Insight into the mechanism of IncA, a type III secreted effector protein in Chlamydia trachomatis

Erik Ronzone, Thomas Jefferson University


Members of the family Chlamydiaceae are responsible for a range of pathologies in eukaryotes including respiratory disease and pelvic inflammatory disease in humans. These obligate intracellular pathogens enter host cells and remodel nascent phagosomes into specialized vacuoles known as inclusions, which, through poorly understood processes, separate from the canonical endolysosomal maturation pathway. Bacteria replicate from within these parasitophorous compartments and undergo a complex, multi-stage lifecycle culminating in the release of infectious progeny that begin the cycle anew. Currently, there are no commercially available vaccines targeting Chlamydia or any other bacterial sexually transmitted infection. Antibiotics are effective in treating chlamydiosis if detected early; however, the rise of resistant strains of Chlamydia is making this option less effective every year. Moreover, re-infection and/or reactivation of latent (asymptomatic) infections are common occurrences suggesting alternatives to current therapeutic options are desperately needed. Understanding the mechanisms used by Chlamydia to avoid destruction could aid in the identification of novel targets for a new generation of antimicrobial compounds. To this end, our lab has recently identified the chlamydial protein IncA (Inclusion protein A) as being a potent regulator of endocytic SNARE-mediated fusion. The endocytic SNARE complex regulates the terminal step of phagolysosome formation. Consequently, its blocking by IncA suggests this protein could be playing a key role in the avoidance of innate immune destruction by Chlamydia. Interestingly, there is also data showing IncA is involved in the homotypic fusion of inclusions within the same cell. Thus, IncA may act to inhibit or promote membrane fusion events in a context-dependent manner. We hypothesized that IncA accomplishes its dual roles during infection by functionally and structurally mimicking eukaryotic SNARE proteins, which are also capable of activating and inhibiting membrane fusion events. In support of this hypothesis, we found the dual functions of IncA are encoded in a relatively long 149-residue alpha-helical region of the C-terminal cytoplasmic domain. Biophysical and biochemical analyses, including preliminary X-ray diffraction data, reveal this region assembles into elongated dimers stabilized by coiled-coil motifs. In all, our data suggest that Chlamydia deploys SNARE-like proteins during infection to alter membrane fusion events within the host.

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Recommended Citation

Ronzone, Erik, "Insight into the mechanism of IncA, a type III secreted effector protein in Chlamydia trachomatis" (2014). ProQuest ETD Collection - Thomas Jefferson University. AAI3642838.