Palytoxin-induced Na+ influx into yeast cells expressing the mammalian sodium pump is due to the formation of a channel within the enzyme.

Palytoxin forms ionic channels in animal cell membranes but does not have similar effects on bacteria or yeast cells. These channels appear to be associated with the sodium pump. Using a heterologous expression system for the mammalian sodium pump in the yeast Saccharomyces cerevisiae, we recently demonstrated palytoxin-induced K+ efflux from yeast cells. Using the same system, we now show that the palytoxin-induced Na+ influx measured by others in animal cells is also directly associated with the sodium pump. Under the influence of palytoxin, yeast cells that express the mammalian sodium pump exchange extracellular Na+ ions for intracellular K+ ions with a stoichiometry of approximately 1:1. Both fluxes can be inhibited by ouabain. K+ efflux can also be observed when extracellular Na+ is replaced by Li+, Cs+, or NH4+. These data suggest that all palytoxin-induced ion fluxes measured so far in various cell systems are directly associated with the sodium pump. Palytoxin-induced Na+ influx or K+ efflux does not occur with yeast cells that express a truncated form of the sodium pump that is missing 44 of the carboxyl-terminal amino acids of the alpha 1 subunit. Scatchard analysis reveals only a slightly lower affinity of the truncated form for [3H]ouabain compared with the affinity of the native enzyme. Yeast cells expressing the truncated enzyme can bind [3H]ouabain, which can be displaced by palytoxin. Therefore, the inability of the truncated form to conduct ions under the influence of palytoxin is not due to the removal of the palytoxin binding site but rather to the removal of a part of the enzyme that participates in a direct or indirect way in the formation of the palytoxin-induced channel. Based on these findings, we conclude that palytoxin opens a channel within and not merely in the vicinity of the sodium pump. This might be the same channel that under normal conditions actively transports Na+ and K+ ions.