The CorA Mg2+ transport protein of Salmonella typhimurium - Mutagenesis ofconserved residues in the second membrane domain

Salmonella typhimurium CorA is the archetypal member of the largest family of Mg2+ transporters of the Bacteria and Archaea, It contains three transme mbrane segments. There are no conserved charged residues within these segme nts indicating electrostatic interactions are not used in Mg2+ transport th rough CorA, Previous mutagenesis studies of CorA revealed a single face of the third transmembrane segment that is important for Mg2+ transport. In th is study, we mutated hydroxyl-bearing and other conserved residues in the s econd transmembrane segment to identify residues involved in transport. Res idues Ser(260), Thr(270) and Ser(274) appear to be important for transport and are oriented such that they would also line a face of an alpha-helix, I n addition, the sequence (276)YGMNF(280), found in virtually all CorA homol ogues, is critical for CorA function because even conservative mutations ar e not tolerated at these residues. Finally, mutations of residues in the se cond transmembrane segment, unlike those in the third transmembrane segment , revealed cooperative behavior for the influx of Mg2+. We conclude that th e second transmembrane segment forms a major part of the Mg2+ pore with the third transmembrane segment of CorA.