Annuaire
Acinetobacter type VI secretion system comprises a non-canonical membrane complex.
PLoS Pathog, 19(9):e1011687.
A. baumannii can rapidly acquire new resistance mechanisms and persist on abioticsurface, enabling the colonization of asymptomatic human host. In Acinetobacterthe type VI secretion system (T6SS) is involved in twitching, surface motilityand is used for interbacterial competition allowing the bacteria to uptake DNA.A. baumannii possesses a T6SS that has been well studied for its regulation andspecific activity, but little is known concerning its assembly and architecture.The T6SS nanomachine is built from three architectural sub-complexes. Unlike thebaseplate (BP) and the tail-tube complex (TTC), which are inherited frombacteriophages, the membrane complex (MC) originates from bacteria. The MC is themost external part of the T6SS and, as such, is subjected to evolution andadaptation. One unanswered question on the MC is how such a gigantesque molecularedifice is inserted and crosses the bacterial cell envelope. The A. baumannii MClacks an essential component, the TssJ lipoprotein, which anchors the MC to theouter membrane. In this work, we studied how A. baumannii compensates the absenceof a TssJ. We have characterized for the first time the A. baumannii's specificT6SS MC, its unique characteristic, its membrane localization, and assemblydynamics. We also defined its composition, demonstrating that its biogenesisemploys three Acinetobacter-specific envelope-associated proteins that define anintricate network leading to the assembly of a five-proteins membranesuper-complex. Our data suggest that A. baumannii has divided the function ofTssJ 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 buildthe T6SS channel. We believe that the atypical species-specific features wereport in this study will have profound implication in our understanding of theassembly and evolutionary diversity of different T6SSs, that warrants futureinvestigation.
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