The type VI secretion system protein AsaA in Acinetobacter baumannii is a periplasmic protein physically interacting with TssM and required for T6SS assembly

Type VI secretion system (T6SS) is described as a macromolecular secretion machine that is utilized for bacterial competition. The gene clusters encoding T6SS are composed of core tss genes and tag genes. However, the clusters differ greatly in different pathogens due to the great changes accumulated during the long-term evolution. In this work, we identified a novel hypothetical periplasmic protein designated as AsaA which is encoded by the first gene of the T6SS cluster in the genus Acinetobacter. By constructing asaA mutant, we delineated its relative contributions to bacterial competition and secretion of T6SS effector Hcp. Subsequently, we studied the localization of AsaA and potential proteins that may have interactions with AsaA. Our results showed that AsaA in Acinetobacter baumannii (A. baumannii) localized in the bacterial periplasmic space. Results based on bacterial two-hybrid system and protein pull-down assays indicated that it was most likely to affect the assembly or stability of T6SS by interacting with the T6SS core protein TssM. Collectively, our findings of AsaA is most likely a key step in understanding of the T6SS functions in A. baumannii.