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 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 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.
Wu et al. BMC Microbiology 2011 11:87 doi:10.1186/1471-2180-11-87