Interaction between Yersinia pestis YopM protein and human alpha-thrombin.

YopM, a 41.5 kDa virulence protein of Yersinia pestis is believed to have an anti-inflammatory role in bubonic plague. It has been shown previously that YopM binds human alpha-thrombin but not prothrombin and inhibits thrombin-induced platelet aggregation in vitro. In the present studies we carried out crosslinking reactions between purified YopM and alpha-thrombin or its blocked form FPR-alpha-thrombin in the presence of various competitors to identify where on thrombin YopM binds. We found that thrombin cleaves YopM at the C-terminus, indicating that this part of YopM must interact with thrombin's catalytic site. Hirudin, a 65 amino acid natural thrombin inhibitor, prevents both the YopM degradation and the formation of a ca. 75 kDa crosslinking complex between YopM and alpha-thrombin. A similar effect is observed when hirugen, a short peptide corresponding to hirudin's C-terminus (amino acids 58-64), is used as a synthetic thrombin inhibitor. A 15 bp long specific oligonucleotide known to block alpha-thrombin successfully competes with YopM for the thrombin-binding site, whereas a control, scrambled sequence aptamer does not. As these competitors block a fibrinogen binding site (also called anion binding exosite I), our crosslinking data indicate that YopM binds not only to the active site of alpha-thrombin but also to the abeI.