Protein Analysis and Purification

Choosing a Tag for Your Protein

Posted on by Sara Klink

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.

Immunofluorescent detection of HiBiT-tagged proteins in CRISPR-edited cell pools and clones using the Anti-HiBiT Monoclonal Antibody.
CRISPR-Cas9 editing knocked-in HiBiT at the endogenous locus of proteins with varying subcellular localization. Fixed CRISPR-modified clones or pools of cells were imaged by immunofluorescent staining using the Anti-HiBiT Monoclonal Antibody (red) and Hoechst dye (blue). Panel A. VCL-HiBiT pool. Panel B. SMARCA4-HiBiT clone. Panel C. HDAC2-HiBiT clone. Panel D. HSP90B1-HiBiT pool.

Affinity Tags

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.

Continue reading “Choosing a Tag for Your Protein”

Looking Back: Cell-Free Expression Systems Helped to Characterize Proteins Involved in Hypoxia Response

Posted on by Promega
Structur of a HIF-1a-pVHL-ElonginB-ElonginC complex
Structure of a HIF-1a-pVHL-ElonginB-ElonginC complex

William G. Kaelin Jr., Sir Peter J. Ratcliffe and Gregg L. Semenza were awarded the 2019 Nobel Prize in Physiology or Medicine for their discoveries of how cells sense and adapt to oxygen availability.

Kaelin and Ratcliffeโ€™s labs focused their efforts on the transcription factor HIF (hypoxia-inducible factor). This transcription factor is critical in the cellular adaptation of to changes in oxygen availability.

When oxygen levels are elevated cells contain very little HIF. Ubiquitin is added to the HIF protein via the VHL complex and it is degraded in the proteasome.  When oxygen levels are low (hypoxia) the amount of HIF increases.

In 2001 both groups published articles characterizing the interaction between VHL and HIF, and these articles were referenced by the Nobel Prize Organization in their press release about this year’s award. (1,2). Both studies demonstrated that under the normal oxygen conditions hydroxylation of proline residue P564 enabled VHL to recognize and bind to HIF.

The use of cell free expression (i.e., TNT Coupled Transcription/Translation System) by both labs was key in the characterization of the VHL:HIF interaction The labs utilized HIF and VHL 35-S labeled proteins generated via the TNT system under both normal or in a hypoxic work station to:

  • Determine the affect of ferrous chloride and cobaltous chloride on the interaction
  • Map the specific region of HIF required for the interaction to occur (556-574)
  • Determine the effect of HIF point mutations on the interaction
  • Use synthetic peptides to block the interaction
  • Conclude that a factor in mammalian cells was necessary for the interaction to occur.

Literature Cited

  1. Ivan, M et al. (2001) HIF Targeted for VHL-Mediated Destruction by Proline Hydroxylation: Implications for O2 Sensing. Science 292: 464โ€“67.
  2. Jaakkola, P. et al. (2001) Targeting of HIF-ฮฑ to the von Hippel-Lindau Ubiquitylation of Complex by O2– Regulated Prolyl Hydroxylation. Science 202, 468โ€“72 .

Related Posts

All You Need is a Tether: Improving Repair Efficiency for CRISPR-Cas9 Gene Editing

Posted on by Sara Klink

Ribonucleoprotein complex with Cas9, guide RNA and donor ssDNA. Copyright Promega Corporation.

With the advent of genome editing using CRISPR-Cas9, researchers have been excited by the possibilities of precisely placed edits in cellular DNA. Any double-stranded break in DNA, like that induced by CRISPR-Cas9, is repaired by one of two pathways: Non-homologous end joining (NHEJ) or homology-directed repair (HDR). Using the NHEJ pathway results in short insertions or deletions (indels) at the break site, so the HDR pathway is preferred. However, the low efficiency of HDR recombination to insert exogenous sequences into the genome hampers its use. There have been many attempts at boosting HDR frequency, but the methods compromise cell growth and behave differently when used with various cell types and gene targets. The strategy employed by the authors of an article in Communications Biology tethered the DNA donor template to Cas9 complexed with the ribonucleoprotein and guide RNA, increasing the local concentration of the donor template at the break site and enhancing homology-directed repair. Continue reading “All You Need is a Tether: Improving Repair Efficiency for CRISPR-Cas9 Gene Editing”

Brazilian University Swatting at Leishmaniasis Parasite

Posted on by Julia Nepper

The Medicinal Chemistry Center (CQMED), headquartered at Campinas State University in Brazil, recently started a project in partnership with Promega to develop drugs that can be used against Leishmania. This genus of protozoans is the etiological agent of leishmaniasis, transmitted to humans by sandflies.

