Delivery determinants of an Acinetobacter baumannii type VI secretion system bifunctional peptidoglycan hydrolase.

Abstract

Acinetobacter baumannii is a Gram-negative opportunistic pathogen that is a common cause of nosocomial infections. The increasing development of antibiotic resistance in this organism is a global health concern. The A. baumannii clinical isolate AB307-0294 produces a type VI secretion system (T6SS) that delivers three antibacterial cargo effector proteins (Tse15, Tde16 and Tae17) that give this strain a competitive advantage against other bacteria in polymicrobial environments. These effectors are delivered via specific non-covalent interactions with the T6SS needle tip proteins VgrG15, VgrG16 and VgrG17, respectively. Here we determine the molecular function of the Tae17 effector protein and define the regions of Tae17 and VgrG17 essential for its delivery. Specifically, we show that Tae17 is a multidomain, bifunctional peptidoglycan-degrading enzyme. Tae17 has both lytic transglycosylase activity, which targets the peptidoglycan sugar backbone, and amidase activity, which targets the sugar-peptide bonds. Moreover, we show that the transglycosylase activity was more important for killing Escherichia coli. Using deletion constructs and bacterial two-hybrid analyses, we identify that amino acids 1051-1085 of the VgrG17 needle tip protein and amino acids 1-162 of the Tae17 effector protein are necessary for the Tae17:VgrG17 interaction. Furthermore, we identify the VgrG17 amino acids G1069 and W1075 as crucial for the delivery of Tae17; the first time such specific delivery determinants of T6SS cargo effectors have been defined. This study provides molecular insight into how the T6SS allows A. baumannii strains to gain dominance in polymicrobial communities and thus improve their chances of survival and transmission. IMPORTANCEWe have shown that the Acinetobacter baumannii T6SS effector Tae17 is a modular, bifunctional, peptidoglycan-degrading enzyme that has both lytic transglycosylase and amidase activity. Both activities contribute to the ability to degrade peptidoglycan, but the glycosyltransferase activity was more important for the interbacterial killing of Escherichia coli. We have defined the specific regions of Tae17 and its cognate delivery protein VgrG17 that are necessary for the non-covalent interactions and, for the first time, identified specific amino acids essential for delivery. This work contributes to our molecular understanding of bacterial competition strategies in polymicrobial environments and may provide a window to the design of new therapeutic approaches for combating infection by A. baumannii.