Introduction
His-tags are one of the most widely used tools in recombinant protein expression, particularly across antibody engineering, nanobody discovery, and early-stage drug discovery workflows.
Their popularity is not driven by complexity, but by practicality. A short, standardised tag can be applied across different proteins and experiments, allowing researchers to move quickly from expression to initial evaluation without building new assays each time.
A Simple, Flexible Design
A His-tag is a short sequence of histidine residues (most commonly 6×His) fused to a protein of interest. In most cases, it has minimal impact on protein structure and can be easily introduced during construct design.
It can be placed at either the N- or C-terminus, often with a linker to improve accessibility, and is frequently combined with a cleavage site so that it can be removed later if required.
This flexibility is one of the main reasons His-tags are so widely adopted—they are easy to design, easy to use, and adaptable across different systems.
One Tag, Multiple Uses
What makes His-tags particularly useful is that they are not limited to a single step in the workflow.
The same tag can be used for:
- purification using metal affinity systems such as Ni-NTA
- detection using anti-His antibodies
- general tracking of expression across different constructs
This means a single construct design can support multiple stages of a workflow without requiring new reagents or assay development for each protein. In early-stage work, where many variants are being tested, this consistency is especially valuable.
Widely Used in Antibody and Nanobody Workflows
In antibody engineering and nanobody discovery, His-tags are commonly included during expression and screening.
Antibody fragments such as Fab or scFv, as well as nanobodies, are often expressed with His-tags so that they can be purified and detected using the same approach across many variants. At this stage, the goal is usually to compare expression levels and identify promising candidates, rather than perform detailed analytical characterisation.
Using a shared tag helps maintain consistency across samples and simplifies comparison between constructs.
Measuring His-Tagged Proteins in Practice
Because His-tags can be recognised by anti-His antibodies, they are often used for quantifying protein expression, not just purification.
In many labs, this is done using:
- ELISA-based assays
- Western blot (semi-quantitative)
- SDS-PAGE for quick visual comparison
- Measurement after purification
SDS-PAGE is widely used to check expression, but it is generally not considered a reliable quantitative method, as band intensity can vary depending on loading, staining, and imaging conditions. Western blot can provide more specificity, but still tends to be semi-quantitative.
ELISA, on the other hand, can provide more robust quantification of His-tagged proteins, although it typically involves multiple steps and longer assay times.
On Amperia™, His-tagged proteins are measured using an anti-His antibody-based assay within a single workflow, allowing protein concentration to be read out more directly from the sample.
This doesn’t replace established methods, but provides a more streamlined option when comparing expression across multiple samples.
Explore more:
- Tagged Protein Quantification Kit | His-Tag
- Application note: Enabling His-tagged Protein Analysis in Expression Workflows
- Resource library
References
- Bornhorst, J.A. & Falke, J.J. (2000). Purification of proteins using polyhistidine affinity tags. Methods in Enzymology.
- Terpe, K. (2003). Overview of tag protein fusions: from molecular and biochemical fundamentals to commercial systems. Applied Microbiology and Biotechnology.
Frequently asked questions
Can I quantify His-tagged proteins without purification?
Yes. Many workflows use direct measurement from crude lysates to compare expression levels before purification.
Why is crude lysate measurement challenging?
Crude samples contain host cell proteins, nucleic acids, and other components that can interfere with detection and increase variability.
Is SDS-PAGE sufficient for quantification?
SDS-PAGE is useful for visual confirmation but is not considered a reliable quantitative method due to variability in staining and loading.
What is the advantage of antibody-based detection?
It provides specificity for the His-tagged protein, allowing target quantification even in complex mixtures.