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

The evolution of Na ⁺ -selective four-domain voltage-gated channels (4D-Na _v s) 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 _v s, 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 _v s. 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.
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