Secondary transport of metalcitrate complexes: The CitMHS family

Primary and secondary transport of citrate has been extensively studied in pathogenic and non-pathogenic bacteria. Primary transporters of citrate complexed with metal ions, particularly Fe, have also garnered attention, with the fec system of E. coli being a classic example. In contrast, little is known about secondary transporters of metalcitrate complexes. Recently, a family of proteins responsible for secondary metalcitrate transport in bacteria was discovered and designated as the CitMHS transporter family. Several members have been functionally characterized to date and serve as the foundation for understanding this family. Three subfamilies have been categorized, depending on the main metal ion transported. These subfamilies are the Mg²citrate transporter, the Ca²citrate transporter, and the Fe ³citrate transporter. Each subfamily is believed to be substrate-selective due to the metalcitrate complexes being abundantly present in their environment and/or the ability of the complex to be metabolized by the organism. The implication of this family in the pathogenic access to Fe, information about transcriptional control, putative structure, predicted family members, members characterized to date and potential use in bioremediation are discussed.