Tag-Free Recombinant Proteins: Benefits, Production and Applications

When and how to obtain a protein that better reflects the target itself

Affinity and fusion tags make recombinant proteins easier to express, purify, and detect. Yet a tag can also become an experimental variable. For studies that depend on the target’s structure, activity, or molecular interactions, a tag-free final product may be preferable. This article explains what tag-free means, when tag removal is useful, how it is done, and which properties still need to be verified after purification.

A tag-free recombinant protein contains no engineered affinity or fusion tag in the final product. Common tags include polyhistidine (His), GST, MBP, FLAG, Strep, and SUMO. Typically placed at the N or C terminus, they can assist expression, affinity purification, detection, or solubility.

Tag-free does not mean that a tag cannot be used during production. A common approach expresses and captures a tagged fusion protein, cleaves off the tag, removes the tag and protease, and then polishes the purified target. The tag serves as a production aid without being retained in the final preparation. Proteases used for this purpose include TEV, HRV 3C, thrombin, factor Xa, and SUMO-specific proteases; the choice depends on the construct and target.

Figure 1  A typical workflow for producing a tag-free recombinant protein

A His tag enables immobilized metal affinity chromatography (IMAC), while larger partners such as GST or MBP can improve the soluble yield of some proteins. These benefits are often worth keeping. Nevertheless, a tag adds an engineered sequence and may affect a target’s conformation, solubility, activity, or interactions in a construct-dependent manner. A flexible fusion partner can also complicate some structural studies, although tagged constructs can be suitable for crystallography in other cases [1].

The useful question is therefore whether the tag could influence the specific readout. When it might obscure an epitope, alter a binding surface, affect an assay, or introduce unwanted heterogeneity, a tag-free preparation is worth evaluating. Removing a tag alone does not establish a native fold or full biological activity: sequence, integrity, oligomeric state, and function must be assessed for the intended application.

Loss of solubility or stability

A fusion partner may help the target express, fold, or remain soluble. A protein that is stable as a fusion can precipitate or aggregate after cleavage, reducing recovery. The construct, cleavage conditions, and post-cleavage buffer may need to be optimized for the untagged form.

More demanding purification

Once the affinity handle is removed, the target may no longer have a general capture method. A second separation must remove free tag, cleavage protease, uncleaved fusion, and other impurities. Depending on the target, this may involve subtractive affinity chromatography, ion-exchange chromatography, size-exclusion chromatography (SEC), hydrophobic interaction chromatography, or a target-specific method.

Cleavage and sequence design

TEV, HRV 3C, thrombin, factor Xa, enterokinase, and SUMO-specific proteases differ in recognition sequence, cleavage efficiency, specificity, and buffer requirements. The junction should be designed with the desired final terminus in mind: some constructs retain extra residues after cleavage. Even TEV, which is widely used for its specificity, has been reported to cleave certain human proteins at noncanonical sites [2]. Assess the target sequence and confirm the identity of the final product.

Fusion expression followed by protease cleavage

Express and purify a tagged fusion, cleave it with a suitable site-specific protease, and separate the target from the tag, enzyme, and uncleaved material. This is a well-established and adaptable route. TEV is frequently used when its cleavage junction and reaction conditions are compatible with the target.

SUMO fusion and cleavage

SUMO can improve expression or solubility for some targets and is removed by a SUMO-specific protease. With an appropriate N-terminal fusion design, cleavage can yield the target without additional N-terminal residues [3]. The benefit still depends on the particular protein and expression system.

Intein-mediated cleavage

Engineered inteins can release a target from a fusion partner under defined conditions, offering an alternative to adding a separate cleavage protease. Feasibility and any residues left at the terminus depend on the intein construct and cleavage chemistry.

Direct expression without a tag

Some targets can be expressed without a fusion tag and purified by ion exchange, SEC, or another suitable method. This route avoids a cleavage step but places greater demands on expression yield, protein stability, and the purification strategy.

For many projects, tagged expression followed by cleavage provides a practical balance between production efficiency and a tag-free final product. The best route should be selected on the basis of the target and the intended assay.

Structural biology

X-ray crystallography and NMR often require a sufficiently pure, homogeneous, and stable sample. Removing a flexible tag may reduce a source of heterogeneity for some constructs. The tag-free version must still be assessed for structural integrity and stability.

Protein interactions and binding assays

For interactions with antibodies, receptors, ligands, or other proteins, eliminating a tag can remove a potential source of steric interference or nonspecific binding. In SPR, BLI, and ITC, however, the assay format also matters: immobilization chemistry, orientation, and site accessibility can affect the result. Choose the protein format and assay design together.

Enzyme and functional studies

If the readout is intrinsic enzyme activity, receptor behavior, or ligand binding, comparing tagged and tag-free preparations may help establish whether the tag changes the measurement. Tag removal should be justified by the target and experimental system, rather than treated as a universal requirement.

Antibody discovery and drug research

Antigen conformation and epitope accessibility can influence antibody screening. If a tag is close to the binding region or alters antigen presentation, a tag-free antigen can reduce that variable during selection and binding assessment. Orthogonal validation against the relevant native target remains valuable.

Complex membrane proteins

For GPCRs, ion channels, and transporters, tag status is only one consideration. Folding, transmembrane topology, lipid or detergent environment, stability, and functional state can be equally important. A tag-free membrane protein is useful only if its preparation supports the conformation and activity required by the experiment.

No single expression and purification protocol fits every target. Protein size, domain architecture, subcellular location, transmembrane segments, expression system, and downstream use all influence construct and process design. A tag-free project should therefore plan the expression construct, tag placement, cleavage site, removal strategy, and final quality assessment as one workflow.

DIMA BIOTECH offers a custom tag-free recombinant protein service. Based on the target’s properties and intended applications, the workflow may combine tagged expression and affinity capture with tag cleavage, removal of process-related components, and polishing purification. The final preparation can then be assessed for the attributes relevant to the project, such as identity, purity, aggregation state, and activity. This approach supports structural studies, functional assays, interaction analysis, antibody discovery, and work on complex proteins while keeping the final protein format aligned with the research question.

Explore more information of tag-free Recombinant protein service

References

1. Smyth DR et al. Crystal structures of fusion proteins with large-affinity tags. Protein Science. 2003;12:1313–1322.
2. Unexpected tobacco etch virus (TEV) protease cleavage of recombinant human proteins. Protein Expression and Purification. 2024;220:106488.
3. Discovery and engineering of enhanced SUMO protease enzymes. Journal of Biological Chemistry. 2018;293:13224–13233.