Type III Secretion: More Systems Than You Think

摘要

The type III secretion (T3S) pathway allows bacteria to inject effector proteins into the cytosol of target animal or plant cells. T3S systems evolved into seven families that were distributed among Gram-negative bacteria by horizontal gene transfer.There are probably a few hundred effectors interfering with control and signaling in eukaryotic cells and offering a wealth of new tools to cell biologists. Exotoxins, in particular A-B toxins, which allow bacteria to destroy or disturb animal cells at adistance from the site of infection or colonization, have been known for a very long time. In contrast, type III secretion (T3S), which allows direct communication between bacteria and animal or plant cells, was only discovered in the 1990s. With T3S, abacterium docked at the surface of a cell or even residing within a vacuole injects effector proteins into the cytosol across the cellular or vacuolar membrane and modulates the cell functions to its advantage (30, 31, 51, 66, 74, 112, 124). The T3S apparatus, called the "injectisome," consists of two pairs of rings spanning the two bacterial membranes, linked by a rod, and a ~60-nm-long needle protruding outside the bacterial body (10, 81, 84) (FIGURE 1). The length of the needle is genetically controlled (78) and is adjusted in relation to the length of other macromolecules present at the bacterial and cellular surfaces (95).