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Journal of Bacteriology, July 2004, p. 4605-4612, Vol. 186, No. 14
0021-9193/04/$08.00+0     DOI: 10.1128/JB.186.14.4605-4612.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

The CorA Mg2+ Transporter Is a Homotetramer

Mary Ann Warren,{dagger} Lisa M. Kucharski, Alexander Veenstra, Liang Shi,{ddagger} Paul F. Grulich, and Michael E. Maguire*

Department of Pharmacology, Case School of Medicine, Case Western Reserve University, Cleveland, Ohio 44106-4965

Received 2 March 2004/ Accepted 6 April 2004

CorA is a primary Mg2+ transporter for Bacteria and Archaea. The C-terminal domain of ~80 amino acids forms three transmembrane (TM) segments, which suggests that CorA is a homo-oligomer. A Cys residue was added to the cytoplasmic C terminus (C317) of Salmonella enterica serovar Typhimurium CorA with or without mutation of the single periplasmic Cys191 to Ser; each mutant retained function. Oxidation of the Cys191Ser Cys317 CorA gave a dimer. Oxidation of Cys317 CorA showed a dimer plus an additional band, apparently cross-linked via both Cys317 and C191. To determine oligomer order, intact cells or purified membranes were treated with formaldehyde or carbon disulfide. Higher-molecular-mass bands formed, consistent with the presence of a tetramer. Cross-linking of the Bacillus subtilis CorA expressed in Salmonella serovar Typhimurium similarly indicated a tetramer. CorA periplasmic soluble domains from both Salmonella serovar Typhimurium and the archaeon Methanococcus jannaschii were purified and shown to retain structure. Formaldehyde treatment showed formation of a tetramer. Finally, previous mutagenesis of the CorA membrane domain identified six intramembrane residues forming an apparent pore that interacts with Mg2+ during transport. Each was mutated to Cys. In mutants carrying a single intramembrane Cys residue, spontaneous disulfide bond formation that was enhanced by oxidation with Cu(II)-1,10-phenanthroline was observed between monomers, indicating that these Mg2+-interacting residues within the membrane are very close to their cognate residue on another monomer. Thus, CorA appears to be a homotetramer with a TM segment of one monomer physically close to the same TM segment of another monomer.

* Corresponding author. Mailing address: Department of Pharmacology, School of Medicine, Case Western Reserve University, 10900 Euclid Ave., Cleveland, OH 44106-4965. Phone (216) 368-6186. Fax: (216) 368-6187. E-mail: mem6{at}cwru.edu.

{dagger} Present address: Department of Pharmacology and Therapeutics, Roswell Park Cancer Institute, Buffalo, NY 14263.

{ddagger} Present address: Microbiology Group, Pacific Northwest National Laboratory, Richland, WA 99352.

Journal of Bacteriology, July 2004, p. 4605-4612, Vol. 186, No. 14
0021-9193/04/$08.00+0     DOI: 10.1128/JB.186.14.4605-4612.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.



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