Structure-function analysis of PorX_Fj, the PorX homolog from Flavobacterium johnsioniae, suggests a role of the CheY-like domain in type IX secretion motor activity

The type IX secretion system (T9SS) is a large multi-protein transenvelope complex distributed into the Bacteroidetes phylum and responsible for the secretion of proteins involved in pathogenesis, carbohydrate utilization or gliding motility. In Porphyromonas gingivalis, the two-component system PorY sensor and response regulator PorX participate to T9SS gene regulation. Here, we present the crystal structure of PorX_Fj, the Flavobacterium johnsoniae PorX homolog. As for PorX, the PorX_Fj structure is comprised of a CheY-like N-terminal domain and an alkaline phosphatase-like C-terminal domain separated by a three-helix bundle central domain. While not activated and monomeric in solution, PorX_Fj crystallized as a dimer identical to active PorX. The CheY-like domain of PorX_Fj is in an active-like conformation, and PorX_Fj possesses phosphodiesterase activity, in agreement with the observation that the active site of its phosphatase-like domain is highly conserved with PorX.