Structural analysis of a forkhead-associated domain from the type III secretion system protein YscD

The type III secretion system, T3SS, is used by many bacterial pathogens to evade the host immune response. The T3SS utilizes a multiprotein complex, the injectisome, to transport virulence proteins directly into the host cytosol. Injectisome proteins extend from the bacterial cytosol, across the bacterial inner and outer membranes, and into the extracellular space. A crucial injectisome protein from the Yersinia pseudotuberculosis T3SS is YscD. YscD is an inner membrane protein that has both cytoplasmic and periplasmic domains. The X-ray crystallographic structure of the cytoplasmic domain of YscD, YscDc, is presented here. YscDc has a forkhead- associated (FHA) domain fold. Based on structural comparisons between the FHA domain of YscDc and FHA domains of known function, two regions of possible importance were identified within YscDc : loops [beta]3[beta]4 and [beta]4[beta]5. Type III secretion in Y. pseudotuberculosis was abolished when loop [beta]3[beta]4 and [beta]4[beta]5 amino acids were substituted by alanines. The FHA domain of YscD was determined to be crucial for the function of the T3SS in Y. pseudotuberculosis. The FHA domain of YscDc is highly similar in sequence to YscD homologs of other T3SS- encoding bacteria. This domain may be significant for injectisome structural integrity or may be engaged in critical protein-protein interactions with cytosolic T3SS proteins. Understanding the role that this FHA domain plays in the T3SS will provide insight into the role of a protein crucial for injectisome assembly and T3SS functio