Chlamydiae are obligate intracellular bacterial pathogens with medical and veterinary
importance. Their principal virulence mechanism is their ability to invade different
cell types both in vivo and in vitro. Whilst the human species, Chlamydia trachomatis
and Chlamydia pneumoniea, are limited to infection of mucosal epithelium, the
veterinary species of Chlamydia caviae, Chlamydia psittaci, Chlamydia abortus,
amongst others, tend to establish a more systemic infection causing organ failure,
reproductive defects, and spontaneous abortion.
The cell invasion mechanism of Chlamydia trachomatis is the best studied,
implicating the chlamydial protein TarP and, specifically, the N-terminal domain that
is phosphorylated by the Src-family of non-receptor tyrosine kinases. The veterinary
species also possess their respective TarP homologues, but they differ from the
Chlamydia trachomatis version by lacking the N-terminal phosphodomain. To
resolve this issue, and begin to elucidate the alternative invasion mechanism
exhibited by the non-trachomatis species, a bioinformatics-based search of potential
signalling motif within the highly conserved C-terminal half of TarP was performed. A
candidate Vinculin Binding Domain (VBD) and Focal Adhesion Kinase (FAK)-binding
motif (LD) were indentified, which were determined to function in recruiting vinculin
and FAK to the sites of invasion, respectively. In addition, the VBD positively
influenced the actin-recruiting function of the LD motif. Structural modelling revealed
the formation of the hydrophobic interface in VBD/vinculin or LD/FAK interaction.
Finally, knockout mouse embryonic fibroblasts confirmed a role for vinculin and FAK
in invasion. Altogether, the data indicate that TarP-mediated exploitation of focal adhesion
signalling represents a pan-chlamydial invasion mechanism. Interestingly, this
signalling mechanism was found to have a post-invasion function. Chlamydia
infection led to changes in focal adhesions and adherens junctions, which may be
important in apoptosis resistance, counteracting the exfoliation cues provided by
neighbouring cells, or cell motility and dissemination of infection within the host.