Microscopic image of Leishmania parasite
Microscopic image of Leishmania tropica. Credit: Brian E. Keas at Michigan State University.

Leishmaniasis is classified as a neglected tropical disease that mainly affects poor communities. Symptoms include large skin sores and an enlarged spleen. The challenge in developing drugs to treat Leishmania is finding appropriate therapeutic targets. These targets are normally proteins whose inhibition leads to death of the parasite. In addition to pharmaceutical company Eurofarma, whose goal is to develop drugs for Leishmania, Promega was chosen to help solve this problem because of our NanoBRETโ„ข Target Engagement (TE) assay*, a well-established technique for measuring protein interactions. In this assay, NanoLucยฎ luciferase is attached to the protein of interest, and a fluorescent NanoBRETโ„ข tracer molecule is added to the cells. This produces a BRET signal. When a competing ligand is added, it will displace the tracer molecule, enabling quantification of the strength of the interaction compared to the tracer molecule..

A challenge that researchers will face will be ensuring that the NanoBRETโ„ข tracer reaches the inside of the parasite cells; because Leishmania is an intracellular parasite, molecules need to cross the host cell membrane, the membrane of the vacuole containing the parasites, and the membrane of the parasite itself. Another challenge the slow reproduction of Leishmania within macrophages. On top of that is the fact that the parasiteโ€™s metabolism varies depending on its biological cycle, meaning that there could be long periods of time during which a drugโ€™s therapeutic target is not expressed in the cell, during which time the drug would have no effect. The ideal target would be expressed at high levels throughout the cell cycle.

The project is being led by Rafael Couรฑago, a researcher at CQMED, and Promega scientists Matt Robers and Jean-Luc Vaillaud.

*An earlier version of this blog incorrectly said that these experiments are based on the NanoBRETโ„ข assay using HaloTagยฎ protein.

Targeted Degradation: It’s Not Just for PROTACs (or Proteins) Any More

Posted on by Promega
PROTACs for Targeted Protein Degradation
An illustration of PROTAC structure and the proteins it binds.

Updated June 30, 2025

Targeting a single protein and making it disappear from the cell is quite the magic trick, and there are various molecular tools available for this task. You can use RNA interference, which prevents a protein from being made, inhibitors that bind the protein, rendering it unavailable for use or even gene editing tools like CRISPR that can remove the encoding gene from the genome. Also, you can target an existing protein for destruction, using the cellโ€™s own garbage disposal system to degrade the protein, and you can even expand the concept of targeted degradation to other kinds of macromolecules, such as RNA.

Continue reading “Targeted Degradation: It’s Not Just for PROTACs (or Proteins) Any More”

Lighting Up GPCR Research with Bioluminescent Tagging

Posted on by Julia Nepper

G Protein-Coupled Receptors (GPCRs) are a very large, diverse family of transmembrane receptors in eukaryotes. These receptors detect molecules outside the cell and activate internal signaling pathways by coupling with G proteins. Once a GPCR is activated, ฮฒ-arrestins translocate to the cell membrane and bind to the occupied receptor, uncoupling it from G proteins and promoting its internalization.

Reporter tags are useful for studying the dynamics of GPCRs and associated proteins, but large tags can disrupt the receptorsโ€™ native functioning, and often overexpression of the tagged protein is required to obtain sufficient signal. Here is one example of how researchers have used the small, bright NanoLucยฎ luciferase to overcome these common challenges and answer questions about GPCRs.

Continue reading “Lighting Up GPCR Research with Bioluminescent Tagging”

Bottom-up Proteomics: Need Help?

Posted on by Kassidy Steyer

The use of mass spectrometry for the characterization of individual or complex protein samples continues to be one of the fastest growing fields in the life science market.

