Human Clara cell 10-kDa protein is the counterpart of rabbit uteroglobin

Human Clara cell 10-kDa protein has been suggested to be a counterpart of rabbit uteroglobin, an immunomodulatory and antiinflammatory secretory protein. Since this human protein is not readily available in substantial quantity for detailed characterization of its biochemical, biological, and pharmacological properties, we sought to express it in Escherichia coli in order to study its structure-function relationship. However, bacterial overproduction of homodimeric proteins with interchain disulfide bonds, such as Clara cell 10-kDa protein, was thought to be impossible until we achieved expression of native uteroglobin (Miele, L., CordellaMiele, E., and Mukherjee, A. B. (1990) J. Biol. Chem. 265, 6427-6435). Here, we report high level production of recombinant native dimeric human Clara cell 10-kDa protein in E. coli and its characterization. Recombinant human Clara cell 10-kDa protein forms its disulfide bonds within the bacterial cytoplasm. The purified protein possesses two of the most important activities characteristic of uteroglobin: (i) it is an excellent substrate of transglutaminase, and (ii) it is a potent inhibitor of porcine pancreatic and, more importantly, human synovial phospholipase A₂. These results demonstrate that human Clara cell 10-kDa protein and rabbit uteroglobin have very similar biochemical properties. Our data suggest that this protein may possess immunomodulatory and antiinflammatory activities of potential physiological and pharmacological importance.