Inhibition of Mg2+ current by single-gene mutation in Paramecium.
"Eccentric" is a newly-isolated mutant of Paramecium tetraurelia that fails to swim backwards in response to Mg2+. In the wild type, this backward swimming results from Mg2+ influx via a Mg(2+)-specific ion conductance (IMg). Voltage-clamp analysis confirmed that, as suspected, step changes in membrane potential over a physiological range fail to elicit IMg from eccentric. Further electrophysiological investigation revealed a number of additional ion-current defects in eccentric: (i) The Ca2+ current activated upon depolarization inactivates more slowly in eccentric than in the wild type, and it requires longer to recover from this inactivation. (ii) The Ca(2+)-dependent Na+ current deactivates significantly faster in the mutant. (iii) The two K+ currents observed upon hyperpolarization are reduced by > 60% in eccentric. It is difficult to envision how these varied pleiotropic effects could result from loss of a single ion current. Rather, they suggest that the eccentric mutation affects a global regulatory system. Two plausible hypotheses are discussed.
Preston
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RR
Laboratory of Molecular Biology, University of Wisconsin-Madison 53706.
Kung
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C
D000200:Action Potentials; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002465:Cell Movement / Q000187:drug effects; D017125:Genes, Protozoan; D005808:Genes, Recessive; D005809:Genes, Regulator; D007473:Ion Channels / Q000378:metabolism*; D008274:Magnesium / Q000378:metabolism* / Q000494:pharmacology; D008954:Models, Biological; D009154:Mutation; D016805:Paramecium tetraurelia / Q000235:genetics* / Q000378:metabolism; D010641:Phenotype; D011188:Potassium / Q000378:metabolism; D012964:Sodium / Q000378:metabolism
D007473:Ion Channels; D012964:Sodium; D008274:Magnesium; D011188:Potassium; D002118:Calcium