Co2+ Selectivity of Thermotoga maritima CorA and Its Inability to Regulate Mg2+ Homeostasis Present a New Class of CorA Proteins

CorA is a family of divalent cation transporters ubiquitously present in bacteria and archaea. Although CorA can transport both Mg2+ and Co2+ almost equally well, its main role has been suggested to be that of primary Mg2+ transporter of prokaryotes and hence the regulator of Mg2+ homeostasis. The reason is that the affinity of CorA for Co2+ is relatively low and thus considered non-physiological. Here, we show that Thermotoga maritima CorA (TmCorA) is incapable of regulating the Mg2+ homeostasis and therefore cannot be the primary Mg2+ transporter of T. maritima. Further, our in vivo experiments confirm that TmCorA is a highly selective Co2+ transporter, as it selects Co2+ over Mg2+ at > 100 times lower concentrations. In addition, we present data that show TmCorA to be extremely thermostable in the presence of Co2+. Mg2+ could not stabilize the protein to the same extent, even at high concentrations. We also show that addition of Co2+, but not Mg2+, specifically induces structural changes to the protein. Altogether, these data show that TmCorA has the role of being the transporter of Co2+ but not Mg2+. The physiological relevance and requirements of Co2+ in T. maritima is discussed and highlighted. We suggest that CorA may have different roles in different organisms. Such functional diversity is presumably a reflection of minor, but important structural differences within the CorA family that regulate the gating, substrate selection, and transport.