Engineering of TEV protease variants with redesigned substrate specificity.

Due to their ability to catalytically cleave proteins and peptides, proteases present unique opportunities for the use in industrial, biotechnological, and therapeutic applications. Engineered proteases with redesigned substrate specificities have the potential to expand the scope of practical applications of this enzyme class. We here apply a combinatorial protease engineering-based screening method that links proteolytic activity to the solubility and correct folding of a fluorescent reporter protein to redesign the substrate specificity of tobacco etch virus (TEV) protease. The target substrate EKLVFQA differs at three out of seven positions from the TEV consensus substrate sequence. Flow cytometric sorting of a semi-rational TEV protease library, consisting of focused mutations of the substrate binding pockets as well as random mutations throughout the enzyme, led to the enrichment of a set of protease variants that recognize and cleave the novel target substrate.

A fluorometric lead(II) assay by using a DNA dendrimer as a carrier for the immobilization of the signal probe.

A DNA dendrimer was constructed by combination of rolling circle amplification (RCA) and hybridization chain reaction (HCR). A fluorescence resonance energy transfer (FRET) pair consisting of 6-carboxyfluorescein (FAM) and 5-carboxytetramethylrhodamine (TAMRA) was then created as a signaling probe for the ultrasensitive detection of Pb2+. Firstly, the DNAzyme released by Pb2+-induced recycling amplification as a primer induces RCA to open two different hairpins for generating repeated "Y" structures. Next, two other hairpins (labeled with FAM and TAMRA, respectively) are opened by the "Y" structures to trigger the HCR. As a result, a DNA dendrimer is generated. It is high loading with FRET pair and also promotes FRET pair mutually close to produce a remarkable FRET signal. Hence, ultrasensitive detection of Pb2+ is accomplished by measurement of the ratio of the yellow fluorescence of TAMRA (peaking at 588 nm) and the green fluorescence of FAM (at 525 nm). The method works in the 0.001 to 10 nM Pb2+ concentration range and has a 0.3 pM detection limit. It was applied to determination of intracellular Pb2+ with convincing performance. Graphical abstractSchematic representation of novel DNA dendrimer produced by the combination of rolling circle amplification (RCA) and hybridization chain reaction (HCR) for immobilization of the fluorescence resonance energy transfer (FRET) pair as signal probe for sensitive determination of Pb2+.

Directed evolution of the 3C protease from coxsackievirus using a novel fluorescence-assisted intracellular method.

Proteases are crucial for regulating biological processes in organisms through hydrolysis of peptide bonds. Recombinant proteases have moreover become important tools in biotechnological, and biomedical research and as therapeutics. We have developed a label-free high-throughput method for quantitative assessment of proteolytic activity in Escherichia coli. The screening method is based on co-expression of a protease of interest and a reporter complex. This reporter consists of an aggregation-prone peptide fused to a fluorescent protein via a linker that contains the corresponding substrate sequence. Cleavage of the substrate rescues the fluorescent protein from aggregation, resulting in increased fluorescence that correlates to proteolytic activity, which can be monitored using flow cytometry. In one round of flow-cytometric cell sorting, we isolated an efficiently cleaved tobacco etch virus (TEV) substrate from a 1:100 000 background of non-cleavable sequences, with around 6000-fold enrichment. We then engineered the 3C protease from coxsackievirus B3 (CVB3 3Cpro) towards improved proteolytic activity on the substrate LEVLFQ↓GP. We isolated highly proteolytic active variants from a randomly mutated CVB3 3Cpro library with up to 4-fold increase in activity. The method enables simultaneous measurement of proteolytic activity and protease expression levels and can therefore be applied for protease substrate profiling, as well as directed evolution of proteases.

A new sensitive and quantitative chemiluminescent assay to monitor intracellular xanthine oxidase activity for rapid screening of inhibitors in living endothelial cells.

Xanthine oxidase (XO) is an important enzyme, expressed at high levels in the vasculature in endothelial cells, that catalyzes the hydroxylation of hypoxanthine to xanthine and xanthine to uric acid. Excessive production of uric acid results in hyperuricemia linked to gout and cardiovascular diseases. Testing inhibition of XO is important for detection of potentially effective drugs or natural products that could be used to treat diseases caused by increased XO activity. In the present study, for the first time, we developed an in vitro chemiluminescent bioassay to determine XO activity in living endothelial cells and the IC50 value of oxypurinol, the active metabolite of the inhibitor drug allopurinol. Intracellular XO activity was measured in less than 20 min with a luminol/catalyst-based chemiluminescence assay able to measure XO with a limit of 0.4 µU/mL. Oxypurinol addition to 5 × 103 cells (ranging from 5.0 to 0.0 µM) caused a linear decrease in XO activity, with an IC50 of 1.0 ± 0.5 µM. The detection system developed was low-cost, rapid, reproducible, and easily miniaturizable so suitable to be used on small quantities of cells.