THE ROLE OF AN ENOLASE-RELATED MOLECULE IN PLASMINOGEN BINDING TO CELLS

The a isoform of enolase is a candidate plasminogen receptor on U937 m onocytoid cells [Miles, L. A., Dahlberg, C. L., Plescia, J., Felez, J. , Kato, K. & Plow, E. F. (1991) Biochemistry 30, 1682-1691]. In the pr esent study, an enolase-related molecule was detected on the surfaces of peripheral blood monocytes and neutrophils by fluorescence-activate d cell sorting. A mRNA transcript encoding a unique membrane form of a n enolase-related molecule was not detected by Northern-blotting and p rimer-extension analyses, consistent with the cell-surface protein bei ng authentic alpha-enolase. Both the alpha and beta isoforms of purifi ed enolase, bound plasminogen with an affinity similar to that of the cell surface. Moreover, immunopurified alpha-enolase enhanced plasmino gen activation by tissue plasminogen activator and blocked the binding of plasminogen to alpha(2)-antiplasmin, mimicking functions arising f rom the association of plasminogen with cells. The interaction of the enolase isoforms with plasminogen was dependent upon recognition of th e C-terminal lysyl residue of the enolases by the lysine-binding sites of plasminogen, as the interaction was blocked by (a) peptides with C -terminal lysine residues and (b) an antibody to the C-terminal aspect of enolase. A monoclonal antibody was developed, characterized and ut ilized to quantify the enolase molecules present on the surface of U93 7 cells. A substantial number of molecules, 1.8X10(6)/ cell, was prese nt, accounting for approximately 10% of the plasminogen-binding capaci ty of these cells. These studies clearly establish the role of enolase as a cell-surface plasminogen-binding site with profibrinolytic funct ions.