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Proteins of this family participate mainly in the type I secretion pathway (Sec-independent), functioning in conjunction with cytoplasmic membrane transporters of various families (like MFS, ABC, RND and PET) and periplasmic proteins. The complex formed in this way, allows the export of various solutes (metals, drugs, proteins) across the two envelopes of the Gram-negative bacterial cell envelope in a single energy-coupled step. The crystal structure of Escherichia coli TolC has been solved. 3 TolC monomers form a continuous channel with length over 140 Å spanning both the outer membrane with a ß-barrel of 12 beta strands (4 per monomer) and the periplasmic space (with 12 coil-coiled alpha helices). Similar structure has been observed in the VceC of Vibrio cholerae, and it is assumed for other members of the family (OprN, OprM etc). Such proteins are found in nearly all classes of Proteobacteria, in Cyanobacteria, Spirochetes and in species of Deinococcus, Aquifex and Porphyromonas. Due to their multimeric nature, prediction algorithms are not so successful in proteins of this family.
Type I secretion in gram-negative bacteria
Biochim Biophys Acta. 2004 Nov 11;1694(1-3):149-61. doi: 10.1016/j.bbamcr.2004.05.001.
Three's company: component structures bring a closer view of tripartite drug efflux pumps
Curr Opin Struct Biol. 2004 Dec;14(6):741-7. doi: 10.1016/j.sbi.2004.10.003.
Structure and function of TolC: the bacterial exit duct for proteins and drugs
Annu Rev Biochem. 2004;73:467-89. doi: 10.1146/annurev.biochem.73.011303.074104.
Trimeric structure of OprN and OprM efflux proteins from Pseudomonas aeruginosa, by 2D electron crystallography
J Struct Biol. 2005 Apr;150(1):50-7. doi: 10.1016/j.jsb.2005.01.001.
The crystal structure of the outer membrane protein VceC from the bacterial pathogen Vibrio cholerae at 1.8 A resolution
J Biol Chem. 2005 Apr 15;280(15):15307-14. doi: 10.1074/jbc.M500401200. Epub 2005 Jan 31.