Substrate recognition by the bacterial type II secretion system: more than a simple interaction

Type II secretion system (T2SS) is a multiprotein trans-envelope complex that translocates fully folded proteins through the outer membrane of Gram-negative bacteria. Although T2SS is extensively studied in several bacteria pathogenic for humans, animals and plants, the molecular basis for exoprotein recruitment by this secretion machine as well as the underlying targeting motifs remain unknown. To address this question, we used bacterial two-hybrid, surface plasmon resonance, in vivo site-specific photo-cross-linking approaches and functional analyses. We showed that the fibronectin-like Fn3 domain of exoprotein PelI from Dickeya dadantii interacts with four periplasmic domains of the T2SS components GspD and GspC. The interaction between exoprotein and the GspC PDZ domain is positively modulated by the GspD N1 domain, suggesting that exoprotein secretion is driven by a succession of synergistic interactions. We found that an exposed 9-residue-long loop region of PelI interacts with the GspC PDZ domain. This loop acts as a specific secretion signal that controls exoprotein recruitment by the T2SS. Concerted in silico and in vivo approaches reveal the occurrence of equivalent secretion motifs in other exoproteins, suggesting a plausible general mechanism of exoprotein recruitment by the T2SS.