Increased Protein Encapsulation in Polymersomes with Hydrophobic Membrane Anchoring Peptides in a Scalable Process

Hollow vesicles made from a single or double layer of block-copolymer molecules, called polymersomes, represent an important technological platform for new developments in nano-medicine and nano-biotechnology. A central aspect in creating functional polymersomes is their combination with proteins, especially through encapsulation in the inner cavity of the vesicles. When producing polymersomes by techniques such as film rehydration, significant proportions of the proteins used are trapped in the vesicle lumen, resulting in high encapsulation efficiencies. However, because of the difficulty of scaling up, such methods are limited to laboratory experiments and are not suitable for industrial scale production. Recently, we developed a scalable polymersome production process in stirred-tank reactors, but the statistical encapsulation of proteins resulted in fairly low encapsulation efficiencies of around 0.5%. To increase encapsulation in this process, proteins were genetically fused with hydrophobic membrane anchoring peptides. This resulted in encapsulation efficiencies of up to 25.68%. Since proteins are deposited on the outside and inside of the polymer membrane in this process, two methods for the targeted removal of protein domains by proteolysis with tobacco etch virus protease and intein splicing were evaluated. This study demonstrates the proof-of-principle for production of protein-functionalized polymersomes in a scalable process.

Exploring the Mutations on Improving Oxidative Stability of Tobacco Etch Virus Protease for Tag-removal in Refolding of Two Disulfide-rich Proteins

Tobacco etch virus protease (TEVp) is a useful tool for removing fusion tag, but wild type TEVp shows less oxidative stability, which limits its application under the oxidized redox state to facilitate disulfide bonds formation for refolding disulfide-bonded proteins. Previously, we combined six mutations into the TEVp to generate the TEVp5M for obviously increasing the protein solubility and decreasing the auto-cleavage. In this work, we introduced and combined C19S, C110S and C130S mutations into the TEVp5M to generate seven variants, analyzed protein solubility and the cleavage activity of the constructs in each of three E. coli strains including BL21(DE3), BL21(DE3)pLys, and Rossetta(DE3), and those of the optimized soluble variants in the oxidative cytoplasm of Origami(DE3) under the same induction conditions. The results suggested that desirable protein solubility, cleavage activity and oxidative stability are not combined. Unlike that of the C19S, introduction of the C110S and/or C130S less affected protein solubility but increased tolerance to the oxidative redox state. Use of the TEVp5MC110S/C130S variant, the refolded disulfide-rich bovine enteropeptidase or maize peroxidase was released via cleaving the sequence between the target protein and the cellulose-binding module bound to regenerated amorphous cellulose.

Selection of tobacco etch virus protease variants with enhanced oxidative stability for tag-removal in refolding of two disulfide-rich proteins

Tobacco etch virus protease (TEVp) is a powerful enzymatic reagent for removing fusion tag. In this work, we constructed nine TEVp variants with introducing one to three mutations of C19S, C110S and C130S into the soluble TEVp variant, TEVp5M. Using the C-terminal green fluorescent protein (GFP) variant reporter, all constructs showed different solubility levels among four E. coli strains. The TEVp5M containing the C110S and/or C130S mutations in the hyperoxic strain showed the enhanced the cleavage activity. Addition of dithiothreitol to the cultural medium increased the activity of certain constructs produced in the BL21(DE3), contrary to the added hydrogen peroxide, due to cytoplasmic redox change measured by the redox sensitive GFP construct. The more cysteine residues in the purified TEVp5M were modified specifically than those in the other variants. All purified constructs showed similar specific activities in the presence of 5 mM dithiothreitol. In the buffer containing the compounds to aid disulfide bond formation of the refolded protein, the double mutant TEVp5MC110S/C130S exhibited the highest cleavage efficiency. This variant was efficient for removing the fusion tag after refolding of cellulose-binding module tagged disulfide-rich proteins including bovine enteropeptidase and maize peroxidase absorbed on the regenerated amorphous cellulose.

