Voltage-gated calcium channels of Paramecium cilia.

Paramecium cells swim by beating their cilia， and make turns by transiently reversing their power stroke. Reversal is caused by Ca entering the cilium through voltage-gated Ca (Ca) channels that are found exclusively in the cilia. As ciliary Ca levels return to normal， the cell pivots and swims forward in a new direction. Thus， the activation of the Ca channels causes cells to make a turn in their swimming paths. For 45 years， the physiological characteristics of the Paramecium ciliary Ca channels have been known， but the proteins were not identified until recently， when the P. tetraurelia ciliary membrane proteome was determined. Three Caα1 subunits that were identified among the proteins were cloned and confirmed to be expressed in the cilia. We demonstrate using RNA interference that these channels function as the ciliary Ca channels that are responsible for the reversal of ciliary beating. Furthermore， we show that Pawn (pw) mutants of Paramecium that cannot swim backward for lack of Ca channel activity do not express any of the three Ca1 channels in their ciliary membrane， until they are rescued from the mutant phenotype by expression of the wild-type PW gene. These results reinforce the correlation of the three Ca channels with backward swimming through ciliary reversal. The PwB protein， found in endoplasmic reticulum fractions， co-immunoprecipitates with the Ca1c channel and perhaps functions in trafficking. The PwA protein does not appear to have an interaction with the channel proteins but affects their appearance in the cilia.