Difference between revisions of "Team:Freiburg/Results/Surface"
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<h1>Results: Binding on Surface</h1> | <h1>Results: Binding on Surface</h1> | ||
<div class="level2"> | <div class="level2"> | ||
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<div class="todo_box">Bitte Diagramme (Fig 1, 4, 8, 9) etwas größer machen | <div class="todo_box">Bitte Diagramme (Fig 1, 4, 8, 9) etwas größer machen | ||
</div> | </div> | ||
− | |||
<p> | <p> | ||
− | Optimizing cell-free expression as such is already a challenging task. But it becomes increasingly complicated when the expressed proteins should be immobilized on a glass surface directly after expression: The Cell-free mix is a complex system mainly consisting of proteins not that different to the target proteins. To overcome this hurdle we decided to fuse a tag to the proteins of interest that specifically | + | Optimizing cell-free expression as such is already a challenging task. But it becomes increasingly complicated when the expressed proteins should be immobilized on a glass surface directly after expression: The Cell-free mix is a complex system mainly consisting of proteins not that different to the target proteins. To overcome this hurdle we decided to fuse a tag to the proteins of interest that specifically bind to a chemically treated surface. To get such a tag-system to work, the surface has to be optimized in order to minimize unspecific binding. |
</p> | </p> | ||
+ | |||
<h1> Effective - The Unspecific Surface </h1> | <h1> Effective - The Unspecific Surface </h1> | ||
<div class="image_box left"> | <div class="image_box left"> | ||
− | <div class="thumb2 tcenter" style="width:310px"><div class="thumbinner"><a class="lightbox_trigger" href="https://static.igem.org/mediawiki/2015/f/fa/Freiburg_labjournal-surchem-20150907_pditc_vs_gopts_evaluation.png" title="labjournal:surchem:20150907_pditc_vs_gopts_evaluation.png"><img alt="" class="mediabox2" src="https://static.igem.org/mediawiki/2015/f/fa/Freiburg_labjournal-surchem-20150907_pditc_vs_gopts_evaluation.png" width="300"/></a><div class="thumbcaption"><div class="magnify"><a class="internal" href="https://static.igem.org/mediawiki/2015/f/fa/Freiburg_labjournal-surchem-20150907_pditc_vs_gopts_evaluation.png" title="vergrößern"><img alt="" height="11" src="/igem2015/lib/plugins/imagebox/magnify-clip.png" width="15"/></a></div><strong>Figure 1: | + | <div class="thumb2 tcenter" style="width:310px"><div class="thumbinner"><a class="lightbox_trigger" href="https://static.igem.org/mediawiki/2015/f/fa/Freiburg_labjournal-surchem-20150907_pditc_vs_gopts_evaluation.png" title="labjournal:surchem:20150907_pditc_vs_gopts_evaluation.png"><img alt="" class="mediabox2" src="https://static.igem.org/mediawiki/2015/f/fa/Freiburg_labjournal-surchem-20150907_pditc_vs_gopts_evaluation.png" width="300"/></a><div class="thumbcaption"><div class="magnify"><a class="internal" href="https://static.igem.org/mediawiki/2015/f/fa/Freiburg_labjournal-surchem-20150907_pditc_vs_gopts_evaluation.png" title="vergrößern"><img alt="" height="11" src="/igem2015/lib/plugins/imagebox/magnify-clip.png" width="15"/></a></div><div <strong>Figure 1: Comparison of mean fluorescence intensity of immobilized GFP on GOPTS and PDITC surfaces.</strong> The mean fluorescence for diffenrent concentrations of GFP on GOPTS is representet by the red bars. GFP on PDITC is represented by blue bars.</div></div></div> |
</div> | </div> | ||
<p> | <p> | ||
− | For measuring in iRIf the target proteins have to be fixed to a glass | + | For measuring in iRIf the target proteins have to be fixed to a glass slide, therefore we used silanes to introduce reactive groups on the surface. These Silanes can bind to Siliciumdioxide (glass) after reactive hydroxy groups were created by oxygen plasma activation. Adding either the silane GOPTS (3-Glycidyloxypropyltrimethoxysilane) or APTES (3-Aminopropyltriethoxysilane) allows us to bind certain chemical groups to the surface. The epoxy-group of GOPTS reacts covalently with amino-, hydroxy- and thiol-groups so that a large spectrum of molecules can be coupled. APTES on the other hand is not reactive itself but can be covalently fused to amino-groups with the homobifunctional cross-linker PDITC (<em>p</em>-Phenyldiisothiocyanate). For further information on chemistry and methods see <a href="https://2015.igem.org/Team:Freiburg/Project/Surface_Chemistry"> surface chemistry methods </a>. |
