Crystal structure of a bacterial CNNM magnesium transporter

CBS-pair domain divalent metal cation transport mediators (CNNMs) are a broadly conserved family of integral membrane proteins with close to 90,000 protein sequences known. CNNM proteins are associated with Mg2+ transport but it is not known if they mediate transport themselves or regulate other transporters. Here, we determined the crystal structure of an archaeal CNNM protein with Mg2+-ATP bound. The structure reveals a novel transmembrane fold for the DUF21 domain, the largest family of domains of unknown function. The protein has a negatively charged cavity that penetrates halfway through the membrane suggesting it functions as a cation transporter. The cytosolic portion of the protein is comprised of highly charged four-helix bundle and a CBS-pair domain. HDX-MS experiments, molecular dynamics, and additional crystal structures show that the cytosolic domains undergo large conformational changes upon nucleotide binding suggesting a mechanism of regulation shared between human and bacterial orthologs. The molecular characterization of CNNM proteins has profound implications for understanding their biological functions in human diseases, including cancer, and in animals, bacteria and plants.