Systematic deletion analysis and characterization of the type III secretion apparatus component YscD of Yersinia pestis

YscD is an essential component of the plasmid pCD1-encoded type III secretion system (T3SS) of Yersinia pestis. YscD has a single transmembrane (TM) domain that connects a small N-terminal cytoplasmic region (residues 1 to 121) to a larger periplasmic region (residues 143 to 419). Deletion analyses demonstrated that both the N-terminal cytoplasmic region and the C-terminal periplasmic region are essential for YscD function. Additional studies demonstrated that a predicted cytoplasmic forkhead-associated (FHA) domain of YscD is also required for function; in contrast, a predicted periplasmic phospholipid binding (BON) domain and a putative periplasmic "ring-building" domain of YscD could be deleted with no significant effect on the T3S process. Although deletion of the putative "ring-building" domain did not disrupt T3S activity per se, the calcium-dependent regulation of the T3S apparatus was affected. The extreme C-terminal region of YscD (residues 354 to 419) was essential for secretion activity and had a strong dominant negative effect on the T3S process when exported to the periplasm of the wild type parent strain. Finally, replacement of the YscD TM domain with a TM domain of dissimilar sequence had no effect on the T3S process, indicating that the TM domain has no sequence-specific function in the assembly or function of the T3SS.