Structural insights into the roles of the IcmS–IcmW complex in the type IVb secretion system of Legionella pneumophila

Significance Type IVb secretion systems are crucial for the pathogenesis ofLegionella pneumophila andCoxiella burnetii . IcmS and IcmW are known as adaptor proteins for theLegionella T4BSS and regulate the translocation of many virulent effector proteins into host cells. However, the mechanism by which IcmS–IcmW recognizes its substrates and facilitates their delivery is unclear. We performed structural and biochemical analyses of the IcmS–IcmW complex. We found that the IcmS–IcmW complex harbors a distinct structure and binds its cognate effectors via an extended hydrophobic surface. IcmS–IcmW also functions as an inseparable partner of DotL to assemble a unique type IV coupling protein complex. Our results provide mechanistic insights into the dual roles of the IcmS–IcmW complex in T4BSSs.