A novel protein purification strategy mediated by the combination of CipA and Ssp DnaB intein

Self-assembly systems to troubleshoot metabolic engineering challenges

Enzyme self-assembly is a technology in which enzyme units can aggregate into ordered macromolecules, assisted by scaffolds. In metabolic engineering, self-assembly strategies have been explored for aggregating multiple enzymes in the same pathway to improve sequential catalytic efficiency, which in turn enables high-level production. The performance of the scaffolds is critical to the formation of an efficient and stable assembly system. This review comprehensively analyzes these scaffolds by exploring how they assemble, and it illustrates how to apply self-assembly strategies for different modules in metabolic engineering. Functional modifications to scaffolds will further promote efficient strategies for production.

In Vitro Biosynthesis of Isobutyraldehyde Through the Establishment of a One-Step Self-Assembly-Based Immobilization Strategy

The in vitro biosynthesis of high-value compounds has become popular and attractive. The convenient and simple strategy of enzyme immobilization has been significant for continuous and efficient in vitro biosynthesis. On the basis of that, this work established a one-step self-assembly-based immobilization strategy to efficiently biosynthesize isobutyraldehyde in vitro. Isobutyraldehyde is a crucial precursor for the synthesis of foods and spices. The established CipA scaffold-based strategy can express and immobilize enzymes at the same time, and purification requires only one centrifugation step. Structural simulations indicated that this scaffold-dependent self-assembly did not influence the structure or catalytic mechanisms of the isobutyraldehyde production-related enzymes leucine dehydrogenase (LeuDH) and ketoisovalerate decarboxylase (Kivd). Immobilized LeuDH and Kivd displayed a higher conversion capacity and thermal stability than the free enzymes. Batch conversion experiments demonstrated that the recovered immobilized LeuDH and Kivd have similar conversion capacities to the enzymes used in the first round of reaction. The continuous production of isobutyraldehyde was achieved by filling the immobilized enzymes into the column of a constructed device. This study not only expands the application range of self-assembly systems but also provides guidance for the in vitro production of value-added compounds.

mem-iLID, a fast and economic protein purification method

Protein purification is the vital basis to study the function, structure and interaction of proteins. Widely used methods are affinity chromatography-based purifications, which require different chromatography columns and harsh conditions, such as acidic pH and/or adding imidazole or high salt concentration, to elute and collect the purified proteins. Here we established an easy and fast purification method for soluble proteins under mild conditions, based on the light-induced protein dimerization system improved light-induced dimer (iLID), which regulates protein binding and release with light. We utilize the biological membrane, which can be easily separated by centrifugation, as the port to anchor the target proteins. In Xenopus laevis oocyte and Escherichia coli, the blue light-sensitive part of iLID, AsLOV2-SsrA, was targeted to the plasma membrane by different membrane anchors. The other part of iLID, SspB, was fused with the protein of interest (POI) and expressed in the cytosol. The SspB-POI can be captured to the membrane fraction through light-induced binding to AsLOV2-SsrA and then released purely to fresh buffer in the dark after simple centrifugation and washing. This method, named mem-iLID, is very flexible in scale and economic. We demonstrate the quickly obtained yield of two pure and fully functional enzymes: a DNA polymerase and a light-activated adenylyl cyclase. Furthermore, we also designed a new SspB mutant for better dissociation and less interference with the POI, which could potentially facilitate other optogenetic manipulations of protein-protein interaction.

Novel intein-based self-cleaving affinity tag for recombinant protein production in Escherichia coli

We evaluated several intein-based self-cleaving affinity tags for expression and single-step affinity chromatography purification of recombinant proteins produced in Escherichia coli. We used human growth hormone (hGH) as target protein that contains two internal disulfide bridges and an N-terminal phenylalanine. Use of N-terminal thiol-induced Sce VMA1 intein affinity tag resulted in purified hGH deficient in disulfide bonds. Inteins with self-cleavage inducible by pH and/or temperature shift were analyzed. N-terminal Ssp DnaX intein affinity tag resulted in a completely cleaved cytosolic protein, whereas N-terminal Ssp DnaB intein affinity tag resulted in a cytosolic fusion protein incapable of releasing hGH. Periplasmic expression of target protein was analyzed using an N-terminal signal peptide and C-terminal Ssp DnaX pH-inducible self-cleaving affinity tag. The fusion protein was properly expressed in pH 8 buffered culture medium. Fusion of a periplasmic signal peptide to the N-terminus of the POI allowed secretion to the periplasmic region and presence of the natural N-terminal amino acid of the POI following cleavage. Periplasmic expression of hGH fused to this novel C-terminal DnaX intein-based self-cleaving affinity tag made possible expression and purification of hGH protein containing disulfide bonds and free of extra amino acids.