GFP Protein Purification: After FPLC, there are always two bands in SDS-PAGE. How to achieve precise purification?

If GFP protein shows two bands on SDS-PAGE after purification by fast protein liquid chromatography (FPLC), there could be several reasons:

1. Partial protein degradation:

This may lead to an additional band smaller than the expected molecular weight.

2. Post-translational modifications:

For example, phosphorylation or glycosylation may increase the molecular weight of the protein.

3. Isomers or polymorphism:

Certain forms of GFP may exist in different conformations or isomers.

4. Other impurities:

Although purified by FPLC, it may still contain impurities with similar molecular weights to GFP.

To further purify GFP more precisely, the following strategies can be considered:

1. Optimize FPLC conditions:

Depending on the FPLC column you are using (e.g., gel filtration, ion exchange, or affinity chromatography), you can try a more stringent elution gradient or a more suitable buffer to improve separation efficiency.

2. Use a secondary chromatography step:

For example, perform a rough purification using affinity chromatography first, then further purify with gel filtration or ion exchange chromatography.

3. Improve sample handling and extraction conditions:

Ensure enough protease inhibitors are added during extraction and handling, and process the sample as quickly as possible to reduce protein degradation.

4. Perform heat treatment:

Some proteins are heat-stable, whereas some impurities may not be. Consider briefly heating at a certain temperature, then cooling and centrifuging to remove unstable proteins.

5. Conduct a control experiment:

By expressing the GFP gene in a different host system, you can determine if two bands still appear. This can help ascertain if the issue is related to a specific cellular system.

6. Protein digestion test:

Partially digest the GFP sample with proteases, then run it on SDS-PAGE to see if there is any overlap between the two bands. This can help determine if there are indeed two different protein forms present.

By employing the above strategies, you can attempt to further purify GFP protein and understand why two bands appear on SDS-PAGE.

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