Different sets of components, consisting of 1 to more than 40 elements, distinguish each pathway. In gram-negative bacteria there are five conserved pathways or mechanisms (designated types I to V) that collectively mediate the processes required for recognition of secretion substrates at the cytoplasmic face of the inner membrane (IM) and active transport of the substrates across the outer membrane (OM) (reviewed in reference 29). The gram-negative cell envelope presents a formidable hydrophobic barrier against protein secretion to the microbial cell surface or into the extracellular matrix. We confirmed that MxiM-MxiD and MxiJ-MxiD interactions occur in vivo in the cell envelope, and we present evidence that together these base elements can form a transmembrane structure which is likely an important intermediary in the process of needle complex assembly. Interestingly, many of the effects were also elicited by the inner-membrane-associated base element, MxiJ. MxiM affects several features of MxiD, including its stability, envelope association, and assembly into homomultimeric structures. Here we identify a role for another base element in Shigella, MxiM, in interactions with the major outer-membrane-associated ring-forming protein, MxiD. Several studies have shown that the initial steps in needle complex assembly require interactions among the base proteins, although specific details of this process remain unknown. The general structure of the needle complex consists of a transenvelope base containing at least three ring-forming proteins (MxiD, MxiJ, and MxiG in Shigella) that is connected to a hollow needle-like extension that projects away from the cell surface. While the effectors, as well as the host responses elicited, differ among type III systems, they all utilize a conserved set of 9 to 11 proteins that together form a bacterial envelope-associated secretory organelle or needle complex. The type III secretion pathway is broadly distributed across many parasitic bacterial genera and serves as a mechanism for delivering effector proteins to eukaryotic cell surface and cytosolic targets.
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