Paramecium tetraurelia, which is a cell covered in cilia, responds to sensory stimuli with changes in swimming behavior. Membrane potential controls beat frequency, hence speed, and graded Ca2+ action potentials cause the cell to turn. The physiology of the action potentials caused by activation of ciliary voltage-gated Ca2+ (Cav) channels has been known for 50 or more years. It has also been known that the Cav channels which are necessary for the action potential are found exclusively in the cilia. The Ca2+ entering the cilium through these Cav channels affects the ciliary beating, reversing the power stroke and transiently sending the cell swimming backward. As ciliary Ca2+ levels return to normal, the cell pivots in place and then swims forward in a new direction. Thus, the activation of the ciliary Cav channels causes cells to swim backward transiently and make a turn in their swimming paths. The pore-forming subunit proteins (Cavα1) were not identified until recently when we determined the P. tetraurelia ciliary membrane proteome. Three Cavα1 subunits were subsequently cloned and confirmed to be expressed in the cilia. We also demonstrated using interference RNA (RNAi) that these channels function as the ciliary Cav channels that are responsible for the reversal of ciliary beating. The ciliary membrane proteome included plasma membrane Ca2+ ATPases (PMCAs) that are in high abundance. RNAi for these PMCAs (PMCA 18/19) increased the duration of backward swimming, suggesting that these calcium pumps play a role in controlling the ciliary Ca2+ levels following the activation of the Cav channels. These PMCAs can be reciprocally co-precipitated with the Cavα1, and can be found in the same lipid raft fractions as the Cavα1. Therefore, we postulate that an important mechanism for the immediate removal of Ca2+ upon activation of the Cav channels is the physical proximity and interaction of the ciliary calcium pumps with the channels.
Acknowledge P20GM103449.