More than forty years ago, important new mutants of Paramecium tetraurelia were named Pawns (pw) for the chess piece because they had lost the ability to swim backward (Kung, Chang et al. 1975). Subsequently they were shown to lack the calcium conductance of the voltage gated calcium channels (CaV), which have been shown by electrophysiology to be limited to the ciliary membrane. Otherwise, pawns have normal conductances (Satow and Kung 1974). The genes for pwA and pwB were identified by complementation 25 years later, but they did not appear to code for channels or subunits. The channels were also elusive until 2012 when we identified three CaV1 in the ciliary membrane proteome. We cloned, epitope tagged and expressed these very large (>250 kd) proteins, and showed that they are in the ciliary membrane. These advances allowed us to demonstrate by RNAi that these channels were responsible for most if not all of the backward swimming in depolarizing conditions. This in turn set the stage for answering the question of why pwA and pwB mutants cannot swim backward. When we express tagged CaV1 a, b, or c in wild type cells, these channels can be found in the ciliary membrane. When these tagged proteins are expressed in pwA or pwB cells, these channels are not detected in the ciliary membrane. However, these channels can be found again in the ciliary membranes, when these mutants are “rescued” with the wild type copy of their mutant gene. Therefore, pwA or pwB mutants do not swim backward because they do not have CaV channels in their cilia where they must reside in order to generate the action potential that drives backward swimming.
Kung, C., S. Chang, Y. Satow, J. Van Houten and H. Hansma (1975). "Genetic dissection of behavior in Paramecium." Science 188: 898-904.
Satow, Y. and C. Kung (1974). "Genetic dissection of active electrogenesis in Paramecium aurelia." Nature 247: 69-71.
Acknowledge P20GM103449.