The bacterial type VI secretion machine: yet another player for protein transport across membranes
Alain Filloux, Abderrahman Hachani, and Sophie Bleves





Type I–V secretion systems in Gram-negative bacteria. Type I, type III and type IV SSs (left) are believed to transport proteins in one step from the bacterial cytosol to the bacterial cell surface and external medium. In the case of type III and type IV SSs, the proteins are transported from the bacterial cytoplasm to the target cell cytosol. One exception for type IV is the pertussis toxin, which is secreted in two steps and released into the extracellular medium. This exception is represented by the dotted arrow, which connects Sec and the type IV SS. Type II and type V SSs transport proteins in two steps. In that case, proteins are first transported to the periplasm via the Sec or Tat system before reaching the cell surface. Type Va is a putative autotransporter, indicating that the C-terminus of the protein forms the outer-membrane channel (cylinder) whereas the N-terminus (pink line) is exposed to the surface or released by proteolytic cleavage (scissors). C, bacterial cytoplasm; IM, bacterial inner membrane; P, bacterial periplasm; OM, bacterial outer membrane; ECM, extracellular milieu. PM (brown zone), host cell plasma membrane. When appropriate, coupling of ATP hydrolysis to transport is highlighted. Arrows indicate the route followed by transported proteins.

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