Excretion of proteins by gram-negative bacteria: export of bacteriocins and fimbrial proteins by Escherichia coli.

In gram-negative bacteria only few proteins are exported across both the cytoplasmic membrane and the outer membrane which forms an extra barrier for protein excretion. In this review we describe the mechanisms of production and export of two types of plasmid-encoded proteins in Escherichia coli. These proteins are the bacteriocin cloacin DF13 and the K88ab and K99 fimbrial subunits. Specific so-called helper proteins located at different positions in the cell envelope play an essential role in the export of these proteins. The genetic organization, subcellular location and functions of these helper proteins, as well as the effects of mutations and culture conditions on the export of the proteins are described. Models for the export mechanisms are presented and future application possibilities for engineering foreign protein excretion in E. coli with these export systems are discussed.