The OEP family (Outer membrane efflux protein) form trimeric channels that allow export of a variety of substrates in Gram negative bacteria. Each member of this family is composed of two repeats. The trimeric channel is composed of a 12stranded all beta sheet barrel that spans the outer membrane, and a long all helical barrel that spans the periplasm. Examples include the <taxon tax_id="562">Escherichia coli</taxon> TolC outer membrane protein, which is required for proper expression of outer membrane protein genes; the Rhizobium nodulation protein; and the Pseudomonas FusA protein, which is involved in resistance to fusaric acid.