Expression, purification, and biological activity evaluation of cathepsin L in mammalian cells

Improved recombinant expression of soluble cathepsin B and L in Escherichia coli

Cysteine cathepsins such as cathepsin B and L play an important role in numerous diseases like acute pancreatitis or SARS-CoV-2 and therefore have high potential for the development of new therapeutics. To be able to screen for potent and selective inhibitors sufficient amounts of protein are required. Here, we present an easy and efficient protocol for the recombinant expression of soluble and active murine cathepsin B and L. For this, we used the strain E. coli SHuffle® T7 Express which is capable of forming disulfide bridges in the cytoplasm. The enzymes were purified by immobilized nickel ion-affinity chromatography. Using different constructs and media, expression levels were significantly improved and expression yields of 80 ± 2 mg L-1 for procathepsin B, which is 16-fold better than previously reported expression yields for procathepsin B, and 37 ± 2 mg L-1 for procathepsin L, were achieved. After activation with dithiothreitol at slightly acidic pH, in vitro kinetic parameters of both cathepsins were determined using the commonly used synthetic substrates Arg-Arg-AMC or Phe-Arg-AMC. Moreover, to investigate the impact of the short C-terminal propeptide of procathepsin B, it was deleted by site-directed mutagenesis, the shortened target protein was expressed and purified, activated in vitro, and its activity was similar to the variant bearing this C-terminal propeptide. KEY POINTS: • Recombinant gene expression of cathepsin B and L in E. coli SHuffle® T7 Express • Soluble cathepsin expression with high expression yields • Investigation of the short C-terminal propeptide of cathepsin B.

Cathepsin B- and L-like Protease Activities Are Induced During Developmental Barley Leaf Senescence

Leaf senescence is a developmental process allowing nutrient remobilization to sink organs. Previously cysteine proteases have been found to be highly expressed during leaf senescence in different plant species. Using biochemical and immunoblotting approaches, we characterized developmental senescence of barley (Hordeum vulgare L. var. 'GemCraft') leaves collected from 0 to 6 weeks after the onset of flowering. A decrease in total protein and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) large subunits occurred in parallel with an increase in proteolytic activity measured using the fluorogenic substrates Z-RR-AMC, Z-FR-AMC, and casein labeled with fluorescein isothiocyanate (casein-FITC). Aminopeptidase activity detected with R-AMC peaked at week 3 and then decreased, reaching a low level by week 6. Maximal proteolytic activity with Z-FR-AMC and Z-RR-AMC was detected from pH 4.0 to pH 5.5 and pH 6.5 to pH 7.4, respectively, while two pH optima (pH 3.6 to pH 4.5 and pH 6.5 to pH 7.4) were found for casein-FITC. Compound E-64, an irreversible cysteine protease inhibitor, and CAA0225, a selective cathepsin L inhibitor, effectively inhibited proteolytic activity with IC50 values in the nanomolar range. CA-074, a selective cathepsin B inhibitor, was less potent under the same experimental conditions, with IC50 in the micromolar range. Inhibition by leupeptin and phenylmethylsulfonyl fluoride (PMSF) was weak, and pepstatin A, an inhibitor of aspartic acid proteases, had no effect at the concentrations studied (up to 0.2 mM). Maximal proteolytic activity with the aminopeptidase substrate R-AMC was detected from pH 7.0 to pH 8.0. The pH profile of DCG-04 (a biotinylated activity probe derived from E-64) binding corresponded to that found with Z-FR-AMC, suggesting that the major active proteases are related to cathepsins B and L. Moreover, immunoblotting detected increased levels of barley SAG12 orthologs and aleurain, confirming a possible role of these enzymes in senescing leaves.