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J Biol Chem, Vol. 273, Issue 44, 28663-28669, October 30, 1998

The CorA Mg²⁺ Transport Protein of Salmonella typhimurium
MUTAGENESIS OF CONSERVED RESIDUES IN THE THIRD MEMBRANE DOMAIN IDENTIFIES A Mg²⁺ PORE

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

The CorA transport system is the major Mg²⁺ influx pathway for bacteria and the Archaea. CorA contains three C-terminal transmembrane segments. No conserved charged residues are apparent within the membrane, suggesting that Mg²⁺ influx does not involve electrostatic interactions. We have mutated conserved residues within the third transmembrane segment to identify sites involved in transport. Mutation of conserved aromatic residues at either end of the membrane segment to alternative aromatic amino acids did not affect total cation uptake or cation affinity. Mutation to alanine greatly diminished uptake with little change in cation affinity implying that the conserved aromatic residues play a structural role in stabilizing this membrane segment of CorA at the interface between the bilayer and the aqueous environment. In contrast, mutation of Tyr²⁹², Met²⁹⁹, and Tyr³⁰⁷ greatly altered the transport properties of CorA. Y292F, Y292S, Y292C, or Y292I mutations essentially abolished transport, without effect on expression or membrane insertion. M299C and M299A mutants exhibited a decrease in cation affinity for Mg²⁺, Co²⁺, or Ni²⁺ of 10-50-fold without a significant change in uptake capacity. Mutations at Tyr³⁰⁷ had no significant effect on cation uptake capacity; however, the affinity of Y307F and Y307A mutations for Mg²⁺ and Co²⁺ was decreased 3-10-fold, while affinity for Ni²⁺ was unchanged compared with the wild type CorA. In contrast, the affinity of the Y307S mutant for all three cations was decreased 2-5-fold. Projection of the third transmembrane segment as an -helix suggests that Tyr²⁹², Met²⁹⁹, and Tyr³⁰⁷ all reside on the same face of the -helix. We interpret the transport data to suggest that a hydroxyl group is important at Tyr³⁰⁷, and that these three residues interact with Mg²⁺ during transport, forming part of the cation pore or channel within CorA.

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