7 mmol EDTA per  mmol Mg2+ ions present in the reaction. This sub-stock solution is then used in a dissociation assay format which indicates competition for exchange site with unlabeled nucleotide. The reaction is monitored by fluorescence measurement at 485nm Ex / 535nm Em.  Stringent quality control ensures that the exchange rate of Bodipy-GTP or mant-GTP is enhanced at least five fold in the presence of 0.8 µM Vav2 DH.

Reagents

1. Vav2 DH protein (Cat. # CS-GE06)

2. Rac1,2 or 3 protein (Cat. # RC01, CS-RC02 or CS-RC03)

3. Exchange buffer 2 (20 mM Tris-HCl pH 7.5, 50 mM NaCl, 1 mM DTT, 2 mM EDTA, 100 µg/ml BSA, and 0.75 µM Bodipy-FL-GDP), note - make fresh.

4. 50 mM MgCl₂ in 20 mM Tris-HCl pH 7.5, 50 mM NaCl.

5. 5 mM GTP in 20 mM Tris-HCl pH 7.5, 50 mM NaCl. 

Equipment

1. Fluorescence spectrometer (λex=485nm, λem=525nm, both with<15nm bandwidth)

2. Corning 96-well half area plates (Cat # 3686) or other plate with low protein binding surface.

Method

1. Place Vav2 vial on ice and dilute to 0.30 µg/µl (8 µM) with ice cold Exchange Buffer.

2. Dilute Rac protein to 1.25 µg/µl (50 µM) with ice cold Exchange Buffer..

3. Add the following components together in a fresh 15 ml Falcon tube and mix well by pipetting or gentle vortex:

     Component           µl per well 

     Exchange Buffer       75      50 µM Rac                  5      8 µM Vav2                10  

Note: For a total mixture volume, multiply the volume of reagents per well by the number of wells in the experiment, plus add 20% volume for pipetting losses.

4. Incubate for 20 min at room temperature (RT).

5. Lock in the nucleotide by adding 10 µl (per well) of 50 mM MgCl₂.

6. Set up the fluorimeter with Excitation wavelength at 485 nm +/-15 nm and emission wavelength at 525 nm +/- 15 nm at RT.

7. Aliquot the pre-loaded mixture to the assigned wells and place the plate in the fluorimeter. 

8. After 5 cycles (150 seconds), place the program on Hold or Pause, and remove the plate.

9. Pipette 10 µl of a) 5 mM GTP solution, b) a small compound, c) a test protein, d) 4 mM EDTA (+ve exchange control) or e) Dilution Buffer (negative control) in respective wells and immediately pipette up and down twice and resume reading for 20 minutes.

10. Save the readings after the kinetic protocols are finished. The exchange rate can be calculated by reducing the data to max slope (using 12 pts) or Vmax with the software that accompanies the plate reader. The exchange curve can be achieved by export to Microsoft Excel.

Figure 2. Vav2 DH GEF protein domain mediated Bodipy-FL-GDP dissociation from Rac1.