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Porin activity in the osmotic shock fluid of Escherichia coli

Osmotic shock fluid of Escherichia coli exhibited pore-forming activity. This activity could be followed by an in vitro assay based on the conductivity increase for ions due to the presence of pores in black lipid membranes. The histogram (the distribution of conductivity increments in a single pore experiment) obtained with osmotic shock fluid from E. coli was identical to the histogram obtained by detergent-solubilized porin isolated from the outer membrane. The osmotic shock fluid from porin-negative mutants also exhibited pore activity, although the histogram and ion specificity were different from those of porin. Antibodies raised against detergent-solubilized porin were able to form precipitin lines by the Ouchterlony immunodiffusion technique when shock fluids, but not detergent-solubilized porin, were used. These antibodies prevented the formation of pores when shock fluids contained porin but not when shock fluids obtained from porin-negative mutants were used. Macroscopic membrane conductivity of shock fluids due to porin exhibited a concentration dependence, in contrast to detergent-solubilized porin. These results indicate that the hydrodynamic properties of periplasmic or "soluble" porin are different from those of the detergent-solubilized porin of the outer membrane. Periplasmic porin comprises about 0.7% of total protein in the osmotic shock fluid.