Protein Traffic in Gram-negative bacteria – how exported and secreted proteins find their way

bacterial protein export; inner membrane protein insertion; outer membrane insertion; protein secretion. Gram-negative bacteria assemble many proteins into the inner and outer mem-branes and export a large number of proteins to the periplasm or to the extra-cellular medium. During the billions of years bacteria have been around, they have evolved a number of different pathways with sophisticated machines to accurately and efficiently move proteins from one location to another. In this review, we first introduce specific proteins that are representative substrates of the protein transport pathways and describe their function. Then, their specific routes from synthesis to their destinations are described mentioning the signal peptide that may initiate their export and discuss what is known about the folding state of the substrates during transport. The membrane translocation device involved, the energy source required for transport, and whether a chap-erone is needed will be discussed