Abstract
ABSTRACT
Maintaining proper cation homeostasis is essential for the survival of all living organisms. During infection,
Salmonella
must adapt to host-imposed magnesium (Mg²⁺) limitation. To compensate,
Salmonella
induces high-affinity Mg²⁺ transporters and synthesis of polyamines, organic cations that partially substitute for Mg²⁺. Once Mg²⁺ levels are restored and cells enter stationary phase, excess cations must be exported; failure to do so results in loss of viability. The CorC-domain protein PaeA exports excess polyamines, whereas MgpA and CorC reduce intracellular Mg²⁺ levels. Here, we identify two new CorC-domain proteins, MgpB (formerly YegH) and CorB, as mediators of Mg²⁺ homeostasis, showing that they also reduce intracellular Mg²⁺ accumulation and protect cells from endogenous excess-cation stress. Genetic analyses revealed the functional redundancy among these proteins in conferring resistance to exogenous high Mg²⁺, endogenous stress induced after Mg²⁺ starvation, and egg whites. Deletion of
mgpB
and/or
corB
conferred little to no phenotype. However, these mutations showed synergistic phenotypes in strains lacking MgpA. This synergistic relationship was also evident in mouse competition assays. Lack of MgpA, CorC, MgpB, and CorB leads to an increase in intracellular Mg²⁺ levels. Moreover, expression of
mgpB
and
corB
are increased in high Mg²⁺, all consistent with their role as putative Mg²⁺ exporters. Together, these findings identify MgpB and CorB as new mediators of Mg²⁺ homeostasis and reveal a multilayered system that enables
Salmonella
to finely balance intracellular cation levels during environmental and host-imposed stress.
IMPORTANCE
Mg²⁺ neutralizes negative charges within cells and influences macromolecular structure, stability, function, and intermolecular interactions. Mg²⁺ cannot be synthesized or degraded, requiring tight regulation through coordinated influx and efflux. However, the mechanisms that downregulate Mg²⁺ levels remain poorly understood. In this work, we identify two CorC-domain proteins, MgpB and CorB, as critical components of Mg²⁺ homeostasis that protect cells from both endogenous and exogenous excess-Mg
2+
stress. Importantly, these proteins also contribute to virulence and egg white tolerance, highlighting cation homeostasis as a key target of host antimicrobial defenses. These findings and previous work establish the CorC-domain family of proteins as a layered, partially redundant system for maintaining cation balance.