The Paramecium system has been developed to understand the intracellular actions of Ca2+ and the molecular mechanisms of ion-channel regulation. Two sets of behavioral mutants of Paramecium were found to be defective in the structural gene for calmodulin. Mutational substitutions at the C-terminal lobe of the calmodulin dumbbell abolish Ca2+-dependent K+ channel activities, while substitutions at the N-terminal lobe abolish Ca2+-dependent Na+ channel activity. Rescue experiments showed that channel activations result from calmodulin-binding into the membrane, presumably to the channel, and not through phosphorylation. These observations will be greatly extended to understand how single amino-acid substitutions at specific loci of the calmodulin molecule lead to channel failure. Two-electrode voltage clamp will be employed to examine the two Ca2+ currents and the various Ca2+-dependent currents in various mutants. Experiments in vitro, with these channels captured in excised patches, will also be carried out. These channels will be challenged with isolated or site-directed mutant calmodulins, foreign calmodulins, and calmodulin half-molecules to further dissect the specific requirement in the activations of these Ca2+-dependent channels. The characteristics and activation mechanism of a Ca2+-dependent Mg2+ current will be scrutinized. Mutants, whose defects most likely lie in the channel proteins and not calmodulin, will also be examined in detail. Ion-channel regulations account for physiological phenomena such as heart-beat regulation and short-term memory, as well as pathological conditions such as cystic fibrosis and alcohol intoxication. The Paramecium system is used to identify and further study such regulations in molecular and electric terms.
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