Yersinia pestis YopM: thrombin binding and overexpression

In previous studies, Yersinia pestis YopM has been shown through mutational analysis to be necessary for virulence in mice and found to have homology with the thrombin-binding domain of the platelet receptor GPIb alpha. In this study, YopM was purified and shown by dot blot and chemical cross-linking tests to bind to human alpha-thrombin. No cross-linked product could be detected when human prothrombin was incubated with YopM. As a functional test of thrombin binding, it was shown that native but not boiled YopM inhibits thrombin-induced aggregation of human platelets. Control tests showed that YopM did not inactivate the platelets themselves, nor was its effect a nonspecific consequence of its very acidic isoelectric point. Microsequencing of YopM revealed an intact N terminus, indicating that functional YopM is not processed at the N terminus or secreted by a mechanism involving a cleavable signal sequence. Further characterization was made of an interesting effect on yopM expression that had been noticed in a previous study. A 1.5-kb HaeIII subclone overexpressed YopM in both Y. pestis and Escherichia coli compared with a larger clone containing the 5.3-kb HindIII-F fragment. To search for a possible regulator of YopM expression, the HindIII-F fragment was sequenced, revealing several open reading frames and three large repeated sequences. Deletional analysis showed that these were not involved in regulation of yopM. The data implicated a DNA structure 5' to yopM in moderating yopM expression.