α-Enolase Resides on the Cell Surface of Mycoplasma fermentans and Binds Plasminogen

Mycoplasmas (class Mollicutes) are wall-less prokaryotes widely distributed in nature. Most mycoplasmas are parasites, exhibiting strict host and tissue specificities, and almost all adhere to the surfaces of eukaryotic cells (18, 19). The adherence of these organisms to host cells is an initial and essential step in tissue colonization and the subsequent development of disease, and adherence-deficient mutants are avirulent (19). The human pathogen Mycoplasma fermentans was isolated from the urogenital tract several decades ago. The interest in this organism has recently increased because of its possible role in the pathogenesis of rheumatoid arthritis (11). Plasminogen (Plg) is a 92-kDa eukaryotic glycoprotein activated in vivo into the broad-spectrum serine protease plasmin that degrades fibrin and noncollagenous proteins. Plasmin activity results in several physiological and pathophysiological processes, such as fibrinolysis, pericellular proteolysis, tissue penetration of cancer cells, and neuronal cell death (17, 20). Many eukaryotic cells express surface structures that interact with Plg, and specific receptors have been described previously (17). Lysine or lysine analogs such as ɛ-aminocaproic acid (ɛACA) mimic COOH-terminal lysine and thereby inhibit the interaction (20). Recently, it has become evident that Plg is also capable of interacting with a vast number of both gram-positive (14, 15) and gram-negative (21) pathogenic bacteria. M. fermentans is a typical extracellular microorganism able to adhere to human epithelial cells. Recently, we have shown that M. fermentans binds Plg (23) and that Plg binding markedly increases the adherence of M. fermentans to HeLa cells (24). Furthermore, in the presence of the urokinase-type Plg activator, M. fermentans cells were detected within host cells, suggesting that the ability to bind and activate Plg into plasmin enables M. fermentans to invade host cells (24). Plasmin generated on various bacteria has been shown to degrade mammalian extracellular matrices and, in a few instances, to enhance bacterial metastasis in vitro through reconstituted basement membrane or epithelial cell monolayers (13). Bacteria expressing Plg receptors on their cell surfaces enhance the activation of Plg by prokaryotic or eukaryotic Plg activators. In essence, Plg receptors and activators turn bacteria into proteolytic organisms by using a host-derived system. In gram-negative bacteria, the filamentous surface appendages fimbriae and flagella form a major group of Plg receptors (21). In gram-positive bacteria, surface-bound enzyme molecules as well as M protein-related structures have been identified as Plg receptors (1). The glycolytic enzymes α-enolase and glyceraldehyde-3-phosphate dehydrogenase are the nonclassical cell surface Plg binding proteins of Streptococcus pneumoniae (2, 3, 16). Plg binds to streptococcal α-enolase through the interaction of the amino-terminal lysine binding domain of Plg with both the carboxy-terminal lysines and the internal motif FYDKERKVYD on the surface-displayed α-enolase (4, 7). In the present study, we have isolated, identified, and described a membrane-bound α-enolase as a key M. fermentans surface protein that mediates Plg binding.