To evaluate these two chemistries, we compared the binding capacities of the respective surfaces on simple microscope glass slides. Self-purified GFP in different concentrations was spotted on both surfaces and the resulting fluroescence intensity was compared (Figure 1). Especially for low concentrations it is clearly visible that APTES/PDITC outcompetes the GOPTS surface. Therefore, we continued our experiments using the PDITC-chemistry. | To evaluate these two chemistries, we compared the binding capacities of the respective surfaces on simple microscope glass slides. Self-purified GFP in different concentrations was spotted on both surfaces and the resulting fluroescence intensity was compared (Figure 1). Especially for low concentrations it is clearly visible that APTES/PDITC outcompetes the GOPTS surface. Therefore, we continued our experiments using the PDITC-chemistry. | ||
− | With these results we decided to switch on special glass slides, suitable for iRIf. With this method we were able to detect the interaction of anti-GFP antibodies with spotted GFP of different concentrations. To check for variations in different GFP-purification protocols, we compared | + | With these results we decided to switch on special glass slides, suitable for iRIf. With this method we were able to detect the interaction of anti-GFP antibodies with hand-spotted GFP of different concentrations. To check for variations in different GFP-purification protocols, we compared GFPs obtained from different groups (GFP1 to GFP3). |
</p> | </p> | ||
<div class="table sectionedit2"><table class="inline"> | <div class="table sectionedit2"><table class="inline"> | ||
<tr class="row0"> | <tr class="row0"> | ||
− | <td class="col0">GFP 1 | + | <td class="col0">GFP 1, 50 µg/ml</td><td class="col1">GFP 1, 17 µg/ml</td><td class="col2">GFP 1, 5 µg/ml</td> |
</tr> | </tr> | ||
<tr class="row1"> | <tr class="row1"> | ||
− | <td class="col0">GFP 2 | + | <td class="col0">GFP 2, 50 µg/ml</td><td class="col1">GFP 2, 17 µg/ml</td><td class="col2">GFP 2, 5 µg/ml</td> |
</tr> | </tr> | ||
<tr class="row2"> | <tr class="row2"> | ||
− | <td class="col0">GFP 3 | + | <td class="col0">GFP 3, 50 µg/ml</td><td class="col1">GFP 3, 17 µg/ml</td><td class="col2">GFP 3, 5 µg/ml</td> |
</tr> | </tr> | ||
<tr class="row3"> | <tr class="row3"> | ||
− | <td class="col0"> </td><td class="col1">bBSA (0.5 mg/mL)</td><td class="col2">BSA (10 mg/mL)</td> | + | <td class="col0"> </td><td class="col1">bBSA (0.5 mg/mL)</td><td class="col2">BSA (10 mg/mL)</td> |
</tr> | </tr> | ||
</table></div> | </table></div> | ||
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<p> | <p> | ||
Immobilized GFP was first blocked in the iRIf device with 10 mg/mL BSA. Anti-GFP in buffer solution was flushed over the slide and leads to a change in the optical thickness at the GFP spots. The binding was detected in real-time. Figure 2 shows a quotient picture from this step, thus the increase in optical thickness due to antibody binding. | Immobilized GFP was first blocked in the iRIf device with 10 mg/mL BSA. Anti-GFP in buffer solution was flushed over the slide and leads to a change in the optical thickness at the GFP spots. The binding was detected in real-time. Figure 2 shows a quotient picture from this step, thus the increase in optical thickness due to antibody binding. | ||
