Bacterial interaction with plants and animals, symbiotic or pathogenic, involves the transfer to host cells and tissues of a range of bacterial proteins whose biochemical activities are key to establishing both commensalism and infection. Export of protein molecules across the bacterial membranes takes place via a variety of mechanisms, from simple one-component systems to complex multicomponent pathways, with five types of nonhomologous protein secretion system characterized in pathogenic bacteria so far. In the October 3 Cell, Sun Nyunt Wai and colleagues at Umeå University describe an additional protein transfer strategy in gram-negative bacteria that furthers our understanding of host–bacteria interactions (Cell, 115:25-35, October 3, 2003).

Wai et al. dissected the secretion pathway of cytolysin A (ClyA), a pore-forming toxin with hemolytic and cytolytic properties produced by the gram-negative bacterium Escherichia coli and closely related species. Gram-negative bacteria have two distinct membrane bilayers separated by a periplasmic space...

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