TSGIT: An N- and C-terminal tandem tag system for purification of native and intein-mediated ligation-ready proteins

Essential factors, advanced strategies, challenges, and approaches involved for efficient expression of recombinant proteins in Escherichia coli

Producing recombinant proteins is a major accomplishment of biotechnology in the past century. Heterologous hosts, either eukaryotic or prokaryotic, are used for the production of these proteins. The utilization of microbial host systems continues to dominate as the most efficient and affordable method for biotherapeutics and food industry productions. Hence, it is crucial to analyze the limitations and advantages of microbial hosts to enhance the efficient production of recombinant proteins on a large scale. E. coli is widely used as a host for the production of recombinant proteins. Researchers have identified certain obstacles with this host, and given the growing demand for recombinant protein production, there is an immediate requirement to enhance this host. The following review discusses the elements contributing to the manifestation of recombinant protein. Subsequently, it sheds light on innovative approaches aimed at improving the expression of recombinant protein. Lastly, it delves into the obstacles and optimization methods associated with translation, mentioning both cis-optimization and trans-optimization, producing soluble recombinant protein, and engineering the metal ion transportation. In this context, a comprehensive description of the distinct features will be provided, and this knowledge could potentially enhance the expression of recombinant proteins in E. coli.

Proteomic applications in identifying protein-protein interactions

There are many things that can be used to characterize a protein. Size, isoelectric point, hydrophobicity, structure (primary to quaternary), and subcellular location are just a few parameters that are used. The most important feature of a protein, however, is its function. While there are many experiments that can indicate a protein's role, identifying the molecules it interacts with is probably the most definitive way of determining its function. Owing to technology limitations, protein interactions have historically been identified on a one molecule per experiment basis. The advent of high throughput multiplexed proteomic technologies in the 1990s, however, made identifying hundreds and thousands of proteins interactions within single experiments feasible. These proteomic technologies have dramatically increased the rate at which protein-protein interactions (PPIs) are discovered. While the improvement in mass spectrometry technology was an early driving force in the rapid pace of identifying PPIs, advances in sample preparation and chromatography have recently been propelling the field. In this chapter, we will discuss the importance of identifying PPIs and describe current state-of-the-art technologies that demonstrate what is currently possible in this important area of biological research.

Factors involved in heterologous expression of proteins in E. coli host

The production of recombinant proteins is one of the most significant achievements of biotechnology in the last century. These proteins are produced in the eukaryotic or prokaryotic heterologous hosts. By increasing the omics data especially related to different heterologous hosts as well as the presence of new amenable genetic engineering tools, we can artificially engineer heterologous hosts to produce recombinant proteins in sufficient quantities. Numerous recombinant proteins have been produced and applied in various industries, and the global recombinant proteins market size is expected to be cast to reach USD 2.4 billion by 2027. Therefore, identifying the weakness and strengths of heterologous hosts is critical to optimize the large-scale biosynthesis of recombinant proteins. E. coli is one of the popular hosts to produce recombinant proteins. Scientists reported some bottlenecks in this host, and due to the increasing demand for the production of recombinant proteins, there is an urgent need to improve this host. In this review, we first provide general information about the E. coli host and compare it with other hosts. In the next step, we describe the factors involved in the expression of the recombinant proteins in E. coli. Successful expression of recombinant proteins in E. coli requires a complete elucidation of these factors. Here, the characteristics of each factor will be fully described, and this information can help to improve the heterologous expression of recombinant proteins in E. coli.

Simplified cloning and isolation of peptides from "sandwiched" SUMO-peptide-intein fusion proteins

BACKGROUND

Some peptides are targets for degradation when heterologously expressed as fusion proteins in E. coli, which can limit yields after isolation and purification. We recently reported that peptide degradation may be prevented by production of a "sandwiched" SUMO-peptide-intein (SPI) fusion protein, which protects the target peptide sequence from truncation and improves yield. This initial system required cloning with two commercially available vectors. It used an N-terminal polyhistidine tagged small ubiquitin-like modifier (SUMO) protein and a C-terminal engineered Mycobacterium xenopii DNA Gyrase A intein with an inserted chitin binding domain (CBD) to create "sandwiched" fusion proteins of the form: His₆-SUMO-peptide-intein-CBD. However, the major drawback of this previously reported fusion protein "sandwich" approach is the increased time and number of steps required to complete the cloning and isolation procedures, relative to the simple procedures to produce recombinant peptides in E. coli from a single (non-"sandwiched") fusion protein system.

