phoe

PhoE is an outer membrane porin protein found in Escherichia coli and many related enteric bacteria. It is encoded by the phoE gene, part of the phosphate (Pho) regulon whose expression is controlled by the PhoR/PhoB two-component system. PhoE is expressed primarily under conditions of inorganic phosphate limitation and repressed when phosphate is abundant.

Functionally, PhoE forms a trimeric channel in the outer membrane that enables passive diffusion of small hydrophilic solutes. Its primary role is to facilitate inorganic phosphate uptake when phosphate is scarce, but the porin also permits passage of other small nutrients. In this way PhoE acts as an alternative porin to the major channels OmpF and OmpC under phosphate-starved conditions, expanding the outer membrane’s permeability to a range of small solutes.

Structurally, PhoE is a beta-barrel outer membrane protein. Each monomer forms a 16-stranded beta-barrel, and three monomers assemble into a homotrimer that spans the outer membrane. Like other OMPs, PhoE contains an N-terminal signal sequence that directs the protein to the Sec pathway for translocation into the periplasm, followed by insertion into the outer membrane via the Bam complex. Loops surrounding the pore, particularly the constriction region, contribute to its substrate selectivity.

Regulation is driven by the PhoB response regulator activated under phosphate limitation. PhoB induces transcription of phoE and other Pho regulon genes, while high extracellular phosphate reduces PhoR signaling, decreasing PhoE expression.

PhoE’s significance lies in its influence on nutrient uptake and outer membrane permeability, which can affect bacterial fitness under nutrient stress and, indirectly, antibiotic susceptibility to drugs that utilize porin channels. See also OmpF, OmpC, and PhoA.