MxiM and MxiJ, base elements of the Mxi-Spa type III secretion system of Shigella, interact with and stabilize the MxiD secretin in the cell envelope

The type III secretion pathway is broadly distributed across many parasitic bacterial genera and serves as a mechanism for delivering effector protein s to eukaryotic cell surface and cytosolic targets. While the effectors, as well as the host responses elicited, differ among type III systems, they a ll utilize a conserved set of 9 to 11 proteins that together form a bacteri al envelope-associated secretory organelle or needle complex. The general s tructure of the needle complex consists of a transenvelope base containing at least three ring-forming proteins (MAD, MxiJ, and MAG in Shigella) that is connected to a hollow needle-like extension that projects away from the cell surface. Several studies have shown that the initial steps in needle c omplex assembly require interactions among the base proteins, although spec ific details of this process remain unknown. Here we identify a role for an other base element in Shigella, MxiM, in interactions with the major outer- membrane-associated ring-forming protein, MxiD. MxiM affects several featur es of MxiD, including its stability, envelope association, and assembly int o homomultimeric structures. Interestingly, many of the effects were also e licited by the inner-membrane-associated base element, MxiJ. We confirmed t hat 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 tran smembrane structure which is likely an important intermediary in the proces s of needle complex assembly.