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Structural and Biochemical Characterization of a Periplasmic Lipoprotein with a Role in Adaptation of Salmonella to Magnesium Limitation

Living cells require magnesium ions for many fundamental biochemical processes. Certain pathogenic bacteria can thrive in environments with limiting magnesium, and the ability to adapt to magnesium limitation can be critical for bacterial virulence. We have used genetic and biochemical approaches to gain insight into how Salmonella enterica, a major bacterial cause of food‐borne illness, adapts to low magnesium. Using a genetic screen in a Salmonella mutant defective for adaptation to low magnesium, we identified a periplasmic lipoprotein protein that promotes growth in low magnesium. This Salmonella protein, which has a monomeric molecular weight of 19.7 kDa, contains a periplasmic localization sequence followed by a single domain of unknown function that is conserved among enterobacteria. To elucidate its function, we have purified and crystallized this protein as well as obtained atomic‐resolution X‐ray diffraction data. Circular dichroism spectroscopy indicates that the protein consists predominantly of beta‐sheet secondary structure, and the diffraction data indicate there is likely more than one protein monomer in the asymmetric unit. Overall, our results demonstrate a physiological function in magnesium ion homeostasis for a previously uncharacterized periplasmic lipoprotein in Salmonella, and we have obtained the first atomic‐resolution structural data for a member of this family of proteins. These discoveries will provide insight into molecular mechanisms of bacterial adaptation to magnesium limitation.