RESULTS

In this work we generate the plasmid pSPIH6, which improves upon the previous system by encoding both the SUMO and intein proteins and allows facile construction of a SPI protein in a single cloning step. Additionally, the Mxe GyrA intein encoded in pSPIH6 contains a C-terminal polyhistidine tag, resulting in SPI fusion proteins of the form: His₆-SUMO-peptide-intein-CBD-His₆. The dual polyhistidine tags greatly simplify isolation procedures compared to the original SPI system, which we have here demonstrated with two linear bacteriocin peptides: leucocin A and lactococcin A. The yields obtained for both peptides after purification were also improved compared to the previous SPI system as a result of this streamlined protocol.

CONCLUSIONS

This modified SPI system and its simplified cloning and purification procedures described here may be generally useful as a heterologous E. coli expression system to obtain pure peptides in high yield, especially when degradation of the target peptide is an issue.

Mechanism of human Lig1 regulation by PCNA in Okazaki fragment sealing

During lagging strand synthesis, DNA Ligase 1 (Lig1) cooperates with the sliding clamp PCNA to seal the nicks between Okazaki fragments generated by Pol δ and Flap endonuclease 1 (FEN1). We present several cryo-EM structures combined with functional assays, showing that human Lig1 recruits PCNA to nicked DNA using two PCNA-interacting motifs (PIPs) located at its disordered N-terminus (PIP_N-term) and DNA binding domain (PIP_DBD). Once Lig1 and PCNA assemble as two-stack rings encircling DNA, PIP_N-term is released from PCNA and only PIP_DBD is required for ligation to facilitate the substrate handoff from FEN1. Consistently, we observed that PCNA forms a defined complex with FEN1 and nicked DNA, and it recruits Lig1 to an unoccupied monomer creating a toolbelt that drives the transfer of DNA to Lig1. Collectively, our results provide a structural model on how PCNA regulates FEN1 and Lig1 during Okazaki fragments maturation.

Semi-Biosynthetic Production of Surface-Binding Adhesive Antimicrobial Peptides Using Intein-Mediated Protein Ligation

Microbial infections remain a global health concern, calling for the urgent need to implement effective prevention measures. Antimicrobial peptides (AMPs) have been extensively studied as potential antimicrobial coating agents. However, an efficient and economical method for AMP production is lacking. Here, we synthesized the direct coating adhesive AMP, NKC-DOPA₅, composed of NKC, a potent AMP, and repeats of the adhesive amino acid 3,4-dihydroxyphenylalanine (DOPA) via an intein-mediated protein ligation strategy. NKC was expressed as a soluble fusion protein His-NKC-GyrA (HNG) in Escherichia coli, comprising an N-terminal 6× His-tag and a C-terminal Mxe GyrA intein. The HNG protein was efficiently produced in a 500-L fermenter, with a titer of 1.63 g/L. The NKC-thioester was released from the purified HNG fusion protein by thiol attack and subsequently ligated with chemically synthesized Cys-DOPA₅. The ligated peptide His-NKC-Cys-DOPA₅ was obtained at a yield of 88.7%. The purified His-NKC-Cys-DOPA₅ possessed surface-binding and antimicrobial properties identical to those of the peptide obtained via solid-phase peptide synthesis. His-NKC-Cys-DOPA₅ can be applied as a practical and functional antimicrobial coating to various materials, such as medical devices and home appliances.

Mechanistic investigation of human maturation of Okazaki fragments reveals slow kinetics

The final steps of lagging strand synthesis induce maturation of Okazaki fragments via removal of the RNA primers and ligation. Iterative cycles between Polymerase δ (Polδ) and Flap endonuclease-1 (FEN1) remove the primer, with an intermediary nick structure generated for each cycle. Here, we show that human Polδ is inefficient in releasing the nick product from FEN1, resulting in non-processive and remarkably slow RNA removal. Ligase 1 (Lig1) can release the nick from FEN1 and actively drive the reaction toward ligation. These mechanisms are coordinated by PCNA, which encircles DNA, and dynamically recruits Polδ, FEN1, and Lig1 to compete for their substrates. Our findings call for investigating additional pathways that may accelerate RNA removal in human cells, such as RNA pre-removal by RNase Hs, which, as demonstrated herein, enhances the maturation rate ~10-fold. They also suggest that FEN1 may attenuate the various activities of Polδ during DNA repair and recombination.