Figure 2.

The ordered secretion of the T3SA in Shigella and its main regulators. (a) The early substrates are secreted as soon as expressed to assemble the inner rod and needle. The concomitant secretion of the first switch regulator Spa32 allows secretion of small amounts of middle substrates IpaD and IpaB to yield the inactive T3SA represented in Figure 1a. (b) The contact with host cells activates the T3SA, allowing the secretion of middle substrates and the second switch regulator MxiC. This, in turn, triggers the successive secretion of late substrates A (c) and late substrates B (d). The black arrow represents the secretion by the T3SA. The control of the production of late substrates B by late substrates A is described in Figure 3.