Discussion 
Structural features of SrcA We used a reverse genetics approach to define a new secretion chaperone in S.
Typhimurium that is integrated functionally with the T3SS encoded by SPI-2, a system well described for its role in immune subversion and intracellular infection during host colonization.
Consistent with other class I secretion chaperones, SrcA has extensive electronegative charge distributed over the surface of the molecule.
The exact function of this charge distribution is not known, but data from other systems suggests a docking recognition function with other components of the type III apparatus, possibly the T3SS-accociated ATPase.
For instance, electronegative surface residues on the SigE chaperone in the SPI-1-encoded T3SS negatively affect cargo secretion, but not cargo stability [30].
In enteropathogenic E. coli, a surface-exposed electronegative residue in the CesT chaperone (Glu142) likewise contributes to Tir secretion but not Tir binding [16], suggesting a role in either targeting bound cargo to the T3SS or in the secretion process itself.
Interestingly, SrcA lacks 17-amino acids that make up the carboxyl terminus of CesT, which includes Glu142, and yet it still retains effector binding, ATPase binding and effector secretion functionalities.
Thus, it is likely that other surface charged residues of SrcA are involved in these functions or that SrcA targets effector cargo to the secretion apparatus through a mechanism distinct from CesT.
The interface for the SrcA homodimer is extensive and is more in keeping with the structural features of single-effector class IA chaperones (approximately1100-1300 A2) compared to the reduced dimer interface of Spa15, a multi-cargo class IB chaperone from Shigella [31].
Similar to CesT and SicP, the dimer interface of SrcA adopts a parallel configuration when comparing alpha2 helices of opposing subunits.
In contrast, the subunits of Spa15 undergo a significant relative rotation (80degrees) about the alpha2-axis resulting in a different interface.
These features may relate to biological function in the SPI2 T3SS and/or in vetting effector cargo amongst the >30 effectors identified in Salmonella.
We found no evidence of interactions between SrcA and translocon components of the SPI-2 T3SS and so it appears as though SrcA functions specifically in effector translocation events.
