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The multicomponent type VI secretion system ( T6SS) mediates the transport of effector proteins by puncturing target membranes. T6SSs are suggested to form a contractile nanomachine, functioning similar to the cell-puncturing device of tailed bacteriophages. The T6SS members VipA/ VipB form tubular complexes and are predicted to function in analogy to viral tail sheath proteins by providing the energy for secretion via contraction. The ATPase ClpV disassembles VipA/ VipB tubules in vitro, but the physiological relevance of tubule disintegration remained unclear. Here, we show that VipA/ VipB tubules localize near-perpendicular to the inner membrane of Vibrio cholerae cells and exhibit repetitive cycles of elongation, contraction and disassembly. VipA/ VipB tubules are decorated by ClpV in vivo and become static in Δ clpV cells, indicating that ClpV is required for tubule removal. VipA/ VipB tubules mislocalize in Δ clpV cells and exhibit a reduced frequency of tubule elongation, indicating that ClpV also suppresses the spontaneous formation of contracted, non-productive VipA/ VipB tubules. ClpV activity is restricted to the contracted state of VipA/ VipB, allowing formation of functional elongated tubules at a T6SS assembly. Targeting of an unrelated ATPase to VipA/ VipB is sufficient to replace ClpV function in vivo, suggesting that ClpV activity is autonomously regulated by VipA/ VipB conformation. [ABSTRACT FROM AUTHOR]

In Pseudomonas aeruginosa three type VI secretion systems ( T6SSs) coexist, called H1- to H3-T6SSs. Several T6SS components are proposed to be part of a macromolecular complex resembling the bacteriophage tail. The T6SS protein HsiE1 ( TagJ) is unique to the H1-T6SS and absent from the H2- and H3-T6SSs. We demonstrate that HsiE1 interacts with a predicted N-terminal α-helix in HsiB1 ( TssB) thus forming a novel subcomplex of the T6SS. HsiB1 is homologous to the Vibrio cholerae VipA component, which contributes to the formation of a bacteriophage tail sheath-like structure. We show that the interaction between HsiE1 and HsiB1 is specific and does not occur between HsiE1 and HsiB2. Proteins of the TssB family encoded in T6SS clusters lacking a gene encoding a TagJ-like component are often devoid of the predicted N-terminal helical region, which suggests co-evolution. We observe that a synthetic peptide corresponding to the N-terminal 20 amino acids of HsiB1 interacts with purified HsiE1 protein. This interaction is a common feature to other bacterial T6SSs that display a TagJ homologue as shown here with Serratia marcescens. We further show that hsiE1 is a non-essential gene for the T6SS and suggest that HsiE1 may modulate incorporation of HsiB1 into the T6SS. [ABSTRACT FROM AUTHOR]

Almost all aspects of protein traffic in bacteria were covered at the ASM-FEMS meeting on the topic in Iraklio, Crete in May 2006. The studies presented ranged from mechanistic analysis of specific events leading proteins to their final destinations to the physiological roles of the targeted proteins. Among the highlights from the meeting that are reviewed here are the molecular dynamics of SecA protein, membrane protein insertion, type III secretion needles and chaperones, type IV secretion, the two partner and autosecretion systems, the ‘secretion competent state’, and the recently discovered type VI secretion system. [ABSTRACT FROM AUTHOR]