How Biotinylated Molecules are Purified with Streptavidin Beads

Streptavidin is a bacterial protein that binds the small molecule biotin with exceptionally high affinity. The streptavidin–biotin pair represents one of the strongest noncovalent interactions known. Streptavidin agarose beads take advantage of this interaction as a solid-phase platform to purify biotinylated biomolecules, such as proteins and DNA, and to study molecular interactions (Figure 1).

Biotinylated molecules are purified with streptavidin beads through this strong biotin–streptavidin binding. When the sample is mixed with the beads, the biotinylated targets are captured on the bead surface, while unbound molecules are removed by washing, leaving the desired protein or DNA isolated.

Figure 1. Streptavidin agarose beads with a biotinylated protein bound.

What Are Biotinylated Molecules?

Biotinylated molecules are proteins or nucleic acids that have the small molecule biotin covalently linked to them.

A few proteins are naturally biotinylated in living systems (Niers et al., 2011). However, in most research contexts, when we mention biotinylated molecules, we mean proteins or nucleic acids that have been deliberately modified with biotin by scientists as a tool to study them in more detail.

One example is site-specific biotinylation: researchers can attach biotin to a single lysine residue within an Avi-tag using the enzyme BirA (Figure 2, left) (Fairhead & Howarth, 2015). The Avi-tag is an affinity tag that can be fused to a protein of interest to enable its purification or immobilization. Unlike many other affinity tags, the Avi-tag must first be biotinylated before it is useful for these applications.

Instead of using an enzyme to biotinylate one defined lysine, proteins can also be extensively biotinylated on multiple lysine residues across the protein via a chemical reaction (Figure 2, right) (Kay et al., 2009). This approach has the advantage that the protein does not need to carry an Avi-tag, and multiple regions of the protein can be labeled with biotin. This can be particularly helpful if you plan to later digest or fragment the protein into smaller pieces, as we will discuss in the next section.

Figure 2. Proteins can be enzymatically biotinylated at a single lysine residue within an Avi-tag (left), or chemically biotinylated on multiple lysine residues distributed throughout the protein (right).

How are biotinylated molecules purified?

Affinity purification typically consists of three main steps:

1. binding the target protein to the beads,

2. washing away contaminating proteins, and

3. eluting the protein of interest.

This overall workflow is the same when using biotinylated proteins with streptavidin beads. However, there are several important features of the streptavidin–biotin system that make each of these steps behave somewhat differently.

As noted earlier, the streptavidin–biotin interaction is among the strongest biological noncovalent interactions known. Because it is so tight, the beads can be washed under highly stringent conditions, often yielding higher purity of the target protein than systems such as His-tagged proteins on nickel beads.

The trade-off is that this same strength makes it difficult to elute biotinylated molecules from streptavidin beads. Efficient elution generally requires relatively harsh conditions, such as buffers with very high or low pH, organic solvents, heating the beads, or even cleaving the molecule off the beads.

If you would like more guidance on choosing the most appropriate elution strategy for your biotinylated molecule, this article is a useful resource. How to Elute Biotinylated Proteins and Nucleic Acids from Streptavidin Beads?

What else can biotinylated molecules be used for?

Another powerful application of biotinylated molecules is in mapping their interaction partners—identifying which other molecules they physically bind to.

For instance, imagine you are working on a protein that drives cancer development. In biology, molecules almost never function in isolation; they typically rely on binding partners that help them execute their roles. Therefore, if you want to understand how this protein promotes cancer, one of the first logical steps is to identify other molecules that support or enhance its oncogenic behavior.

Experimentally, one approach is to biotinylate the cancer-associated protein, immobilize it on streptavidin beads, and then use these beads to “fish out” proteins from cancer cells that bind to your protein of interest (Figure 3). After you have identified one or more proteins that interact with the cancer-causing protein, you can then design additional experiments to test which of these interactions are functionally important for oncogenesis.

Figure 3. Once your biotinylated target protein is bound to streptavidin beads, the complex can be used as bait to identify interacting proteins.

This is particularly exciting because, once you identify a crucial protein–protein interaction that drives cancer, you can begin to design drugs that block this interaction and thereby inhibit tumor growth. This exact strategy has already been used to develop several anticancer drugs that are currently prescribed to patients (Nada et al., 2024).

Although the example above focuses on proteins, the same general approach can also be applied to DNA. For instance, imagine there is a single nucleotide variant associated with cancer, but it is not yet clear how this change in DNA sequence contributes to tumor development. In that case, you could biotinylate the DNA fragment and use streptavidin beads to pull down proteins that associate with it.

A particularly powerful way to do this is to compare proteins that bind to the original (wild-type) DNA sequence with those that bind to the mutant cancer-associated sequence. This comparison can help pinpoint proteins that are specifically involved in the cancer-related function (Figure 4). Just as in the protein example above, this type of experiment has been used by researchers to uncover the molecular defects underlying certain cancers (Carrasco Pro et al., 2023).

Figure 4. Streptavidin beads can be used to pull down proteins that interact with biotinylated DNA. In this illustration, a blue protein associates with both the wild-type and mutant DNA, whereas a dark green protein binds exclusively to the mutant DNA. In this hypothetical scenario, the dark green protein would be a prime candidate for further investigation.

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References

1. Niers JM, Chen JW, Weissleder R, Tannous BA. Enhanced in vivo imaging of metabolically biotinylated cell surface reporters. Analytical Chemistry. 2011;83(3):994–999. doi:10.1021/ac102758m.

2. Fairhead M, Howarth M. Site-specific biotinylation of purified proteins using BirA. Methods in Molecular Biology. 2015;1266:171–184. doi:10.1007/978-1-4939-2272-7_12.

3. Kay BK, Thai S, Volgina VV. High-throughput biotinylation of proteins. In: Doyle SA, editor. High Throughput Protein Expression and Purification. Methods in Molecular Biology, vol. 498. Totowa, NJ: Humana Press; 2009. p. 185–198. doi:10.1007/978-1-59745-196-3_13.

4. Nada H, Choi Y, Kim S, Jeong KS, Meanwell NA, Lee K. New insights into protein–protein interaction modulators in drug discovery and therapeutic advance. Signal Transduction and Targeted Therapy. 2024;9:341. doi:10.1038/s41392-024-02036-3.

5. Carrasco Pro S, Hook H, Bray D, Berenzy D, Moyer D, Yin M, et al. Widespread perturbation of ETS factor binding sites in cancer. Nature Communications. 2023;14(1):913. doi:10.1038/s41467-023-36535-8.

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