Final Report Summary - BACTERIAL SYRINGES (Protein Translocation Through Bacterial Syringes)
In the project, BACTERIAL SYRINGES, we unraveled the architecture and molecular mechanism of action of Tc toxins. We found that Tc toxins secrete toxic enzymes into host cells using a unique syringe-like injection mechanism. We showed that they are composed of three subunits, TcA, TcB, and TcC. TcA forms the translocation channel, and the TcB-TcC heterodimer functions as a cocoon that shields the toxic enzyme, which is in a semi-unfolded state. The binding of TcB-TcC to TcA triggers the opening of the cocoon and the translocation of the toxic enzyme into the channel. Our studies revealed that this process involves a conformational transition of the β-propeller of TcB from a closed through an unfolded to an open form. The asymmetric interface between the five-fold symmetric TcA and the pseudo-six-fold symmetric TcB β-propeller results in a high-affinity interaction. The translocated polypeptide has to pass through a narrow constriction site inside the cocoon to enter the translocation channel, which contains concrete bands of negative electrostatic potential. Here, we found that the translocation direction of the toxic enzymes of Tc toxins is C-terminus first, though the driving force for translocation is not yet known. A large conformational transition in the membrane-permeating Tc subunit TcA drives membrane insertion. The opening of the TcA shell, which is pH-induced, allows stretched linkers in the TcA prepore to contract, driving the channel into the target cell membrane, where it opens. By comparing the structure and function of insect and human pathogenic toxins from different bacteria, we demonstrated that the architecture and mechanism of action is conserved in the Tc toxin family. Tc toxins interact with glycans at the target cell through their receptor-binding domains.