Figure 1.

Composition and structure of the type I secretion system (T1SS) in A. baumannii: (a) Bioinformatic analysis has led to the identification of the T1SS in genomes of A. baumannii. Gene locus tags are cited from ATCC 17978. Genes predicted to encode proteins required for the biogenesis of the T1SS are found in three gene clusters, with hlyB overlapping with tolC and hlyD. (b) The three components of the T1SS act together to facilitate the secretion of effectors. TolC is a trimeric outer membrane protein with the α-helical barrel forming a tunnel through the periplasm, and it interacts with HlyD. HlyD has a large periplasmic domain linked by a single transmembrane helix, which anchors in the inner membrane. The energy required for the export of specific T1SS substrates is provided by HlyB, which is an ATP-binding protein. Two putative T1SS effectors, namely, Repeats-in-Toxin (RTX)-serralysin-like toxin and biofilm-associated protein (Bap), are involved in the formation and stability of biofilm.