The Yersinia pestis virulence effector YopM binds to a key regulatory site on the human pyrin death domain to inhibit inflammasome activation and effector-triggered immunity

Bacteria in the genus Yersinia use a type III secretion system to inject effectors into host cells to disrupt multiple signaling processes. Two effectors, YopE and YopT, disrupt host signaling by inactivating RhoA GTPases. Inactivation of RhoA triggers assembly of the pyrin inflammasome in infected phagocytes leading to a protective immune response. A third Yersinia effector, YopM, is an essential virulence factor that counteracts YopE- or YopT-triggered immunity by inactivating pyrin. YopM has a leucine-rich repeat (LRR) domain and hijacks the host kinases PRK and RSK to inactivate pyrin by phosphorylation. Previously, the LRR domain of Yersinia pestis YopM was implicated in binding to an N-terminal region of human pyrin, which includes the eponymous pyrin domain (PYD), a member of the death domain fold family. In addition, the PYD was shown to be required for phosphorylation of pyrin by YopM. However, the mechanism underlying this interaction is not fully understood. Using a bacterial two-hybrid assay, protein biochemistry and x-ray crystallography, we show that the concave surface of the YopM LRR domain binds to the PYD. We identified critical residues in YopM and the PYD involved in their interaction. Furthermore, we show that YopM codon substitution variants defective for PYD binding fail to inhibit the pyrin inflammasome in human monocytes infected with Yersinia. These results give key insights into how Y. pestis YopM targets and inhibits the human pyrin inflammasome through binding a key regulatory site on the PYD. In addition, we found that PYD binding is dispensable for YopM to inhibit the murine pyrin inflammasome, suggesting this effector uses a distinct mechanism to target mouse pyrin.