Proteins in the outer membrane effluxprotein family 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 12 stranded beta-barrel that spans the outer membrane, and a long all helical barrel that spans the periplasm.
Examples include the Escherichia coli 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.
Johnson, J. M.; Church, G. M. (1999). "Alignment and structure prediction of divergent protein families: Periplasmic and outer membrane proteins of bacterial efflux pumps". Journal of Molecular Biology. 287 (3): 695–715. doi:10.1006/jmbi.1999.2630. PMID10092468.