The Interactions Between Plasma Lipoprotein (a) And Group A Streptococcus

Proteolytic activity is an important factor for many pathogenic microorganisms to penetrating tissue barriers. Some bacterial pathogens can bind to plasminogen(Plg) and facilitate the activation of Pig to plasmin(Pm) by host Pig activators(PAs). Several invasive pathogens can secrete PAs which can activate the pathogens surface-bound Pig. Therefore. Pig is a key factor in bacterial infectionsThe apolipoprotein(a) of lipoprotein(a)[Lp(a)] has a high similarity to Pig. It has been reported that Lp(a) is a risk factor for cardiovascular disease since Lp(a) can inhibit the binding of Pig to the Pig receptors (such as fibrinogen) or the activction of Pig. We have previously hypothesiaed that Lp(a) might play an important role in preventing infections by inhibiting pathogens from recruiting host plasmin(ogen).It is demonstrated that Group A Streprococcus (GAS) is an important human pathogen and human Pig has a key role in GAS infections, a-enolase (SEN) of GAS surface is one of major Pig receptors. DNA fragment encoding SEN, which was derived from serotype M6 GAS strains (CMCC32175), was cloned into the plasmid vector pASK-IBA37. Full-length (rSEN) or C-terminal lysine residues-truncated variant of SEN (rSENA434-435) were expressed in E. coli BL21. The 6×His-tag was fused at N-terminus of above recombinant proteins and the recombinant proteins were purified by affinity chromatography with TALON metal affinity resins chelated with cobalt.The binding assay with enzyme-linked immunosorbent assay (ELISA), affinity-chromatography and Western blot revealed that rSEN could bind to Lp(a) and a lysine analogε-amino-hexanoic acid (EACA) inhibited the binding. However, rSENA434-435 could not bind to Lp(a). This result implied that C-terminal lysine residues of rSEN and the lysine binding site (LBS) in Lp(a) are responsible for the binding of rSEN and Lp(a). To further confirm the binding mechanism, KIV!0 was cloned, produced in E. coli 21 and purified by the affinity chromatography. The binding assay with ELISA revealed that rKIV10 only bound to rSEN with concentration-dependent manner but not to rSENA434-435. Furthermore, Lp(a) and rKIV10 could inhibit the binding of rSEN to Pig.Lp(a) could bind to serotype M6 GAS and inhibit the pathogen-Plg binding subsequently reduce the amount and activity of serotype M6 GAS-bound plasmin, which may imply an anti-infective role of Lp(a) in vivo. Further study on anti-infective role of Lp(a) using apo(a)-transgene animal model is warranted.In addition, LBS in Lp(a) might be responsible for the Lp(a)-pathogen interaction since EACA could inhibit the binding of serotype M6 GAS to Lp(a).Although the exact receptors on the surface of serotype M6 GAS was not identified our data clearly showed that some suface proteins on GAS cells were responsible for the GAS cell-Lp(a) interaction since protease K-treated bacterial cells could not bind to the Lp(a). On the other hand, further studies are required to identify the exact Lp(a) receptor(s) on serotype M6 GAS surface since GAS produces several suface Plg receptors.To our knowledge, it is first time to report the Lp(a)-GAS interaction. In addition, these results may preliminarily support our hypothesis that Lp(a) may be an important anti-infective molecule.