The roles of LcrG and LcrV in secretion control of Yops in Yersinia pestis

Yersinia pestis, the causative agent of plague, combats the host immune response by injecting proteins called Yops "Yersinia outer proteins" into the eukaryotic host cells using a type III secretion system "T3SS". The activity of the type T3SS is regulated by several proteins including YscF, YopN, TyeA, SycN, YscB, LcrG, and LcrV. YscF forms the extracellular needle complex of the T3SS and is involved in secretion regulation and in sensing environmental signals for secretion activation. YopN, TyeA, SycN, and YscB form a complex in the cytosol of Y. pestis that blocks the secretion of Yops when conditions do not favor secretion. LcrG is an additional negative regulator of secretion that blocks the secretion of Yops when conditions do not favor secretion. LcrV is a positive regulator that binds to LcrG and negates LcrG's blocking activity allowing the secretion of Yops to occur when conditions favor secretion. The interaction between LcrG and LcrV is crucial for the activation of Yops secretion. The goal of this work is to understand how the interaction between LcrG and LcrV regulates the secretion of Yops. First the region in LcrV required for interaction with LcrG was identified. This information was used to construct an LcrV mutant that constitutively blocked Yops secretion when expressed in Y. pestis. This mutant was then studied for its ability to activate Yops secretion in a strain lacking YopN. The information obtained suggests that LcrG's role in secretion blockage maybe the modulation of YopN's function. In addition, genetic evidence suggests that secretion control through the interaction of LcrG and LcrV occurs in the cytoplasm and a second location. Recently, LcrV was demonstrated to form a structure at the tip of the YscF needle. The observation of LcrV at the tip of the needle combined with a genetic analysis of LcrV-LcrG function suggests that the interaction between LcrG and LcrV maybe occurring at the tip of the YscF needle.