Staphylococcal superantigen‐like protein 10 binds to phosphatidylserine and apoptotic cells

Abstract

Staphylococcal superantigen‐like proteins (SSLs) are a family of exoproteins that have structural similarities to staphylococcal superantigens. Although SSLs do not have superantigenic activity, some of them have been reported to bind to host immune related molecules and they have been implicated in immune evasion by S. aureus. In this study, we showed that SSL10 is capable of binding to phospholipids. SSL10 bound to phosphatidylserine (PS) containing liposome, but not to phosphatidylcholine liposome. SSL10, but not SSL7, bound to PS containing liposome, suggesting that SSL10 specifically binds to PS. Analysis of PS binding ability among recombinant truncated SSL10 fragments revealed that the β‐barrel in the N‐terminal oligonucleotide/oligosaccharide‐binding (OB)‐fold domain contributes to PS binding capacity. Fluorescein isothiocyanate labeled OB‐fold of SSL10 stained hydrogen peroxide treated Jurkat cells. Annexin V is widely utilized for detection of apoptosis. Unlike annexin V, the OB‐fold domain of SSL10 also bound to apoptotic cells in the presence of EDTA, suggesting that the OB‐fold of SSL10 recognizes PS and apoptotic cells in a Ca2+ independent manner. These findings suggest SSL10 and its derived peptides may be a novel detection tool for apoptotic cells.

Staphylococcal Superantigen-like Protein 10 (SSL10) Inhibits Blood Coagulation by Binding to Prothrombin and Factor Xa via Their γ-Carboxyglutamic Acid (Gla) Domain

“…5). We previously reported that SSL10 is able to bind phosphatidylserine (17). It is possible that SSL10 bridges prothrombin with phospholipid in the absence of Ca 2ϩ , but the present data exclude this possibility.…”

“…The competitive effect of Ca 2ϩ on SSL10-prothrombin interaction was limited at 1.25 mM of Ca 2ϩ ; therefore, the interaction between SSL10 and prothrombin will be significant in physiological conditions. We previously reported that SSL10 binds to PS (17). Recombinant GST-annexin V, which interacts with PS, did not bind to SSL10 (data not shown), suggesting that the PS-binding feature does not account for the binding of SSL10 to prothrombin.…”

“…The physiological functions of SSL10 have been revealed (i.e. SSL10 binds to the Fc region of human IgG and inhibits the complement activation through the classical pathway (13) and IgG-Fc␥ receptor interaction to inhibit phagocytosis (14), inhibits CXCL12-induced leukemia cell migration by blocking CXCR4 (12), and binds to phosphatidylserine and recognizes apoptotic cells (17)). Taken together, SSL10 seems to possess pleiotropic effects on host immunity and homeostasis.…”

“…The sensorgrams are representative of three experiments. containing liposomes (phosphatidylcholine/PS/cholesterol ϭ 5.5:1.5:3 molar ratio, 25 nmol of lipid/l) were prepared by a dry film method as described previously (17).…”

Bacterial superantigens and superantigen-like toxins

“…SSL10 binds 1:1 with immunoglobulin of the IgG1 subclass with an affinity of 300 nM and a specificity that is restricted to human and primates [376,377]. Binding of SSL10 to fibrinogen, fibronectin [377], CXCR4 [378], phosphatidylserine [379], prothrombin, and factor Xa [380] has also been observed. SSL10 secreted by S. aureus into its immediate environment binds to IgG1, the most abundant of the four human IgG subclasses found in the blood which also has the highest affinity for multiple FcγRs [390].…”