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J. Biol. Chem., Vol. 282, Issue 4, 2606-2614, January 26, 2007

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The Structure of a Cyanobacterial Bicarbonate Transport Protein, CmpA^*

Nicole M. Koropatkin, David W. Koppenaal, Himadri B. Pakrasi^¶, and Thomas J. Smith¹

From the Donald Danforth Plant Science Center, St. Louis, Missouri 63132, Pacific Northwest National Laboratory, Richland, Washington 99352, and ^¶Department of Biology, Washington University at Saint Louis, St. Louis, Missouri 63130

Cyanobacteria, blue-green algae, are the most abundant autotrophs in aquatic environments and form the base of the food chain by fixing carbon and nitrogen into cellular biomass. To compensate for the low selectivity of Rubisco for CO₂ over O₂, cyanobacteria have developed highly efficient CO₂-concentrating machinery of which the ABC transport system CmpABCD from Synechocystis PCC 6803 is one component. Here, we have described the structure of the bicarbonate-binding protein CmpA in the absence and presence of bicarbonate and carbonic acid. CmpA is highly homologous to the nitrate transport protein NrtA. CmpA binds carbonic acid at the entrance to the ligand-binding pocket, whereas bicarbonate binds in nearly an identical location compared with nitrate binding to NrtA. Unexpectedly, bicarbonate binding is accompanied by a metal ion, identified as Ca²⁺ via inductively coupled plasma optical emission spectrometry. The binding of bicarbonate and metal appears to be highly cooperative and suggests that CmpA may co-transport bicarbonate and calcium or that calcium acts a cofactor in bicarbonate transport.

Received for publication, November 1, 2006 , and in revised form, November 20, 2006.

The atomic coordinates and structure factors (code 2I48, 2I49, 2I4B, and 2I4C) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).

^* This work was supported by National Institutes of Health Grant GM078800 and the Membrane Biology Environmental Molecular Sciences Laboratory Scientific Grand Challenge project at the W. R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the United States Department of Energy, Office of Biological and Environmental Research program located at Pacific Northwest National Laboratory (operated for the Department of Energy by Battelle). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: Donald Danforth Plant Science Ctr., 975 N. Warson Rd., St. Louis, MO 63132. Tel.: 314-587-1451; Fax: 314-587-1551; E-mail: tsmith{at}danforthcenter.org.

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