Inhibition of human endogenous retrovirus-K10 protease in cell-free and cell-based assays

A full-length and C-terminally truncated version of human endogenous retrovirus (HERV)-K10 protease were expressed in Escherichia coli and purified to homogeneity. Both versions of the protease efficiently processed HERV-K10 Gag polyprotein substrate. HERV-K10 Gag was also cleaved by human immunodeficiency virus, type 1 (HIV-1) protease, although at different sites. To identify compounds that could inhibit protein processing dependent on the HERV-K10 protease, a series of cyclic ureas that had previously been shown to inhibit HIV-1 protease was tested. Several symmetric bisamides acted as very potent inhibitors of both the truncated and full-length form of HERV-K10 protease, in subnanomolar or nanomolar range, respectively. One of the cyclic ureas, SD146, can inhibit the processing of in vitro translated HERV-K10 Gag polyprotein substrate by HERV-K10 protease. In addition, in virus-like particles isolated from the teratocarcinoma cell line NCCIT, there is significant accumulation of Gag and Gag-Pol precursors upon treatment with SD146, suggesting the compound efficiently blocks HERV-K Gag processing in cells. This is the first report of an inhibitor able to block cell-associated processing of Gag polypeptides of an endogenous retrovirus.

Cathepsin B/cystatin C complex levels in sera from patients with lung and colorectal cancer

A sandwich-type ELISA has been developed for quantification of the complex between the cysteine proteinase cathepsin B (CB) and its reversible tight-binding inhibitor cystatin C (CC) in normal and pathological sera. The assay is based on a combination of catching Ab (3E1), raised against CB, and a horseradish peroxidase-labelled detection Ab (1A2), raised against CC. Only the CB/CC complex is able to evoke a signal in this assay. The detection limit of the assay was 15.5 nM and the working range between 31.3-200 nM. The within and between-run coefficients of variance (CV) varied from 4.7% to 9.4% and 11% to 12.8%, respectively, demonstrating satisfactory reproducibility of the method. The concentration of the CB/CC complex was determined in sera from 90 healthy controls, 32 patients with non-cancerous lung diseases, 148 patients with lung and 32 patients with colorectal cancer. The CB/CC complex was significantly less abundant in sera of patients bearing malignant lung tumours than in those with non-cancerous lung diseases or healthy controls (p<0.001). In colorectal cancer sera its level was significantly lower in advanced stages C and D than in early Dukes' stages A and B (p=0.02). Our results show that the increased levels of CB in malignant sera are not impaired effectively by CC and support the hypothesis of hindered inhibitory capability during cancer progression.

Autocatalytic processing of recombinant human procathepsin B is a bimolecular process

Cathepsin B and other lysosomal cysteine proteinases are synthesized as inactive zymogens, which are converted to their mature forms by other proteases or by autocatalytic processing. Procathepsin B autoactivation was shown in vitro at pH 4.5 to be a bimolecular process with K(s) and k(cat) values of 2.1+/-0.9 microM and 0.12+/-0.02 s(-1)6.0. However, in the presence of 0.5 microg/ml of dextran sulfate, relatively rapid processing is observed even at pH 6.5 (t(1/2) approximately 90 min), suggesting that glycosaminoglycans are involved in in vivo processing of lysosomal cysteine proteases.

Crystal structure of the wild-type human procathepsin B at 2.5 A resolution reveals the native active site of a papain-like cysteine protease zymogen

The structure of the wild-type human procathepsin B has been refined to a crystallographic R-value of 0.18 and R-free of 0.23 exploiting the data obtained from new crystals that diffract beyond 2.5 A resolution. The structure confirms two previously presented, lower-resolution structures. The structure of the propeptide chain folds on the surface of the enzyme domains and blocks access of substrate to the already formed active site. Abundant solvent molecules fill the cavities between the propeptide and the enzyme part of the molecule. The propeptide structure is compared with a substrate model in the S2, S1, S1' and S2' binding sites. In this crystal form the cathepsin B occluding loop residues adopt yet another conformation. The structures show that the occluding loop region between the residues Cys108 and Cys119 behaves quite independently from the rest of the structure and easily adapts to changes in environment. The variety of the observed conformations of the occluding loop is in agreement with other data showing that the loop is responsible for limiting cathepsin B activity to that of a carboxydipeptidase. The region before Cys108 is essentially the same as in the mature structure, whereas the region from Cys119 to Thr125 is raised compared to the mature form by the propeptide squeezed between it and the enzyme domains, surface. The structure strongly suggests that processing of procathepsin B during its autoactivation is not unimolecular.

Crystal structures of human procathepsin B at 3.2 and 3.3 Angstroms resolution reveal an interaction motif between a papain-like cysteine protease and its propeptide

A wild-type human procathepsin B was expressed, crystallized in two crystal forms and its crystal structure determined at 3.2 and 3.3 Angstroms resolution. The structure reveals that the propeptide folds on the cathepsin B surface, shielding the enzyme active site from exposure to solvent. The structure of the enzymatically active domains is virtually identical to that of the native enzyme [Musil et al. (1991) EMBO J. 10, 2321-2330]: the main difference is that the occluding loop residues are lifted above the body of the mature enzyme, supporting the propeptide structure.

The preparation of catalytically active human cathepsin B from its precursor expressed in Escherichia coli in the form of inclusion bodies

A cDNA clone encoding human procathepsin B was expressed at a high level in Escherichia coli using a T7 polymerase expression system, resulting in the formation of insoluble cytoplasmic protein aggregates (inclusion bodies). The recombinant product was solubilized and renatured by refolding and reoxidation. The proenzyme was subsequently processed with pepsin to produce an enzymically active enzyme. By systematic variation of the parameters influencing the folding, formation of disulphide bonds, and processing of procathepsin B to the catalytically active mature form, a simple renaturation procedure was designed, allowing the production of about 3 mg purified active cathepsin B/l E. coli culture broth. The enzyme obtained in this way consists of a single chain and, as a consequence of pepsin treatment, possesses a three-amino-acid extension at its N-terminus. The enzyme has similar kinetic and immunological properties to native human cathepsin B.