EDUCATION/TRAINING:

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Molecular pathogenesis of Yersinia pestis, the causative agent of Plague.

Yersinia pestis, the etiologic agent of bubonic plague, possesses a large (~75 kb) plasmid called pCD1 which encodes many coordinately regulated virulence functions.  Carriage of pCD1 is necessary for virulence of Y. pestis and is responsible for a virulence-related regulatory response termed the Low Calcium Response (LCR). The LCR is typified by increased expression of several virulence-related operons and the secretion of ~12 virulence proteins including the V antigen (LcrV) and a group of proteins that have been collectively termed Yops (Yersinia outer proteins) at 37°C in the absence of Ca²⁺.  LcrV and the Yops are secreted by a type III (also called contact-dependent) protein secretion system, also encoded on pCD1.  This mechanism also is responsible for the post secretion targeting of several Yops into eukaryotic cells, where they disrupt cell signalling and cytoskeletal functions.

LCR expression and secretion have been linked to two different signals; one is the concentration of Ca²⁺ in the environment and the other is contact with eukaryotic cells in tissue culture infection models.  Removal of Ca²⁺ from the growth medium or contact with eukaryotic cells triggers the LCR.

A popular theory is that a repressor is secreted at the same time as LcrV and the Yops and loss of this putative repressor results in increased LCR transcription; however this putative repressor is yet to be rigorously identified.  Nevertheless, selection of two types of mutants altered in the LCR has allowed the identification of positive and negative regulatory factors that are thought to function by modulating secretion.  LcrV (encoded by pCD1) is a virulence protein proposed to suppress early stages of the host immune response.  LcrV also is required to achieve full induction of the LCR in vitro: lcrV strains of Y. pestis only weakly induce transcription of the stimulon and weakly secrete Yops.  Genetic analysis of lcrV mutants suggested LcrV blocks negative regulation of the LCR rather than acting directly to positively regulate the LCR.  The LCR is regulated negatively by the gene products of lcrE and lcrG encoded by pCD1.  lcrE and lcrG strains express LCR operons and secrete Yops and LcrV constitutively at 37°C regardless of Ca²⁺ concentrations.  Previous studies have proposed that LcrE, a surface-localized protein that is released in the absence of Ca²⁺, may function as a Ca²⁺ sensor and that LcrE and LcrG, a primarily cytoplasmic protein, function together to block secretion.  lcrE and lcrG mutations are epistatic over lcrV mutations, suggesting that lcrV, lcrE, and lcrG function in the same pathway and implying that LcrV may interact with LcrE or LcrG.

My recent studies have shown that LcrV interacts stably with LcrG, and have led to the hypothesis that LcrV functions to unblock secretion by removal of an LcrG-mediated block of the secretion machinery (The LcrG-titration model).  My current work is focused on understanding the roles of LcrG and LcrV in the control of Yop secretion and translocation into eukaryotic cells.  Currently I am using genetic and biochemical tools to characterize the interaction of LcrG and LcrV and to determine their significance in controlling Yop secretion.

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