Figure 2.

Model of the step-wise assembly of the injectisome. SctRST form a stable complex in the inner membrane (IM) to which SctU is recruited. This complex nucleates the assembly of the inner rings integrated by SctJ and SctD, which results in the extraction or “pulling” of the IM components from the bacterial plasma membrane. At the same time, the secretin is independently assembled into the outer ring and the two structures come together to form the needle complex base substructure to which SctV is subsequently recruited. Once the NC base is formed, the cytoplasmic sorting platform is recruited to the cytoplasmic side of the NC base and the system starts to function as a type III secretion machine dedicated to the delivery of early substrates such as the inner rod (SctI) and needle (SctF) subunits to complete the assembly of the entire injectisome.