− | The signal can further be amplified by a second binding step. As the anti-GFP antibody was biotinylated it could easily be attached to streptavidin flushed through the microfluidic chamber (Figure 3). This step also confirms the binding of anti-GFP to the spots and therefore the specificity of the interaction. Biotinylated BSA (bBSA, bottom middle) spotted on the slide that also binds to | + | The signal can further be amplified by a second binding step. As the anti-GFP antibody was biotinylated it could easily be attached to streptavidin flushed through the microfluidic chamber (Figure 3). This step also confirms the binding of anti-GFP to the spots and therefore the specificity of the interaction. Biotinylated BSA (bBSA, bottom middle) spotted on the slide that also binds to Streptavidine served as a positive control. |
</p> | </p> | ||
<div class="flexbox"> | <div class="flexbox"> | ||
− | <div class="thumb2 tcenter" style="width:310px"><div class="thumbinner"><a class="lightbox_trigger" href="https://static.igem.org/mediawiki/2015/3/31/Freiburg_files-20150706_irif_movie_anti-gfp_binding_igem_slide.png" title="files:20150706_irif_movie_anti-gfp_binding_igem_slide.png"><img alt="" class="mediabox2" src="https://static.igem.org/mediawiki/2015/3/31/Freiburg_files-20150706_irif_movie_anti-gfp_binding_igem_slide.png" width="300"/></a><div class="thumbcaption"><div class="magnify"><a class="internal" href="https://static.igem.org/mediawiki/2015/3/31/Freiburg_files-20150706_irif_movie_anti-gfp_binding_igem_slide.png" title="vergrößern"><img alt="" height="11" src="/igem2015/lib/plugins/imagebox/magnify-clip.png" width="15"/></a></div><stong>Figure 2: Quotient picture of iRIf measurement for PDITC surface after anti-GFP step.</stong> Anti-GFP bound to all GFP spots on the slide. There is a slight difference between the different concentrations detectable. The positive control bBSA is not yet visible, because no | + | <div class="thumb2 tcenter" style="width:310px"><div class="thumbinner"><a class="lightbox_trigger" href="https://static.igem.org/mediawiki/2015/3/31/Freiburg_files-20150706_irif_movie_anti-gfp_binding_igem_slide.png" title="files:20150706_irif_movie_anti-gfp_binding_igem_slide.png"><img alt="" class="mediabox2" src="https://static.igem.org/mediawiki/2015/3/31/Freiburg_files-20150706_irif_movie_anti-gfp_binding_igem_slide.png" width="300"/></a><div class="thumbcaption"><div class="magnify"><a class="internal" href="https://static.igem.org/mediawiki/2015/3/31/Freiburg_files-20150706_irif_movie_anti-gfp_binding_igem_slide.png" title="vergrößern"><img alt="" height="11" src="/igem2015/lib/plugins/imagebox/magnify-clip.png" width="15"/></a></div><stong>Figure 2: Quotient picture of iRIf measurement for PDITC surface after anti-GFP step.</stong> Anti-GFP bound to all GFP spots on the slide. There is a slight difference between the different concentrations detectable. The positive control bBSA is not yet visible, because no Streptavidine was flushed over to this point.</div></div></div> |
− | <div class="thumb2 tcenter" style="width:310px"><div class="thumbinner"><a class="lightbox_trigger" href="https://static.igem.org/mediawiki/2015/6/68/Freiburg_files-20150706_irif_movie_strep-cy5_binding_igem_slide.png" title="files:20150706_irif_movie_strep-cy5_binding_igem_slide.png"><img alt="" class="mediabox2" src="https://static.igem.org/mediawiki/2015/6/68/Freiburg_files-20150706_irif_movie_strep-cy5_binding_igem_slide.png" width="300"/></a><div class="thumbcaption"><div class="magnify"><a class="internal" href="https://static.igem.org/mediawiki/2015/6/68/Freiburg_files-20150706_irif_movie_strep-cy5_binding_igem_slide.png" title="vergrößern"><img alt="" height="11" src="/igem2015/lib/plugins/imagebox/magnify-clip.png" width="15"/></a></div><strong>Figure 3: Quotient picture of iRIf measurement for PDITC surface after | + | <div class="thumb2 tcenter" style="width:310px"><div class="thumbinner"><a class="lightbox_trigger" href="https://static.igem.org/mediawiki/2015/6/68/Freiburg_files-20150706_irif_movie_strep-cy5_binding_igem_slide.png" title="files:20150706_irif_movie_strep-cy5_binding_igem_slide.png"><img alt="" class="mediabox2" src="https://static.igem.org/mediawiki/2015/6/68/Freiburg_files-20150706_irif_movie_strep-cy5_binding_igem_slide.png" width="300"/></a><div class="thumbcaption"><div class="magnify"><a class="internal" href="https://static.igem.org/mediawiki/2015/6/68/Freiburg_files-20150706_irif_movie_strep-cy5_binding_igem_slide.png" title="vergrößern"><img alt="" height="11" src="/igem2015/lib/plugins/imagebox/magnify-clip.png" width="15"/></a></div><strong>Figure 3:Quotient picture of iRIf measurement for PDITC surface after Streptavidine step.</stong> Streptavidine bound to the positive control bBSA as expected and also to all the GFP spots, because the anti-GFP was biotinylated.</div></div></div> |
