A high molecular weight protein on human U937 cells interacts specifically with the uPAR-prourokinase complex in a reaction that can be fixed covalently by tissue transglutaminase (Behrendt, N. et al. (1993) FEBS Lett. 336, 394-396). We have now isolated approx. 1 ug of this membrane protein and subjected it to various analyses based on mass spectrometry. Tryptic digestion was found to yield a peptide map distinct from all known proteins in the data base of derived composite peptide masses. Partial sequence information was obtained by nano-electrospray tandem mass spectrometry of tryptic peptides. This analysis confirmed that the protein is distinct from any known human sequence, but it was found to be closely related to a recently cloned murine cDNA hypothesized to code for an integral membrane protein, the function of which is unknown. Based on this information we have isolated a human cDNA clone that spans approx. 90 % of the complete sequence, as judged by comparison with the murine cDNA. Our preliminary studies suggest that this protein serves to bind various extracellular proteins in events that may play a role in adhesion, as well as in presenting substrate proteins for degradation mediated by the PA system. This type of function might have important implications for the role of this proteolytic system in invasive processes.