Isolation and characterization of recombinant human cathepsin E expressed in Chinese hamster ovary cells

The cDNA sequence encoding precursor forms of human cathepsin E (CE), an intracellular aspartic proteinase, was expressed in Chinese hamster ovary cells using an SV40 promotor-driven expression vector. By immunoelectron microscopic studies using an anti-human CE antibody and by Percoll density gradient fractionation, the expressed CE was found to be in two different intracellular fractions; the cytosolic compartment and the vacuolar system. The CEs in both the cytosolic and the vacuolar fractions were highly purified by a simple method involving Percoll density gradient fractionation, chromatography on concanavalin A-Sepharose, Mono Q, and TSK-GelG2000SW, and termed s-CE and v-CE, respectively. The v-CE was further separated into a major (v-CE1) and a minor (v-CE2) form by Mono Q chromatography. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting revealed that the s-CE and v-CE1 consists of two polypeptides of 90 and 84 kDa, whereas v-CE2 is composed of 84- and 82-kDa polypeptides. The NH₂- terminal amino acid sequence analyses showed that the 90- and 84-kDa proteins from both s-CE and v-CE started with Ser³ and Lys³⁰ of the sequence of human gastric CE predicted from its cDNA sequence, respectively, and that the NH₂ terminus of the 82-kDa protein of v-CE2 is the Ile³⁷. Upon acid treatment at pH 3.5 and 37 °C for 5 min, the 90- and 84-kDa forms are rapidly converted to the 82-kDa form, indicating that the 90-, 84- and 82- kDa proteins are the pro-CE, the intermediate form, and the mature CE, respectively. All the forms of CE are N-glycosylated with high-mannose-type oligosaccharides. The catalytic properties of s-CE and v-CE are comparable to those of natural human CE. These results suggest that the recombinant CE is initially synthesized on membrane-bound ribosomes as a N-glycosylated preproenzyme and that, after cleavage of the signal segment, the 90-kDa proenzyme is proteolytically processed to the intermediate (84 kDa) and mature (82 kDa) forms by the transport system.