PhoE protein pore of the outer membrane of Escherichia coli K12 is a particularly efficient channel for organic and inorganic phosphate

This study was undertaken to investigate the proposed in vivo pore function of PhoE protein, an Escherichia coli K12 outer membrane protein induced by growth under phosphate limitation, and to compare it with those of the constitutive pore proteins OmpF and OmpC. Appropriate mutant strains were constructed containing only one of the proteins PhoE, OmpF or OmpC, or none of these proteins at all. By measuring rates of nutrient uptake at low solute concentrations, the proposed pore function of PhoE protein was confirmed as the presence of the protein facilitates the diffusion of P_i through the outer membrane, such that a pore protein deficient strain behaves as a K_m mutant. Comparison of the rates of permeation of P_i, glycerol 3-phosphate and glucose 6-phosphate through pores formed by PhoE, OmpF and OmpC proteins shows that PhoE protein is the most effective pore in facilitating the diffusion of P_i and phosphorus-containing compounds. The three types of pores were about equally effective in facilitating the permeation of glucose and arsenate. Possible reasons for the preference for P_i and P_i-containing solutes are discussed.