Since CPAF was detected in granules in the lumen of inclusions during the early stage of chlamydial intracellular growth, an outer membrane vesicular budding model has been proposed for CPAF secretion into host cell cytosol [62], which may also be suitable for the secretion of cHtrA (Figure 8). Evidence for supporting this hypothesis comes from the observation that cHtrA-laden granules/vesicles that are free of chlamydial organisms were readily #see more randurls[1|1|,|CHEM1|]# detected in the chlamydial inclusions. Although it remains to be determined how exactly cHtrA or CPAF is secreted out of the organisms and into
host cell cytosol, as more effector molecules are identified, more tools will be available for figuring out
the secretion pathways Chlamydia has evolved for exporting virulence factors. Figure 8 A proposed model for C. trachomatis secretion of effectors into host cell cytosol. When an infectious and metabolically inactive elementary body (EB) attaches to an epithelial cell, preexisting effectors such as TARP and CT694 can be injected into Selleck Cilengitide host cell cytosol via a single step type 3 secretion system (T3SS) for facilitating EB invasion. Once the internalized EB is differentiated into a non-infectious but metabolically active reticulate body (RB), newly synthesized chlamydial proteins can be secreted into host cell cytosol via either the single step T3SS (for example, secretion of CT847) or multi-step pathways. The C. trachomatis-secreted proteins (CtSPs) with an N-terminal signal sequence (termed Sec-CtSPs) such as cHtrA & CPAF may be translocated into periplasm via a SecY-dependent pathway while those without any N-terminal signal sequences (Nonsec-CtSPs) may be translocated into the periplasmic space via a novel translocon or a leaking T3SS pathway. The
periplasmically localized CtSPs may exit the chlamydial organisms via an outer membrane vesicle (OMV) budding mechanism. The CtSP-laden vesicles in the inclusion lumen can Org 27569 enter host cell cytosol via vesicle fusion with or passing through the inclusion membrane. That’s why CT621 can be visualized in granules in the lumen of inclusion and its secretion can also be inhibited by C1, a small molecule inhibitor known to target bacterial T3SS. HtrA is a hexamer formed by two trimeric rings staggered on top of each other [46, 47]. It possesses dual functions as both a chaperone and a protease [44]. Whether in eukaryotic ER or prokaryotic periplasmic space, HtrA can transmit the stress signals from unfold proteins into stress responses [48–51]. lt appears that Chlamydia can respond to various stress signals by regulating the expression levels of cHtrA [45]. Although it is still unknown how the periplasmic cHtrA works, these previous observations can at least suggest that cHtrA is functional during chlamydial infection.