Abstract
The Dot/Icm machine in Legionella pneumophila is one of the most versatile type IV secretion systems (T4SSs), with a remarkable capacity to translocate over 330 different effector proteins across the bacterial envelope into host cells. At least 27 Dot and Icm proteins are required for assembly and function of the system, yet the architecture and activation mechanism remain poorly understood at the molecular level. Here, we deploy cryo-electron microscopy to reveal in-situ structures of the Dot/Icm machine at near-atomic resolution. Importantly, two proteins essential for effector translocation, DotA and IcmX, form a pentameric protochannel at an inactive state. Upon activation, the DotA-IcmX protochannel undergoes extensive rearrangements to form an extended transenvelope passage capable of transporting effector proteins from the bacterial cytoplasm into host cells as revealed by cryo-electron tomography. Collectively, our findings suggest that the DotA-IcmX complex functions as the gatekeeper for effector translocation of the Dot/Icm T4SS.