Crystal structure of the type VI secretion system adaptor protein Tla3 from Pseudomonas aeruginosa.

The type VI secretion system (T6SS) is a dynamic protein nanomachine employed by Gram-negative bacteria to secrete toxic effector proteins directly into microbial competitors, eukaryotic host cells, or the environment. The T6SS is primarily involved in interbacterial antagonism, along with host-cell manipulation and resource scavenging, underscoring its central role in microbial community structuring and establishment of infection. Pseudomonas aeruginosa employs the 52 kDa adaptor protein Tla3 for the successful loading of the antibacterial phospholipase effector Tle3 onto the H2-T6SS machinery. Tla3 is reported to contain two DUF2875 domains. To date, no structural information is present for DUF2875 domain-containing adaptor proteins of the T6SS. Here, we report the 1.8 Å resolution crystal structure of the T6SS adaptor protein Tla3 from P. aeruginosa strain UCBPP-PA14. The structure shows a thiolase-like fold, with a conserved five-layered α-β-α-β-α core topology. Detailed atomic insight into the structure is significant, as it adds to the hitherto limited structural knowledge of adaptor-mediated effector-loading mechanisms in the T6SS, thereby contributing to a broader understanding of T6SS functionality.