cDNA for Mo3, a monocyte activation antigen, encodes the human receptor for urokinase plasminogen activator.
We have cloned the cDNA for Mo3, an activation Ag expressed by human monocytes and myelomonocytic cell lines after stimulation by PMA, LPS, muramyl dipeptide, certain cytokines, and cAMP agonists. We have previously shown that Mo3 expression in vivo is associated predominantly with macrophages in inflammatory sites. Mo3 is a highly glycosylated protein of about 50 kDa in monocytes and U-937 cells and is anchored to the plasma membrane by glycosyl-phosphatidylinositol linkage. We purified Mo3 protein by cleavage from the U-937 cell surface with phosphatidylinositol-specific phospholipase C, followed by affinity chromatography using a mAb. An internal peptide sequence was determined and used to design oligonucleotide probes for screening an expression cDNA library. Nucleotide sequencing indicated that the complete coding sequence encodes 335 amino acids, including a predicted signal peptide of 22 residues and a hydrophobic C-terminal portion that is probably cleaved during formation of the GPI linkage. The resulting mature protein of about 290 amino acids is consistent with the 29-kDa molecular mass of deglycosylated Mo3. A Northern blot of RNA from U-937 cells revealed a 1.5-kb band that was induced by PMA treatment. Mo3 cDNA was transfected into Cos cells and surface expression of Mo3 was detected by ELISA using various anti-Mo3 mAb. We performed a computer search of the National Biomedical Research Foundation database and found that Mo3 is identical to the human receptor for the urokinase plasminogen activator (uPA-R). Purified soluble Mo3, as well as anti-Mo3 antibodies, were able to block uPA binding to its receptor on U-937 cells, indicating that Mo3 is indeed uPA-R. The use of these anti-Mo3 antibodies may be helpful in assessing the role of uPA-R in processes such as inflammation and tumor invasion.