The objective of this proposal aims at deciphering the mechanism of action of the recently discovered type IX secretion system (T9SS). The T9SS is responsible for Porphyromonas gingivalis pathogenesis through secretion of virulence factors, and for Flavobacterium johnsoniae motility through secretion of adhesins. Among the 18 identified proteins that are involved in T9SS function, the K-L-M-N proteins are supposed to form the core complex. We propose that the outer membrane ring-shaped K-N complex serves as an interaction platform to recruit the inner membrane L-M complex, and that the L-M complex is a novel rotary motor driven by proton motive force. In this project, we will solve the structures of the isolated soluble domains of L and N proteins, and of the L-M and K-L-M-N membrane complexes from the two bacterial models, by X-ray crystallography and cryo-EM. The function of the L-M complex, and notably its role as a rotary motor, will be addressed by in vivo approaches.