ISOLATION AND CHARACTERIZATION OF HUMAN PROUROKINASE AND ITS MUTANTS ACCUMULATED WITHIN THE YEAST SECRETORY PATHWAY

Human pro-urokinase (pro-UK) and two pro-UK deletion mutants, one lacking the epidermal growth factor(EGF)-like domain, and the other lacking both the EGF-like domain and the kringle domain, were produced in Saccharomyces cerevisiae. This was done using the yeast GAL7 promoter and the prepeptide sequence of a fungal aspartic proteinase, Mucor pusillus rennin (MPR). Although biologically active and heavily glycosylated pro-UKs were secreted into the culture medium, the amounts were extremely small. On the other hand, large amounts of pro-UKs of a single-chain form were accumulated inside the cells, exceeding 3-4% of total cellular proteins. The intracellular pro-UKs were N-glycosylated, probably with a single core carbohydrate unit, and amino acid sequencing of their N termini revealed that the secretion signal of MPR was correctly processed. Biologically active pro-UKs were recovered in high yields by means of solubilization with 4.5 M guanidine. HCl and subsequent dialysis for refolding. The refolded yeast pro-UK was indistinguishable from human kidney-derived pro-UK in terms of specific enzymatic activity and its secondary structure, as determined by circular dichroism spectroscopy.