Protein Not Expressing? You Might Have Chosen the Wrong Tag!-Antibody Detection Service Topic-Tekbiotech-Yeast and Phage Display CRO, Expert in Nano-body and Antibody Drug Development

Protein Not Expressing? You Might Have Chosen the Wrong Tag!

Tags act like ID cards for proteins, used for subsequent detection, tracking, and purification.

I. Introduction to Classic Tags

His Tag (Polyhistidine Tag)

1.Principle: Nickel ions in the chromatography column bind to the imidazole side chains of histidine (His). Finally, a high concentration of imidazole is used to "push" the His tag off.

2.Features: Small tag (typically 6xHis), inexpensive, most versatile, does not affect protein structure or function.

3.Applications: The most commonly used tag, bar none! It is the first choice when the tag needs to be removed for subsequent functional experiments. It has relatively low immunogenicity and can be used directly for immunization. Expression levels are generally high.

4.Disadvantages: Susceptible to contamination by endogenous cellular proteins, resulting in potentially lower purity. Does not help with the solubility of certain proteins. When the target protein expression level is low, it can easily be washed away during purification steps, leading to low yield.

GST Tag (Glutathione S-Transferase Tag)

1.Principle: The GST protein is an enzyme whose native function is to catalyze the reaction of glutathione. The beads in the chromatography column are immobilized with glutathione. Finally, a high concentration of glutathione is used to "compete" the GST tag off.

2.Features: Large tag (~26 kDa), itself an enzyme.

3.Applications: High purification efficiency, strong specificity, and purity is generally better than the His tag. Can be used for pull-down assays to study protein-protein interactions. Can sometimes help the target protein fold correctly.

4.Disadvantages: The large size may affect the structure and function of the target protein. Removal of the tag is relatively cumbersome (commonly using thrombin or PreScission protease).

MBP Tag (Maltose-Binding Protein Tag)

1.Principle: MBP's natural role in bacteria is to bind and transport maltose. The beads in the chromatography column are made of amylose, which is the binding partner for MBP. Finally, using a high concentration of maltose, MBP will elute with the free maltose.

2.Features: Very large tag (~40 kDa), helps protein fold correctly.

3.Applications: When your protein is expressed entirely as inclusion bodies, this is the tag to choose! Its solubilizing effect is top-notch. MBP can generate a higher proportion of active, soluble protein. MBP itself is an affinity tag and can be used directly for certain functional experiments.

4.Disadvantages: The large size significantly impacts protein function and must be removed. Purification cost is relatively higher than other tags, though cheaper than the SUMO tag.

SUMO Tag (Small Ubiquitin-like Modifier Tag)

1.Principle: The SUMO tag itself is typically not used directly for purification! The common setup is: His tag + SUMO tag + Your Protein of Interest. It relies on the preceding His tag to purify the full fusion protein. Subsequently, a SUMO protease cleaves off the SUMO tag, and the mixture is passed over a nickel column again.

2.Features: Small tag (~11 kDa), precise cleavage leaving no residues.

3.Applications: Leaves virtually no extra amino acids, which is critical for structural studies! Excellent solubilizing ability! Its solubilizing effect is comparable to, or even better than, MBP. When you are struggling to cleave off a His tag with proteases, try the SUMO system!

4.Disadvantages: Requires co-expression of SUMO protease or purchase of additional protease (expensive). Purification requires two steps (first His tag purification, then cleavage, then another purification to remove the tag).

Table 1: Purification Principles

Table 2: Lazy Person's Guide - How to Choose?

II. Pitfall Avoidance Guide & Common Problems

1. No Band:

a. Possibly not expressed: Try lowering the induction temperature (e.g., 16°C overnight induction), reducing IPTG concentration, or shortening the expression time.

b. Expression vector construction issues: Missing promoter, frameshift mutation, high GC content, presence of rare codons.

c. Incorrect host strain selection (secretory expression requires a signal peptide).

2. Excessive Contaminating Bands:Use a tag with higher affinity.High molecular weight proteins are prone to degradation; small molecular weight proteins are not only easily degraded but can also be lost.Ensure the imidazole concentration gradient for elution is appropriate.

3. All Proteins are in Inclusion Bodies? :Besides changing the tag (MBP or SUMO), you can also try adding glucose during induction, lowering the temperature, changing the strain, co-expressing chaperones, or performing in vitro denaturation and renaturation.

TekBiotech has established a comprehensive protein expression and purification platform. We are dedicated to providing high-quality protein expression and purification development services to scientists worldwide, including one-stop solutions such as expression system selection (E. coli, yeast, insect, or mammalian cells), vector construction and design, expression condition optimization, downstream purification, and quality validation. Aligned with the evolving needs of life sciences, we develop innovative biological tools and therapeutic proteins, providing robust support for our clients' research projects.

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