The Injectisome, a Complex Nanomachine for Protein Injection into Mammalian Cells

Abstract

Type III protein secretion systems (T3SSs), or injectisomes, are multiprotein nanomachines present in many Gram-negative bacteria that have a sustained long-standing close relationship with a eukaryotic host. These secretion systems have evolved to modulate host cellular functions through the activity of the effector proteins they deliver. To reach their destination, T3SS effectors must cross the multibarrier bacterial envelope and the eukaryotic cell membrane. Passage through the bacterial envelope is mediated by the needle complex, a central component of T3SSs that expands both the inner and outer membranes of Gram-negative bacteria. A set of T3SS secreted proteins, known as translocators, form a channel in the eukaryotic plasma membrane through which the effector proteins are delivered to reach the host cell cytosol. While the effector proteins are tailored to the specific lifestyle of the bacterium that encodes them, the injectisome is conserved among the different T3SSs. The central role of T3SSs in pathogenesis and their high degree of conservation make them a desirable target for the development of antimicrobial therapies against several important bacterial pathogens.

Figure 1

Salmonella Typhimurium SPI-1-encoded type III secretion system. (A) Surface view of the 3D reconstruction of the single-particle cryo-EM map of the needle complex (NC) substructure with the atomic structures of the different NC components docked. OR1, outer ring 1; OR2, outer ring 2; IR1, inner ring 1; IR2, inner ring 2. (B) Central section of an overall cryo-ET structure of the complete injectisome in situ. Of note is the location of IR2 in the cytosolic side of the bacterial envelope. IM, inner membrane; OM, outer membrane. (C) Molecular model of the organization of the injectisome in situ, with available atomic structures fitted into the model. Figure adapted from reference , with permission.

Figure 2

Model of the stepwise assembly of the injectisome. SctRST form a stable complex in the inner membrane, to which SctU is recruited. This complex nucleates the assembly of the IRs integrated by SctJ and SctD, which results in the extraction or “pulling” of the inner membrane components from the bacterial plasma membrane. At the same time, the secretin is independently assembled into the OR and the two structures come together to form the NC base substructure to which SctV is subsequently recruited. Once the NC base is formed, the cytoplasmic sorting platform is recruited to the cytoplasmic side of the NC base and the system starts to function as a type III secretion machine dedicated to the delivery of early substrates, such as the inner rod (SctI) and needle (SctF) subunits, to complete the assembly of the entire injectisome.

Figure 3

Model of the injectisome’s interaction with a eukaryotic host cell. Activation of the injectisome leads to secretion of the translocators, which are deployed on the eukaryotic plasma membrane to form the translocon, which remains in contact with the needle to form a direct conduit between the bacterial and host cell cytosol that serves a passageway for the effector proteins. Figure adapted from reference , with permission.