AbstractType three secretion is the mechanism of protein secretion found in bacterial flagella and injectisomes. At its centre is the export apparatus (EA), a complex of five membrane proteins through which secretion substrates pass the inner membrane. While the complex formed by four of the EA proteins has been well characterised structurally, little is known about the structure of the membrane domain of the largest subunit, FlhA in flagella, SctV in injectisomes. Furthermore, FlhA/SctV is most often studied as a monomer and only a single structure of an SctV homologue assembled into the biologically relevant nonameric ring is available. FlhA has been shown to bind to chaperone-substrate complexes in an open state, but in the assembled ring structure SctV is in a closed state. Here, we identify FlhA and SctV homologues that can be recombinantly produced in the oligomeric state and study them using cryo-electron microscopy. The structures of the cytoplasmic domains from both FlhA and SctV are in the open state and we observe a conserved interaction between a short stretch of residues at the N-terminus of the cytoplasmic domain, known as FlhAL/SctVL, with a groove on the adjacent protomer’s cytoplasmic domain, which stabilises the nonameric ring assembly. This represents the first structure of SctV in the open state, the first observation of the SctVL interaction with the adjacent protomer and confirms the importance of FlhAL for the stability of the FlhA nonameric ring.ImportanceBacterial flagella are assembled from proteins secreted through a type III secretion system. A related type III secretion system is found in injectisomes, molecular syringes that bridge three membranes to secrete proteins directly from the bacterial cytoplasm into eukaryotic host cells. The major protein of the export apparatus of type III secretion is made up of a membrane and a cytoplasmic domain, which in the flagellar system can adopt an open or a closed state, is known to form a nonameric ring in vivo. We produced the full-length proteins from both injectisome and flagellar systems in the assembled state. The structures of the cytoplasmic domains demonstrate the conserved principle of the N-terminus of one subunit binding the membrane proximal face of the adjacent subunit to stabilise the assembled ring. Our structure of the homologue from the injectisome also demonstrates that the open state of the cytoplasmic domain is not unique to flagella.
Type III secretion systems and bacterial flagella: Insights into their function from structural similarities