To interact (often adversely) with the outside world, bacteria have several types of mechanism for transporting toxins and enzymes across complex cell envelopes. A recently described Type 6 secretion system (T6SS) has been added to the list, and its highly conserved gene clusters are found in numerous Gram-negative organisms, including strains of Vibrio cholerae. Pukatzki et al. used the social amoeba Dictyostelium discoideum as an experimental host in which to test T6SS mutants of this pathogen. Four proteins are needed to assemble a complete apparatus: The authors speculate that initially trimers of VgrG proteins penetrate the membrane to form a channel, and then units of a ringlike substrate, Hep, are exported and stack up to form a hollow needle that facilitates its own transport. Remarkably, the components have homologs in the tail-spike membrane-puncturing device of the T4 bacteriophage. After export, the VgrG-1 component promotes pathogenesis by cross-linking host actin.
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