Structural and functional insights into the complex formation of a trimeric autotransporter adhesin with a peptidoglycan-binding periplasmic protein

Trimeric autotransporter adhesin (TAA) is an outer membrane (OM) protein that is widely distributed in gram-negative bacteria and is involved primarily in adhesion to biotic and abiotic surfaces, cell agglutination, and biofilm formation. TAAs are secreted onto the OM by the type Vc secretion system (SS). Because the interactions between TAAs and chaperones or special assistant proteins during secretion are short-lived, it is thought that TAAs reside on the OM without forming complexes with other proteins after secretion. In this study, we aimed to clarify the interactions between an Acinetobacter TAA, AtaA, and a peptidoglycan (PG)-binding periplasmic protein, TpgA, and to identify additional roles of TpgA based on these interactions. Pull-down assays using recombinant proteins identified the interacting domains. X-ray crystallography revealed the A3B3 heterohexameric complex structure of the N-terminal domain of TpgA with the transmembrane domain of AtaA and showed that both electrostatic and hydrophobic interactions contribute to stable complex formation. Bioinformatic analysis suggested that the TAA-TpgA complex is formed in a wide range of species that harbor the taa-tpgA gene cassette in their genome. Furthermore, the absence of TpgA increased the release of AtaA from the cell surface, suggesting that TpgA prevents the release of TAA from the cell surface via its anchoring to the PG. We propose that the TAAs that form a complex with TpgA be assigned to type Vc2 SS, a new subtype of type Vc SS. ### Competing Interest Statement The authors have declared no competing interest.