</div> | </div> | ||
− | <h1> | + | <h1> Selective - The Specific Surface </h1> |
<div class="image_box left"> | <div class="image_box left"> | ||
− | <div class="thumb2 tcenter" style="width:310px"><div class="thumbinner"><a class="lightbox_trigger" href="https://static.igem.org/mediawiki/2015/9/95/Freiburg_labjournal-surchem-20150902_ni-nta_vs_pditc.png" title="https://static.igem.org/mediawiki/2015/9/95/Freiburg_labjournal-surchem-20150902_ni-nta_vs_pditc.png"><img alt="" class="mediabox2" src="https://static.igem.org/mediawiki/2015/9/95/Freiburg_labjournal-surchem-20150902_ni-nta_vs_pditc.png" width="300"/></a><div class="thumbcaption"><div class="magnify"><a class="internal" href="https://static.igem.org/mediawiki/2015/9/95/Freiburg_labjournal-surchem-20150902_ni-nta_vs_pditc.png" title="vergrößern"><img alt="" height="11" src="/igem2015/lib/plugins/imagebox/magnify-clip.png" width="15"/></a></div><strong>Figure 4: Comparison of specific Ni-NTA with unspecific PDITC surfaces</strong> | + | <div class="thumb2 tcenter" style="width:310px"><div class="thumbinner"><a class="lightbox_trigger" href="https://static.igem.org/mediawiki/2015/9/95/Freiburg_labjournal-surchem-20150902_ni-nta_vs_pditc.png" title="https://static.igem.org/mediawiki/2015/9/95/Freiburg_labjournal-surchem-20150902_ni-nta_vs_pditc.png"><img alt="" class="mediabox2" src="https://static.igem.org/mediawiki/2015/9/95/Freiburg_labjournal-surchem-20150902_ni-nta_vs_pditc.png" width="300"/></a><div class="thumbcaption"><div class="magnify"><a class="internal" href="https://static.igem.org/mediawiki/2015/9/95/Freiburg_labjournal-surchem-20150902_ni-nta_vs_pditc.png" title="vergrößern"><img alt="" height="11" src="/igem2015/lib/plugins/imagebox/magnify-clip.png" width="15"/></a></div><strong>Figure 4: Comparison of specific Ni-NTA with unspecific PDITC surfaces.</strong>The red bars show the mean fluorescence intensity for the different GFPs (His-GFP lysate, untagged GFP lysate, purified His-GFP) on PDITC. The blue bars represent the mean fluorescence intensity for the GFPs on Ni-NTA. The grey bar represents the background for both surfaces.</div></div></div> |
</div> | </div> | ||
<p> | <p> | ||
− | All the results presented above show, that a detection of antigen-antibody interaction is possible with | + | All the results presented above show, that a detection of antigen-antibody interaction is possible with the PDITC surface chemistry. But as we need a specific surface to bind antigens out of cell-free expression mix, we wanted to build up a Ni-NTA (Nickel-Nitrolotriacetic acid) surface and therefore linked all our target proteins to a 10x Histidine tag. We established a protocol based on the unspecific PDITC surface and optimized it, till we the unspecific binding of other proteins from the cell free mix was minimal. |
− | To determine the specificity of | + | To determine the specificity of our Ni-NTA surface we compared it to the PDITC surface. To asses the purifcation properties we used complete <i>E. coli</i> lysate transformed with either a tagged or an untagged GFP-construct. Additionally purified GFP-His as used in previous experiments was spotted and fluorescence intensity was measured. Figure 4 shows the intensities obtained for all spots: the intensity for the His-GFP lysate on the Ni-NTA surface is about four times higher than on the PDITC surface, while the values for the purified GFP-His are in a comparable range. |
