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).