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

Dimerization/oligomerization of the extracellular domain of the GLP-1 receptor and the negative cooperativity in its ligand binding revealed by the improved NanoBiT

The glucagon-like peptide-1 receptor (GLP-1R), a family B G-protein coupled receptor (GPCR), regulates the insulin secretion following stimulation by ligands. The transmembrane domain (TM) mediates GLP-1R homodimerization, which modulates its ligand binding and signaling. We investigated the possible involvement of the N-terminal extracellular domain (NTD) in dimerization/oligomerization and dimer-associated ligand binding by NanoLuc Binary Technology (NanoBiT). With improved NanoBiT detection using a decreasing substrate concentration, the negative cooperativity of ligand binding to the NTD was confirmed by accelerated dissociation and Scatchard analysis. The dimerization/oligomerization of the isolated NTD was observed by NanoBiT and validated by analytical ultracentrifugation, deriving the comparable dimerization affinity (~10⁵  M^-1 ). The NTD was also involved in the dimerization/oligomerization of the full-length GLP-1R with mutated TM4 at the cellular level. In an analysis of the parameters of the NTD binding, the K_d for the probe GLP-1 (7-36, A8G) was similar (6-8 μM) in both the 1:1 binding model and the receptor dimerization model. Compared with GLP-1 and dulaglutide, exenatide showed two-site binding with K_i values of 1.4 pM and 8.7 nM. Our study indicates the involvement of NTD in the GLP-1R dimerization/oligomerization and suggests that further investigations on the role in other family B GPCRs are needed.

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.

Enhanced Production of Soluble Recombinant Proteins With an In Situ-Removable Fusion Partner in a Cell-Free Synthesis System

High-yield production of soluble protein is a common concern in diverse fields of biotechnology. In this study, a strategy of using an engineered nucleotide sequence of ubiquitin for enhancing the production of soluble proteins in a cell-free synthesis system is presented. When examined for a series of proteins that otherwise were poorly expressed, N-terminal fusion with ubiquitin significantly increased both the expression levels and solubility of the translational products. The effect of ubiquitin fusion was also markedly augmented by engineering the nucleotide sequence of ubiquitin, leading to several fold enhancements in soluble production of target proteins. Recombinant proteins were produced with their native amino acid sequences through in situ removal of ubiquitin during cell-free synthesis reactions in the presence of a deubiquitinase. The presented strategy could be employed as a facile route to prepare soluble proteins required for various applications.

Enhancing recombinant protein solubility with ubiquitin-like small archeal modifying protein fusion partners

A variety of protein expression tags with different biochemical properties has been used to enhance the yield and solubility of recombinant proteins. Ubiquitin, SUMO (small ubiquitin-like modifier) and prokaryotic ubiquitin like MoaD (molybdopterin synthase, small subunit) fusion tags are getting more popular because of their small size. In this paper we report on the use of ubiquitin-like small archaeal modifier proteins (SAMPs) as fusion tags since they proved to increase expression yield, stability and solubility in our experiments. Equally important, they did not co-purify with proteins of the expression host and there was information that their specific JAB1/MPN/Mov34 metalloenzyme (JAMM) protease can recognize the C-terminal VSGG sequence when SAMPs fused, either branched or linearly to target proteins, and cleave it specifically. SAMPs and JAMM proteases from Haloferax volcanii, Thermoplasma acidophilum, Methanococcoides burtonii and Nitrosopumilus maritimus were selected, cloned, expressed heterologously in Escherichia coli and tested as fusion tags and cleaving proteases, respectively. Investigated SAMPs enhanced protein expression and solubility on a wide scale. T. acidophilum SAMPs Ta0895 and Ta01019 were the best performing tags and their effect was comparable to the widely used maltose binding protein (MBP) and N utilization substance protein A (NusA) tags. Moreover, H. volcanii SAMP Hvo_2619 contribution was mediocre, whereas M. burtonii Mbur_1415 could not be expressed. Out of four investigated JAMM proteases, only Hvo_2505 could cleave fusion tags. Interestingly, it was found active not only on its own partner substrate Hvo_2619, but it also cleaved off Ta0895.