A conserved motif of Porphyromonas Type IX secretion effectors C-terminal secretion signal specifies interactions with the PorKLMN core complex

The Type IX secretion system (T9SS) is a versatile protein transport apparatus restricted to the Bacteroidetes phylum. This multiprotein complex enables secretion of a wide range of effectors, such as protein toxins, filamentous adhesins, enzymes or S-layer subunits. Once translocated in the periplasm through the Sec pathway, recognition and secretion of these cargo proteins rely on a conserved C-terminal domain referred as CTD. However, the precise route followed by the CTD substrates from the periplasm to the cell exterior is yet to be determined. Here we define the interaction network of five different CTDs from the oral pathogen Porphyromonas gingivalis with the PorKLMN T9SS trans-envelope complex. We show that these five CTDs interact with the inner membrane-anchored PorM and the PorN periplasmic proteins. We further determine the contribution of the PorM IgG-like periplasmic domains and CTD conserved motifs for PorM-CTD complex formation. These results showed that all five CTDs interact with the core complex in a similar manner, suggesting a conserved mechanism of substrate selection in the periplasm. Our results thus establish a first model of the path followed by the CTD substrates through the T9SS.