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Microbiology and Molecular Biology Reviews, December 2005, p. 665-695, Vol. 69, No. 4
1092-2172/05/$08.00+0     doi:10.1128/MMBR.69.4.665-695.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

The 2-Hydroxycarboxylate Transporter Family: Physiology, Structure, and Mechanism

Iwona Sobczak{dagger} and Juke S. Lolkema*

Molecular Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Haren, The Netherlands

The 2-hydroxycarboxylate transporter family is a family of secondary transporters found exclusively in the bacterial kingdom. They function in the metabolism of the di- and tricarboxylates malate and citrate, mostly in fermentative pathways involving decarboxylation of malate or oxaloacetate. These pathways are found in the class Bacillales of the low-CG gram-positive bacteria and in the gamma subdivision of the Proteobacteria. The pathways have evolved into a remarkable diversity in terms of the combinations of enzymes and transporters that built the pathways and of energy conservation mechanisms. The transporter family includes H+ and Na+ symporters and precursor/product exchangers. The proteins consist of a bundle of 11 transmembrane helices formed from two homologous domains containing five transmembrane segments each, plus one additional segment at the N terminus. The two domains have opposite orientations in the membrane and contain a pore-loop or reentrant loop structure between the fourth and fifth transmembrane segments. The two pore-loops enter the membrane from opposite sides and are believed to be part of the translocation site. The binding site is located asymmetrically in the membrane, close to the interface of membrane and cytoplasm. The binding site in the translocation pore is believed to be alternatively exposed to the internal and external media. The proposed structure of the 2HCT transporters is different from any known structure of a membrane protein and represents a new structural class of secondary transporters.

* Corresponding author. Mailing address: Molecular Microbiology, University of Groningen, Kerklaan 30, 9751 NN Haren, The Netherlands. Phone: 31 50 363 2155. Fax: 31 50 363 2154. E-mail: j.s.lolkema{at}biol.rug.nl.

{dagger} Present address: Analytical Biochemistry, University Centre for Pharmacy, University of Groningen, Postbus 196, 9700 AD Groningen, The Netherlands.

Microbiology and Molecular Biology Reviews, December 2005, p. 665-695, Vol. 69, No. 4
1092-2172/05/$08.00+0     doi:10.1128/MMBR.69.4.665-695.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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