title: An exchanger-like protein underlies the large Mg2+ current in Paramecium.
abstract: There are very few molecules known to transport Mg(2+) in eukaryotes. The membrane of Paramecium tetraurelia passes a large Mg(2+)-selective current and exhibits a corresponding backward swimming behavior. Both are missing in a group of mutants called eccentric. By sorting an indexed WT genomic library through microinjection into the macronucleus, we have isolated a DNA fragment that complements the eccentric mutations. The Mg(2+) currents and behavior are restored fully in the transformed cells. Surprisingly, the conceptually translated protein is not homologous to any known ion channel but instead has some similarity to K(+)-dependent Na(+)Ca(2+) exchangers. Exchangers are either electrically silent or only pass very small and slow currents compared with ion-channel currents. In light of recent ion-channel crystal structures and considering the need to have narrow ion-selective filters, we speculate on how an exchanger might evolve to show channel-like activities in special circumstances. The significance of finding the molecular basis of a Mg(2+)-specific pathway is also discussed.
authors.affiliation: Laboratory of Molecular Biology and Department of Genetics, University of Wisconsin, Madison 53706, USA. wjhaynes@facstaff.wisc.edu
authors.forename: W John
authors.initials: WJ
authors.lastname: Haynes
authors.forename: Ching
authors.initials: C
authors.lastname: Kung
authors.forename: Yoshiro
authors.initials: Y
authors.lastname: Saimi
authors.forename: Robin R
authors.initials: RR
authors.lastname: Preston
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D017920:Antiporters / Q000235:genetics / Q000502:physiology*; D016054:DNA, Protozoan / Q000235:genetics; D015723:Gene Library; D020868:Gene Silencing; D017125:Genes, Protozoan; D005816:Genetic Complementation Test; D017136:Ion Transport; D008124:Locomotion; D008274:Magnesium / Q000378:metabolism*; D008564:Membrane Potentials; D008845:Microinjections; D008969:Molecular Sequence Data; D016296:Mutagenesis; D016366:Open Reading Frames; D016805:Paramecium tetraurelia / Q000235:genetics / Q000378:metabolism*; D015800:Protozoan Proteins / Q000235:genetics / Q000502:physiology*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D019831:Sodium-Calcium Exchanger / Q000737:chemistry
chemical_list: D017920:Antiporters; D016054:DNA, Protozoan; D015800:Protozoan Proteins; D019831:Sodium-Calcium Exchanger; D008274:Magnesium