Bacterial periplasmic binding protein tertiary structures.

The periplasmic space of Gram-negative bacteria such as Escherichia coli and Salmone~la typh~murium contains a large variety of proteins with a wide range of functions. These proteins are associated with nutrient metabolism, transport, chemotaxis, antibiotic resistance, and energy utilization. There are about a dozen or so periplasmic proteins that participate in transport of small polar substrates across the bacterial inner membrane. These proteins serve to bind the solute in the periplasm and mediate the transfer of the solute across the inner membrane. They are easy to isolate and purify from the periplasmic space using osmotic shock techniques. They have been extensively characterized by the work of many investigators, and it is now becoming clear that they share a great deal of similarity in mechanism and structure. E. coli and S. typhimurium also contain osmotic shock-insensitive permeases which are generally constitutive low affinity systems comprised of a single membrane protein that both recognizes and translocates the substrate from the periplasmic space to the cytosol as well as couples the system to metabolic energy. The osmotic shocksensitive transport systems are more complex, consisting in each case of at least one soluble periplasmic protein, which is the component for substrate recognition.. and two to four inner membrane proteins that play a more direct role in the transfer of the solute across the inner membrane. Reviews of shock-sensitive transport systems describing the genetics and biochemistry of these systems have appeared recently (1-3).