Export of proteins from the bacterial cytoplasm to a final destination in the periplasm and outer membrane is one example of the fundamental process occurring in all cells whereby polypeptides are transferred across biological membranes. Investigations on a variety of different systems have indicated similarities in the mechanism of this process. In the cases of bacterial protein export and the transfer of polypeptides across the endoplasmic reticulum in eukaryotic cells the processes are so

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... ilar that understanding gleaned from studies of the one is usually directly applicable to the other. The study of protein export in E. coli has two advantages over that of eukaryotic secretion. Not only is there the possibility of doing sophisticated genetic experiments, but also one can carry out biochemical investigations in vivo, a facility not so readily available with eukaryotic organisms. Such studies have, for example, shown that membrane translocation can occur both cotranslationally and post-translationally, that export requires protonmotive force, that some component of the export apparatus prevents the exported protein from assuming its native structure in the cytosol, and that there are probably at least two functions for the leader sequence, one in targeting the protein to the export pathway and one in translocation across the membrane.