Abstract
Type III secretion systems (T3SS) are dedicated to targeting anti-host effector proteins into the cytosol of the host cell to promote bacterial infection. Delivery of the effectors requires three specific translocator proteins, of which the hydrophilic translocator, LcrV, is located at the tip of the T3SS needle and is believed to facilitate insertion of the two hydrophobic translocators into the host cell membrane. Here we used Yersinia as a model to study the role of LcrV in T3SS mediated intracellular effector targeting. Intriguingly, we identified N-terminal lcrV mutants that, similar to the wild-type protein, efficiently promoted expression, secretion and intracellular levels of Yop effectors, yet they were impaired in their ability to inhibit phagocytosis by J774 cells. In line with this, the YopH mediated dephosphorylation of Focal Adhesion Kinase early after infection was compromised when compared to the wild type strain. This suggests that the mutants are unable to promote efficient delivery of effectors to their molecular targets inside the host cell upon host cell contact. The significance of this was borne out by the fact that the mutants were highly attenuated for virulence in the systemic mouse infection model. Our study provides both novel and significant findings that establish a role for LcrV in early targeting of effectors in the host cell.
Highlights
Type Three Secretion systems (T3SS) were discovered more than 25 years ago when it was established that Yersinia spp. targeted virulence proteins into eukaryotic cells through a delivery mechanism that required close bacteria-host cell contact (Rosqvist et al, 1991, 1994; Sory and Cornelis, 1994)
We show for the first time that the very N-terminal part of LcrV has a specific role in early targeting of effectors to promote rapid blockage of phagocytosis in Yersinia
We have characterized two lcrV mutants that in contrast to the lcrV mutant, expressed and secreted wild type levels of Yop effectors, but were unique in that they no longer could mediate efficient targeting of YopH to the focal adhesion sites
Summary
Type Three Secretion systems (T3SS) were discovered more than 25 years ago when it was established that Yersinia spp. targeted virulence proteins (effectors) into eukaryotic cells through a delivery mechanism that required close bacteria-host cell contact (Rosqvist et al, 1991, 1994; Sory and Cornelis, 1994). The Role of LcrV in Effector Targeting cells (Finlay et al, 1988; Sasakawa et al, 1988, 1989; Elsinghorst et al, 1989), in other pathogens such as Yersinia spp. and Pseudomonas spp., the T3SS act to block uptake (Rosqvist et al, 1988; Frithz-Lindsten et al, 1997). The injection model is widely used to explain the mode of function of the T3SS It dictates that protein translocation occurs in one step from the bacterial cytosol to the target-cell cytoplasm through a conduit created by the basal body and a needle-like hollow tube that is extended by a tip complex that forms a pore in the host cell membrane (Galán and Wolf-Watz, 2006). A recent study showed that effectors exogenously added to the bacterial surface of both Yersinia and Salmonella could be translocated in a T3SS-dependent manner, suggesting an alternative mechanism to the one-step injection model (Akopyan et al, 2011)
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