A gst fusion protein (Glutathione-S-transferase) is useful for affinity chromatography and immunoprecipitation. The natural form of GST is an enzyme that catalyzes the protective mechanisms of glutathione. Glutathione is an antioxidant that prevents cell damage by reactive oxygen species. However, the GST fusion protein is not natural. It is a genetically engineered protein that has become a useful biotechnological tool.
The affinity between the GST protein and glutathione is what makes it useful for affinity chromatography. Once a capture GST protein is created, it can be used to capture a target protein, and the whole complex can be isolated by running it through a glutathione column. The complex is then eluted by adding an excess of glutathione, which out-competes the bound glutathione and fills the GST binding site resulting in release from the column.