You have identified and cloned your protein of interest, but you want to explore its function. A protein fusion tag might help with your investigation. However, choosing a tag for your protein depends on what experiments you are planning. Do you want to purify the protein? Would you like to identify interacting proteins by performing pull-down assays? Are you interested in examining the endogenous biology of the protein? Here we cover the advantages and disadvantages of some protein tags to help you select the one that best suits your needs.
The most commonly used protein tags fall under the category of affinity tags. This means that the tag binds to another molecule or metal ion, making it easy to purify or pull down your protein of interest. In all cases, the tag will be fused to your protein of interest at either the amino (N) or carboxy (C) terminus by cloning into an expression vector. This protein fusion can then be expressed in cells or cell-free systems, depending on the promoter the vector contains.
Tags with Noncovalent Binding
Polyhistidine or 6XHis tag is a six histidine tag frequently used for creating fusion proteins. Its small size tends not to disrupt protein structure, so you can study the protein in other assays. The polyhistidine tag binds to metal ions like Ni2+ and Zn2+, making it easy to purify the fusion protein when a crude lysate is passed over a resin that contains nickel, binding the polyhistidine-tagged proteins. Proteins fused with polyhistidine are best expressed in bacterial cells where there is little background binding from other proteins. If expressed in insect or mammalian cells, more stringent washing of the metal resin is needed to remove other proteins that have polyhistines in their sequence.
Glutathione-S-Transferase (GST) is another common affinity protein tag that is used for proteins expressed in E. coli to purify the tagged proteins from bacterial lysates. At 26kDa, GST is larger than polyhistidine and a eukaryotic protein, making GST problematic to use with mammalian or insect cells. While the larger tag can interfere with protein function, GST can make fusion proteins more soluble and express at greater levels in E. coli, a benefit for purifying more protein at one time. GST binds to glutathione, and using beads coated in glutathione can capture and purify the fusion protein.
Tags with Covalent Binding
Despite the affinity the polyhistidine tag has for metal ions and GST has for glutathione, the interactions are reversible and can be disrupted when applying stringent washing. A covalent bond offers a method to not only capture a tagged protein but its strength means you can eliminate background binding with stringent washes. HaloTag® Technology is based on the covalent interaction between the 34kDa HaloTag® protein and its ligands. Once you create a N- or C-terminal fusion protein with HaloTag, you can purify the tagged protein, track its movements in the cell, study protein:protein interactions and more. This affinity tag works in both E. coli and mammalian cells. Expressing a tagged protein in mammalian cells means the protein will contain post-translational modifications, more accurately reflecting its cellular function. In addition, once you purify the protein, HaloTag can be removed by TEV protease cleavage, giving you purified, untagged protein for your analyses.
Bioluminescent Reporter Tags
There are bioluminescent tags are available in two options: full-length reporter gene or a tiny peptide (11 amino acids) that generates light in the presence of a second complementary protein. Similar to affinity tags, bioluminescent tags are fused to your protein of interest at the N or C terminus. However, you may be able to skip cloning step and add the tiny HiBiT peptide tag using CRISPR-Cas9 gene editing. This means adding the tag to the endogenous gene and examining your protein of interest in the context of the cell. Most tagging methods use exogeneous expression of the tagged protein, which means the tagged protein is expressed from a plasmid at much greater levels than that found normally in cells. However, this greater expression level does not accurately reflect the amount of protein in the cell and can distort the biology being studied.
In addition, using a bioluminescent reporter like NanoLuc® luciferase, you can detect the light from the protein in the cell, tracking its movements and any translocations (e.g., if a virus containing a bioluminescent reporter successfully infects a cell). The brightness of NanoLuc® luciferase and the HiBiT tag can be measured using both lytic and live-cell methods, offering sensitive detection of the tagged proteins.
Depending on your experimental goals, there a protein fusion tag that will help with your experiments. Whether you are seeking an affinity tag or bioluminescent reporter, you will need to analyze the benefits of each peptide tag to select the right one for your goals.
Want to learn more about protein tagging? This article has more information on the protein tags described in this post, including all their potential applications.
Curious about HaloTag® Technology? Learn about all the ways you can use a single fusion construct to investigate protein function here.
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