The mean intensity for the untagged GFP-lysate spot is in the range of the background for the Ni-NTA surface and just slightly higher for the PDITC surface, which shows, that the GFP cannot bind to Ni-NTA without a His-tag. The low fluorescence intensities for GFP lysate on PDITC is due to the fact, that non-fluorescent proteins in the lysate are bound as efficient as GFP. This way the surface is blocked for further GFP molecules and overall fluorescence decreases compared to the specific Ni-NTA surface. | The mean intensity for the untagged GFP-lysate spot is in the range of the background for the Ni-NTA surface and just slightly higher for the PDITC surface, which shows, that the GFP cannot bind to Ni-NTA without a His-tag. The low fluorescence intensities for GFP lysate on PDITC is due to the fact, that non-fluorescent proteins in the lysate are bound as efficient as GFP. This way the surface is blocked for further GFP molecules and overall fluorescence decreases compared to the specific Ni-NTA surface. | ||
</p> | </p> | ||
<p> | <p> | ||
− | After we showed that we | + | After we showed that we can successfully bind his-tagged proteins to our Ni-NTA surface, we attempted to immobilize our cell-free expressed His-GFP-Lysate on this surface. You can find the results for this crucial part of our project on the <a href="https://2015.igem.org/Team:Freiburg/Results#agfp_results"> main results page</a>. |
</p> | </p> | ||
− | <h1> | + | <h1> Covalent - Other Surface Systems </h1> |
<p> | <p> | ||
− | + | On the way to find the surface best suited for our needs we also worked with the Promega Halo-tag system. The Halo-Tag that can be fused to target proteins binds covalently to chloroalkanes which are immobilized on the surface. We tested several ligands, which differed in length of the alkane chain and surface attachment method. The ligand that worked best for us, was a 3-chloropropylsilane, which we directly immobilized on plasma activated iRIf slides. To test the surface we pipetted our self-expressed Halo-GFP and Halo-mCherry as well as purified Halo-GFP as a positive control and purified untagged GFP as a negative control, which we both got from the group of J. Piehler from Osnabrück. As an additional negative control we spotted bBSA. | |
− | The iRIf measurement showed that the Halo-tagged GFPs were successfully immobilized on the surface. Unfortunately there was a lot of unspecific binding, so that the negative controls nearly bound as much to the surface as the positive control. The | + | The iRIf measurement showed that the Halo-tagged GFPs were successfully immobilized on the surface. Unfortunately there was a lot of unspecific binding, so that the negative controls nearly bound as much to the surface as the positive control. The optimizations that would be necessary for a specific surface could not be performed due to time limitations. We decided to work with the Ni-NTA surface we established for future experiments. |
</p> | </p> | ||
<div class="flexbox"> | <div class="flexbox"> | ||
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</a> | </a> | ||
<div class="thumbcaption"> | <div class="thumbcaption"> | ||
− | <strong>Figure 7: Quotient picture for the Halo-surface measurement.</strong> Spot 1-4 show the binding of biotinylated anti-GFP to successfully immobilized self purified Halo-GFP. On spot 5 to 8 Halo-mCherry, but as no anti-mCherry was flushed over, no signal can be observed. On Spot 9 the positive control (Halo-GFP from J. Piehler) was pipetted and on spot 10 the negative control (His-GFP) both show a clear binding to anti-GFP. Also the second negative control bBSA (spot 11) shows a signal after | + | <strong>Figure 7: Quotient picture for the Halo-surface measurement.</strong> Spot 1-4 show the binding of biotinylated anti-GFP to successfully immobilized self purified Halo-GFP. On spot 5 to 8 Halo-mCherry, but as no anti-mCherry was flushed over, no signal can be observed. On Spot 9 the positive control (Halo-GFP from J. Piehler) was pipetted and on spot 10 the negative control (His-GFP) both show a clear binding to anti-GFP. Also the second negative control bBSA (spot 11) shows a signal after Streptavidine was flushed over. |
</div> | </div> | ||
</div> | </div> | ||
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</a> | </a> | ||
