Strep-Beads from Novagene were used to check the interactions between the Strep-Tag in our Flagellin with the StrepDARPidin and the Strep-Beads themselves.
Theoretically the flagellin should bind to the beads and should be found in the elution in contrast to the StrepDARPidin which should be washed off.
In the third elution there should be less Flagellin or even nothing in case of an interaction with the StrepDARPidin because of the competitive situation. The flagellin should bind totally to the StrepDARPidin and for that reason not/ less to the beads.
3 columns were prepaired and filled with 500 µL GeFi Buffer. 20µL Strep-Beads were added and the columns were spinned down at 4000 rpm for 1 min. Then another 500 µL GeFi buffer was added.
The first (1) probe of 3 µL Hag-D2-Strep was pipetted into the buffer. 20 µL StrepDARPidin were added to column 2. Column 3 was used for loading a mixture of 20 µL StrepDARPidin and 3 µL Flagellin onto the column. The three columns were spinned for 20 min at room temperature in a spinning wheel. After that the columns were washed twice with 500 µL GeFi buffer. For eluting 40 µL Elution Buffer from Novagene (with Desthiobiotin) was used to resuspend the beads. After centrifugation at 13000 rpm the elution was mixed with 10 µL SDS-Loading buffer and analyzed on a SDS-PAGE gel with commassie stain.
As a control 40 µL of a 1:10 dilution of the flagellin concentrate was used on the gel. Unfortunately there was not enough StrepDARPidin left for a second control.
Gel-Analysis:
The gel shows that a very tiny amount of the flagellin binds to the Strep-Beads (line 1). The StrepDARPidin does not (line 2). Surprisingly the mixture shows both proteins in the elution (line 3). Unfortunately this cannot be seen on the scanned picture . The control (line 4) shows us that the flagellin purification was not pure enough and the flagellin concentrate was contaminated with StrepDARPidin from the Ni-NTA purification.
The Strep-Beads might have been not efficient working enough because of their age and usage. The binding of the flagellin might tell us that the Strep-Tag is able to interact with the Streptavidin.
In order to get a better and reliable result we decided to purify the flagellin and the StrepDARPidin newly and repeat the pulldown.
New expression cultures of 1L volume each were induced with lactose and incubated overnight at 30°C and 150 rpm.