A type VI secretion system effector delivery mechanism dependent on PAAR and a chaperone-co-chaperone complex.

The type VI secretion system (T6SS) is used by many Gram-negative bacteria as a molecular weapon to modulate neighbouring bacterial and eukaryotic cells, thereby affecting the dynamics of community structure in multiple species environments. The T6SS injects its inner-needle Hcp tube, the sharpening tip complex consisting of VgrG and PAAR, and toxic effectors into neighbouring cells. Its functions are largely determined by the activities of its delivered effectors. Six mechanisms of effector delivery have been described: two mediated by the inner tube and the others mediated by the VgrG and PAAR tip complex. Here, we report an additional effector delivery mechanism that relies on interaction with a chaperone complex and a PAAR protein as a carrier. The Pseudomonas aeruginosa PAO1 TOX-REase-5 domain-containing effector TseT directly interacts with PAAR4 and the chaperone TecT for delivery, and an immunity protein, TsiT, for protection from its toxicity. TecT forms a complex with its co-chaperone, co-TecT, which is disrupted by the carboxy-terminal tail of PAAR4. In addition, we delineate a complex, multilayered competitive process that dictates effector trafficking. PAAR delivery provides an additional tool for engineering cargo protein translocation.