<div class="thumbcaption"> | <div class="thumbcaption"> | ||
− | <strong>Figure 8: binding curve for the Halo-surface measurement.</strong> The binding curve shows the binding of biotinylated anti-GFP to all The GFPs on the slide (with and without Halo-Tag) and the binding of | + | <strong>Figure 8: binding curve for the Halo-surface measurement.</strong> The binding curve shows the binding of biotinylated anti-GFP to all The GFPs on the slide (with and without Halo-Tag) and the binding of Streptavidine to the bBSA spot and all the GFP spots, because the anti-GFP that was flushed over before is biotinylated. |
</div> | </div> | ||
</div> | </div> |
Revision as of 15:38, 17 September 2015
![](https://static.igem.org/mediawiki/2015/8/85/Freiburg_wiki_background_2_parted_left.png)
![](https://static.igem.org/mediawiki/2015/f/f4/Freiburg_wiki_background_2_parted_right.png)
Results: Binding on Surface
Optimizing cell-free expression as such is already a challenging task. But it becomes increasingly complicated when the expressed proteins should be immobilized on a glass surface directly after expression: The Cell-free mix is a complex system mainly consisting of proteins not that different to the target proteins. To overcome this hurdle we decided to fuse a tag to the proteins of interest that specifically bind to a chemically treated surface. To get such a tag-system to work, the surface has to be optimized in order to minimize unspecific binding.
Effective - The Unspecific Surface
For measuring in iRIf the target proteins have to be fixed to a glass slide, therefore we used silanes to introduce reactive groups on the surface. These Silanes can bind to Siliciumdioxide (glass) after reactive hydroxy groups were created by oxygen plasma activation. Adding either the silane GOPTS (3-Glycidyloxypropyltrimethoxysilane) or APTES (3-Aminopropyltriethoxysilane) allows us to bind certain chemical groups to the surface. The epoxy-group of GOPTS reacts covalently with amino-, hydroxy- and thiol-groups so that a large spectrum of molecules can be coupled. APTES on the other hand is not reactive itself but can be covalently fused to amino-groups with the homobifunctional cross-linker PDITC (p-Phenyldiisothiocyanate). For further information on chemistry and methods see surface chemistry methods . To evaluate these two chemistries, we compared the binding capacities of the respective surfaces on simple microscope glass slides. Self-purified GFP in different concentrations was spotted on both surfaces and the resulting fluroescence intensity was compared (Figure 1). Especially for low concentrations it is clearly visible that APTES/PDITC outcompetes the GOPTS surface. Therefore, we continued our experiments using the PDITC-chemistry. With these results we decided to switch on special glass slides, suitable for iRIf. With this method we were able to detect the interaction of anti-GFP antibodies with hand-spotted GFP of different concentrations. To check for variations in different GFP-purification protocols, we compared GFPs obtained from different groups (GFP1 to GFP3).
GFP 1, 50 µg/ml | GFP 1, 17 µg/ml | GFP 1, 5 µg/ml |
GFP 2, 50 µg/ml | GFP 2, 17 µg/ml | GFP 2, 5 µg/ml |
GFP 3, 50 µg/ml | GFP 3, 17 µg/ml | GFP 3, 5 µg/ml |
bBSA (0.5 mg/mL) | BSA (10 mg/mL) |
Immobilized GFP was first blocked in the iRIf device with 10 mg/mL BSA. Anti-GFP in buffer solution was flushed over the slide and leads to a change in the optical thickness at the GFP spots. The binding was detected in real-time. Figure 2 shows a quotient picture from this step, thus the increase in optical thickness due to antibody binding. The signal can further be amplified by a second binding step. As the anti-GFP antibody was biotinylated it could easily be attached to streptavidin flushed through the microfluidic chamber (Figure 3). This step also confirms the binding of anti-GFP to the spots and therefore the specificity of the interaction. Biotinylated BSA (bBSA, bottom middle) spotted on the slide that also binds to Streptavidine served as a positive control.
