The tip complex of the "Yersinia enterocolitica" injectisome

Type III secretion systems are used by many pathogenic bacteria to deliver effector proteins into host cells. The secretion machinery, also known as the injectisome, is composed of a basal body anchored in the bacterial membranes and an external needle. Delivery of the effectors across the host cell membrane requires, in addition, a set of proteins called the translocators that are exported by the injectisome itself. The interaction between the needle of the injectisome and the translocators is currently not understood in detail. In Yersinia, the translocators are a group of three proteins called YopB, YopD and LcrV. The two hydrophobic proteins YopB and YopD form a pore in the target cell membrane, while LcrV assists the assembly of the pore. LcrV is known since the mid-fifties as the major protective antigen against plague. Using Scanning Transmission Electron Microscopy (STEM) we showed that LcrV forms a distinct structure at the tip of the injectisome needle, the tip complex. The tip complex could be specifically labelled with antibodies directed against LcrV and could be crosslinked to the YscF needle, demonstrating that LcrV forms the tip complex. The unique localization of LcrV at the tip of the injectisome needle explains its crucial role in the translocation process and its role as the main protective antigen against plague. The orientation of LcrV within the tip complex was determined by analysis of tips formed by hybrids between LcrV and its orthologues in Pseudomonas and Aeromonas, PcrV and AcrV, respectively. The N-terminal globular domain and the C-terminus of LcrV were found to form the base structure of the tip complex. By quantitative immunoblot analyses and STEM, we determined that the needle of the Yersinia enterocolitica E40 injectisome consists of 139+/-19 YscF subunits and that the tip complex is formed by three to five LcrV monomers. A pentamer represented the best fit for an atomic model of the tip complex. In addition, the N-terminal globular domain of the V-antigen is crucial for pore formation as it mediates the interaction with the translocator YopB.