A Novel Single-Domain Na⁺-Selective Voltage-Gated Channel in Photosynthetic Eukaryotes

The evolution of Na⁺-selective four-domain voltage-gated channels (4D-Na_vs) in animals allowed rapid Na⁺-dependent electrical excitability, and enabled the development of sophisticated systems for rapid and long-range signaling. While bacteria encode single-domain Na⁺-selective voltage-gated channels (BacNa_v), they typically exhibit much slower kinetics than 4D-Na_vs, and are not thought to have crossed the prokaryote-eukaryote boundary. As such, the capacity for rapid Na⁺-selective signaling is considered to be confined to certain animal taxa, and absent from photosynthetic eukaryotes. Certainly, in land plants, such as the Venus flytrap (Dionaea muscipula) where fast electrical excitability has been described, this is most likely based on fast anion channels. Here, we report a unique class of eukaryotic Na⁺-selective, single-domain channels (EukCatBs) that are present primarily in haptophyte algae, including the ecologically important calcifying coccolithophores, Emiliania huxleyi and Scyphosphaera apsteinii The EukCatB channels exhibit very rapid voltage-dependent activation and inactivation kinetics, and isoform-specific sensitivity to the highly selective 4D-Na_v blocker tetrodotoxin. The results demonstrate that the capacity for rapid Na⁺-based signaling in eukaryotes is not restricted to animals or to the presence of 4D-Na_vs. The EukCatB channels therefore represent an independent evolution of fast Na⁺-based electrical signaling in eukaryotes that likely contribute to sophisticated cellular control mechanisms operating on very short time scales in unicellular algae.