Bottom-up proteomics is the traditional approach to address these questions. Optimization of each the individual steps (e.g. sample prep, digestion and instrument performance) is critical to the overall success of the entire experiment.

To address issues that may arise in your experimental design, Promega has developed unique tools and complementary webinars to help you along the way.

Here you can find a summary of individual webinars for the following topics:

Continue reading “Bottom-up Proteomics: Need Help?”

When Proteins Get Together: Shedding (Blue) Light on Cellular LOV

Posted on by Ken Doyle
NanoBRET

No protein is an island. Within a cell, protein-protein interactions (PPIs) are involved in highly regulated and specific pathways that control gene expression and cell signaling. The disruption of PPIs can lead to a variety of disease states, including cancer.

Two general approaches are commonly used to study PPIs. Real-time assays measure PPI activity in live cells using fluorescent or luminescent tags. A second approach includes methods that measure a specific PPI โ€œafter the factโ€; popular examples include a reporter system, such as the classic yeast two-hybrid system.

Continue reading “When Proteins Get Together: Shedding (Blue) Light on Cellular LOV”

Characterizing Compound Binding in Cell-Free Systems

Posted on by Promega

Dioxins (e.g., 2,3,7,8-Tetrachlorodibenzo-p-dioxin, TCDD) and related compounds (DRCs) are persistent environmental pollutants that gradually accumulate through the food chain, mainly in the fatty tissues of animals. Dioxins are highly toxic and can cause reproductive and developmental problems, damage the immune system, interfere with hormones and also cause cancer. This broad range of toxic and biological effects of DRCs is mostly mediated by the aryl hydrocarbon receptor (AHR).

In animal cells, DRCs bind to AHR in the cytoplasm and then translocate into the nucleus, where they affect the transcription of multiple target genes, including xenobiotic-metabolizing enzymes, such as CYP1A isozymes. AHR is also involved in immune system maintenance, protein degradation and cell proliferation.

The jungle crow (Corvus macrorhynchos) has been considered a suitable indicator for monitoring environmental chemicals such as DRCs. While mammals only have one AHR form, avian species have multiple AHR isoforms such as AHR1 and AHR2. To unveil the functional diversity of multiple avian AHR isoforms in terms of their contribution to responses to DRCs a recent study by Kim et al. investigated the molecular and functional characteristics of jungle crow AHR isoforms, cAHR1 and jcAHR2 (1).

cAHR1 and jcAHR2 proteins were synthesized using AHR proteins were synthesized using the TnT Quick-Coupled Reticulocyte Lysate System  to examine whether these jcAHRs have the potential to bind to TCDD. TCDD-binding affinity of the in vitro-expressed jcAHR protein was analyzed using the velocity sedimentation assay with a sucrose gradient.

The results demonstrate that both jcAHR1and jcAHR2 are capable of binding to TCDD.

Reference
Kim, E-Y (2019) The aryl hydrocarbon receptor 2 potentially mediates cytochrome P450 1A induction in the jungle crow (Corvus macrorhynchos). Ecotoxicology and Environmental Safety 171. 99โ€“111

Mutation Analysis Using HaloTag Fusion Proteins

Posted on by Promega

In a recent reference, Kinoshita and colleagues characterized the phosphorylation dynamics of MEK1 in human cells by using the phosphate affinity electrophoresis technique, Phos-tag sodium dodecyl sulfateโ€“polyacrylamide gel electrophoresis (Phos-tag SDS-PAGE; 1). They found that multiple variants of MEK1 with diferent phosphorylation states are constitutively present in typical human cells.

To investigate the relationships between kinase activity and drug efficacy researchers from the same laboratory group conducted phosphorylation profling of various MEK1 mutants by using Phos-tag SDS- PAGE (2).

They introduced mutations in of the MEK-1 coding gene that are associated with spontaneous melanoma, lung cancer, gastric cancer, colon cancer and ovarian cancer were introduced into Flexi HaloTag clone pFN21AE0668, which is suitable for expression of N-terminal HaloTag-fused MEK1 in mammalian cells. Continue reading “Mutation Analysis Using HaloTag Fusion Proteins”