Type II secretion: a protein secretion system for all seasons.

In Gram-negative bacteria, type II secretion (T2S) is one of five protein secretion systems that permit the export of proteins from within the bacterial cell to the extracellular milieu and/or into target host cells. An analysis of numerous sequenced genomes now reveals that T2S genes are common, but by no means universal, in Gram-negative bacteria. Recent functional studies indicate that T2S can promote the virulence of human, animal and plant pathogens, as well as the physiology of various environmental bacteria. Thus, it is an opportune time to highlight the new and different ways in which T2S serves bacterial function.