Acinetobacter type VI secretion system comprises a non-canonical membrane complex

A. baumannii can rapidly acquire new resistance mechanisms and persist on abiotic surface, enabling the colonization of asymptomatic human host. In Acinetobacter the type VI secretion system (T6SS) is involved in twitching, surface motility and is used for interbacterial competition allowing the bacteria to uptake DNA. A. baumannii possesses a T6SS that has been well studied for its regulation and specific activity, but little is known concerning its assembly and architecture. The T6SS nanomachine is built from three architectural sub-complexes. Unlike the baseplate (BP) and the tail-tube complex (TTC), which are inherited from bacteriophages, the membrane complex (MC) originates from bacteria. The MC is the most external part of the T6SS and, as such, is subjected to evolution and adaptation. One unanswered question on the MC is how such a gigantesque molecular edifice is inserted and crosses the bacterial cell envelope. The A. baumannii MC lacks an essential component, the TssJ lipoprotein, which anchors the MC to the outer membrane. In this work, we studied how A. baumannii compensates the absence of a TssJ. We have characterized for the first time the A. baumannii's specific T6SS MC, its unique characteristic, its membrane localization, and assembly dynamics. We also defined its composition, demonstrating that its biogenesis employs three Acinetobacter-specific envelope-associated proteins that define an intricate network leading to the assembly of a five-proteins membrane super-complex. Our data suggest that A. baumannii has divided the function of TssJ by (1) co-opting a new protein TsmK that stabilizes the MC and by (2) evolving a new domain in TssM for homo-oligomerization, a prerequisite to build the T6SS channel. We believe that the atypical species-specific features we report in this study will have profound implication in our understanding of the assembly and evolutionary diversity of different T6SSs, that warrants future investigation. Assembly of envelope-spanning secretion nanomachines requires the concerted action of architectural proteins whose conservation have helped their identification in bacterial genomes. Specific adaptation occurs to fine tune the nanomachine to specific bacterial environment or biology. The type VI secretion system (T6SS) assembles a trans-envelope channel, the membrane complex (MC) that plays a key role in the transport of the toxin-loaded speargun. Even though the evolutionary pressure has maintained a high degree of conservation of this complex across species, here we demonstrated that Acinetobacter baumannii has evolved two new players to compensate the absence of the highly conserved TssJ lipoprotein. Our finding improves our understanding of the key roles of the lipoprotein: MC stability and multimerization. Understanding the molecular evolution of nanomachines offers the possibility to better understand their architecture and functioning, but also for pathogenic bacteria to identify specific drug target to block their virulence.