Characterization and purification of a soluble protein controlling Ca-channel activity in paramecium

The analysis of the voltage-sensitive Ca⁺⁺ channel of the unicellular eucaryote, Paramecium has been extended to a biochemical level based on recent observations that the transfer of cytoplasm from wild-type cells into mutants lacking Ca⁺⁺-channel function ("pawn" in P. tetraurelia and "CNR" in P. caudatum) causes mutant cells to regain Ca⁺⁺-channel function. We have microinjected various cytoplasmic fractions into mutant cells and measured the restored Ca⁺⁺-channel function using a convenient behavioral assay. Following the "curing" activity, we characterized and purified the component from wild-type cytoplasm that can restore the function missing in cells carrying mutations in the cnrC gene. The curing factor is not an RNA, but a heat-labile, -SH-containing protein that appears to affect existing mutant channels on the ciliary membrane. We have purified this factor over 500-fold from the soluble cytoplasm using conventional techniques. The protein is of low apparent molecular weight (<30,000 daltons), acidic, soluble, and does not have the properties of calmodulin.