Molecular basis for effector protein recognition by the Dot/Icm type IVB coupling protein complex

Abstract

The Legionella pneumophila Dot/Icm type IVB secretion system (T4BSS) is extremely versatile, translocating ~300 effector proteins into host cells. This specialized secretion system employs the Dot/Icm type IVB coupling protein (T4CP) complex, which includes IcmSW that are known to selectively assist the export of a subclass of effectors. Herein, the crystal structure of a four-subunit T4CP subcomplex including a newly identified adaptor LvgA and the structure of the complex bound to the effector protein VpdB are presented. The structures reveal that LvgA and the other Dot/Icm components form an integral complex which functions as a substrate-recognition module, and the interaction occurred between LvgA and a linear motif in the C-terminus of VpdB is critical for the effector recognition. The same binding interface of LvgA also interacted with a C-terminal region of three additional effectors, SidH, SetA and PieA. Mutational analyses identified a FxxxLxxxK binding motif that is shared by VpdB and SidH, but not by SetA and PieA, showing that LvgA recognizes more than one type of binding motif. Together, this work provides a structural basis for how the Dot/Icm T4CP complex recognizes effectors, and highlights the multiple substrate-binding specificities of its adaptor subunit.