Ushers and secretins: Channels for the secretion of folded proteins across the bacterial outer membrane

Gram-negative bacteria have evolved a number of pathways for extracellular protein secretion. Proteins targeted for secretion in Gram-negative bacteria must cross the outer membrane in addition to the cytoplasmic membrane. This must be accomplished without compromising the barrier properties of the outer membrane and is further complicated by the fact that proteins may fold prior to secretion outside the cell. This review will discuss the usher and secretin families of integral outer membrane proteins, which function to allow the secretion of folded proteins in Gram-negative bacteria.