Spa13 of Shigella flexneri has a dual role: chaperone escort and export gate-activator switch of the type III secretion system.

The type III secretion apparatus (T3SA) is used by numerous Gram-negative pathogens to inject virulence factors into eukaryotic cells. The Shigella flexneri T3SA spans the bacterial envelope and its assembly requires the products of ~20 mxi and spa genes. Despite progress made in understanding how the T3SA is assembled, the role of several predicted soluble components, such as Spa13, remains elusive. Here, we show that the secretion defect of the spa13 mutant is associated with lack of T3SA assembly which is partly due to the instability of the needle component MxiH. In contrast to its Yersinia counterpart, Spa13 is not a secreted protein. We identified a network of interactions between Spa13 and the ATPase Spa47, the C-ring protein Spa33, and the inner-membrane protein Spa40. Moreover, we revealed a Spa13 interaction with the inner-membrane MxiA and showed that overexpression of the large cytoplasmic domain of MxiA in the WT background shuts off secretion. Lastly, we demonstrated that Spa13 interacts with the cleaved form of Spa40 and with the translocator chaperone IpgC, suggesting that Spa13 intervenes during the secretion hierarchy switch process. Collectively, our results support a dual role of Spa13 as a chaperone escort and as an export gate-activator switch.