Type III export: new uses for an old pathway

Gram-negative bacteria use type III secretion (TTS) systems to translocate proteins into the extracellular environment or directly into eukaryotic cel ls. These complex secretory systems are assembled from over 20 different st ructural proteins, including 10 that have counterparts in the flagellar exp ort pathway. Secretion substrates are directed to the TTS machinery via mRN A and/or amino acid secretion signals. TTS chaperones bind to select secret ion substrates and assist in the export process. Recent progress in the und erstanding of TTS is reviewed.