Prokaryotic ubiquitin-like ThiS fusion enhances the heterologous protein overexpression and aggregation in Escherichia coli

A new metal affinity NCTR25 tag as a better alternative to the His-tag for the expression of recombinant fused proteins

His-tagging is commonly used in fusion protein production, but the His-tag is usually prohibited in medicinal proteins and must be removed. A fragment (NCTR₂₅-tag) truncated from the N-terminus of human copper transporter 1 was tested for feasibility as a replacement for the His-tag in fusion proteins. The NCTR₂₅-tag and His-tag were separately fused to the transthyretin (TTR) protein, and the expression, affinity purification, refolding and stability of the two kinds of fusions were compared. NCTR₂₅ fusion produced a 63% higher yield of the recombinant protein, which was purified by metal affinity chromatography with an efficiency similar to that of His-tagged protein. NCTR₂₅-tag fusion had much less impact on the foldability, kinetic and thermodynamic stability of tetrameric TTR than His-tag fusion. When the tags were individually fused to enhanced green fluorescent protein (EGFP), NCTR₂₅ fusion yielded 29-128% more product than His-EGFP. NCTR₂₅-EGFP could be purified by metal affinity chromatography and showed better foldability than His-EGFP. Furthermore, tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) fusion with the third disulfide loop of TGF-α (TGF3L-TRAIL) fused with the NCTR₂₅-tag retained the stability and superactivity of His-TGF3L-TRAIL. Therefore, the native tag NCTR₂₅-tag is a feasible alternative to the His-tag in medicinal recombinant proteins.

Ubiquitin-like prokaryotic MoaD as a fusion tag for expression of heterologous proteins in Escherichia coli

Background

Eukaryotic ubiquitin and SUMO are frequently used as tags to enhance the fusion protein expression in microbial host. They increase the solubility and stability, and protect the peptides from proteolytic degradation due to their stable and highly conserved structures. Few of prokaryotic ubiquitin-like proteins was used as fusion tags except ThiS, which enhances the fusion expression, however, reduces the solubility and stability of the expressed peptides in E. coli. Hence, we investigated if MoaD, a conserved small sulfur carrier in prokaryotes with the similar structure of ubiquitin, could also be used as fusion tag in heterologous expression in E. coli.

Results

Fusion of MoaD to either end of EGFP enhanced the expression yield of EGFP with a similar efficacy of ThiS. However, the major parts of the fusion proteins were expressed in the aggregated form, which was associated with the retarded folding of EGFP, similar to ThiS fusions. Fusion of MoaD to insulin chain A or B did not boost their expression as efficiently as ThiS tag did, probably due to a less efficient aggregation of products. Interestingly, fusion of MoaD to the murine ribonuclease inhibitor enhanced protein expression by completely protecting the protein from intracellular degradation in contrast to ThiS fusion, which enhanced degradation of this unstable protein when expressed in E. coli.

Conclusions

Prokaryotic ubiquitin-like protein MoaD can act as a fusion tag to promote the fusion expression with varying mechanisms, which enriches the arsenal of fusion tags in the category of insoluble expression.