Virulence factor required for growth in low Mg(2+) medium and for intramacrophage survival. May be involved in regulating membrane potential by activating Na(+)/K(+)-ATPase. Part of the mgtC/mgtB operon. Induced by low extracellular levels of Mg(2+). Transcriptionally regulated by extracellular magnesium via the two-component regulatory system PhoQ/PhoP. Not required for Mg(2+) influx by MgtB. Belongs to the MgtC/SapB family.