![](https://static.igem.org/mediawiki/2015/3/31/Freiburg_files-20150706_irif_movie_anti-gfp_binding_igem_slide.png)
Selective - The Specific Surface
![](https://static.igem.org/mediawiki/2015/9/95/Freiburg_labjournal-surchem-20150902_ni-nta_vs_pditc.png)
All the results presented above show, that a detection of antigen-antibody interaction is possible with the PDITC surface chemistry. But as we need a specific surface to bind antigens out of cell-free expression mix, we wanted to build up a Ni-NTA (Nickel-Nitrolotriacetic acid) surface and therefore linked all our target proteins to a 10x Histidine tag. We established a protocol based on the unspecific PDITC surface and optimized it, till we the unspecific binding of other proteins from the cell free mix was minimal. To determine the specificity of our Ni-NTA surface we compared it to the PDITC surface. To asses the purifcation properties we used complete E. coli lysate transformed with either a tagged or an untagged GFP-construct. Additionally purified GFP-His as used in previous experiments was spotted and fluorescence intensity was measured. Figure 4 shows the intensities obtained for all spots: the intensity for the His-GFP lysate on the Ni-NTA surface is about four times higher than on the PDITC surface, while the values for the purified GFP-His are in a comparable range. The mean intensity for the untagged GFP-lysate spot is in the range of the background for the Ni-NTA surface and just slightly higher for the PDITC surface, which shows, that the GFP cannot bind to Ni-NTA without a His-tag. The low fluorescence intensities for GFP lysate on PDITC is due to the fact, that non-fluorescent proteins in the lysate are bound as efficient as GFP. This way the surface is blocked for further GFP molecules and overall fluorescence decreases compared to the specific Ni-NTA surface.
After we showed that we can successfully bind his-tagged proteins to our Ni-NTA surface, we attempted to immobilize our cell-free expressed His-GFP-Lysate on this surface. You can find the results for this crucial part of our project on the main results page.
Covalent - Other Surface Systems
On the way to find the surface best suited for our needs we also worked with the Promega Halo-tag system. The Halo-Tag that can be fused to target proteins binds covalently to chloroalkanes which are immobilized on the surface. We tested several ligands, which differed in length of the alkane chain and surface attachment method. The ligand that worked best for us, was a 3-chloropropylsilane, which we directly immobilized on plasma activated iRIf slides. To test the surface we pipetted our self-expressed Halo-GFP and Halo-mCherry as well as purified Halo-GFP as a positive control and purified untagged GFP as a negative control, which we both got from the group of J. Piehler from Osnabrück. As an additional negative control we spotted bBSA. The iRIf measurement showed that the Halo-tagged GFPs were successfully immobilized on the surface. Unfortunately there was a lot of unspecific binding, so that the negative controls nearly bound as much to the surface as the positive control. The optimizations that would be necessary for a specific surface could not be performed due to time limitations. We decided to work with the Ni-NTA surface we established for future experiments.
![](https://static.igem.org/mediawiki/2015/7/7d/Freiburg_labjournal-surchem-exp_42a_slide254.png)
![](https://static.igem.org/mediawiki/2015/e/e4/Freiburg_labjournal-surchem-exp_42a_slide254_binding.png)
Validation of our controls
![](https://static.igem.org/mediawiki/2015/d/d4/Freiburg_GFP_purified.png)
Given that we were using purified GFP-His Tag for evaluation of our self-made surfaces and in further experiments as positive control we validated it by western blot. Therefore we showed the presence of GFP-His Tag with anti-His HRP Conjugate. Additionally we used the specific anti-GFP antibody as for the detection in iRIf.