Evaluation of Structurally Distorted Split GFP Fluorescent Sensors for Cell-Based Detection of Viral Proteolytic Activity

Cell-based assays are essential for virus functional characterization in fundamental and applied research. Overcoming the limitations of virus-labelling strategies while allowing functional assessment of critical viral enzymes, virus-induced cell-based biosensors constitute a powerful approach. Herein, we designed and characterized different cell-based switch-on split GFP sensors reporting viral proteolytic activity and virus infection. Crucial to these sensors is the effective—yet reversible—fluorescence off-state, through protein distortion. For that, single (protein embedment or intein-mediated cyclization) or dual (coiled-coils) distortion schemes prevent split GFP self-assembly, until virus-promoted proteolysis of a cleavable sequence. All strategies showed their applicability in detecting viral proteolysis, although with different efficiencies depending on the protease. While for tobacco etch virus protease the best performing sensor was based on coiled-coils (signal-to-noise ratio, SNR, 97), for adenovirus and lentivirus proteases it was based on GFP11 cyclization (SNR 3.5) or GFP11 embedment distortion (SNR 6.0), respectively. When stably expressed, the sensors allowed live cell biosensing of adenovirus infection, with sensor fluorescence activation 24 h post-infection. The structural distortions herein studied are highly valuable in the development of cellular biosensing platforms. Additionally highlighted, selection of the best performing strategy is highly dependent on the unique properties of each viral protease.

Directed evolution improves the catalytic efficiency of TEV protease

Tobacco etch virus protease (TEV) is one of the most widely-used proteases in biotechnology because of its exquisite sequence-specificity. A limitation, however, is its slow catalytic rate. We developed a generalizable yeast-based platform for directed evolution of protease catalytic properties. Protease activity is read out via proteolytic release of a membrane-anchored transcription factor, and we temporally regulate access to TEV’s cleavage substrate using a photosensory LOV domain. By gradually decreasing light exposure time, we enriched faster variants of TEV over multiple rounds of selection. Our S153N mutant (uTEV1Δ), when incorporated into the calcium integrator FLARE, improved the signal/background ratio by 27-fold, and enabled recording of neuronal activity in culture with 60-second temporal resolution. Given the widespread use of TEV in biotechnology, both our evolved TEV mutants and the directed evolution platform used to generate them, could be beneficial across a wide range of applications.

Towards the application of Tc toxins as a universal protein translocation system

Tc toxins are large bacterial protein complexes that inject cytotoxic enzymes into target cells using a sophisticated syringe-like mechanism. Tc toxins are composed of a membrane translocator and a cocoon that encapsulates a toxic enzyme. The toxic enzyme varies between Tc toxins from different species and is not conserved. Here, we investigated whether the toxic enzyme can be replaced by other small proteins of different origin and properties, namely human Cdc42, herpes simplex virus ICP47, Arabidopsis thaliana iLOV, Escherichia coli DHFR, human Ras-binding domain of CRAF kinase, and tobacco etch virus protease. Using a combination of electron microscopy, X-ray crystallography and in vitro translocation assays, we demonstrate that it is possible to turn Tc toxins into customizable molecular syringes for delivering proteins of interest across membranes. We also infer the guidelines that protein cargos must obey in terms of size, charge, and fold in order to successfully take advantage of this new universal protein translocation system.

A flow extension tethered particle motion assay for single-molecule proteolysis

Regulated proteolysis of signaling proteins under mechanical tension enables cells to communicate with their environment in a variety of developmental and physiologic contexts. The role of force in inducing proteolytic sensitivity has been explored using magnetic tweezers at the single-molecule level with bead-tethered assays, but such efforts have been limited by challenges in ensuring that beads are not restrained by multiple tethers. Here, we describe a multiplexed assay for single-molecule proteolysis that overcomes the multiple-tether problem using a flow extension (FLEX) strategy on a microscope equipped with magnetic tweezers. Particle tracking and computational sorting of flow-induced displacements allows assignment of tethered substrates into singly-captured and multiply-tethered bins, with the fraction of fully mobile, single-tethered substrates depending inversely on the concentration of substrate loaded on the coverslip. Computational exclusion of multiply-tethered beads enables robust assessment of on-target proteolysis by the highly specific tobacco etch virus protease and the more promiscuous metalloprotease ADAM17. This method should be generally applicable to a wide range of proteases and readily extensible to robust evaluation of proteolytic sensitivity as a function of applied magnetic force.