Vicia faba TPC1, a genetically encoded variant of the vacuole Two Pore Channel 1, is hyperexcitable

To fire action-potential-like electrical signals, the vacuole membrane requires the depolarization-activated two-pore channel TPC1, also called <underline>S</underline>lowly activating <underline>V</underline>acuolar SV channel. The TPC1/SV channel, encoded by the TPC1 gene, functions as a voltage-dependent and Ca²⁺-regulated potassium channel. TPC1 currents are activated by a rise in cytoplasmic Ca²⁺ but inhibited by luminal Ca²⁺. In search for species-dependent functional TPC1 channel variants, we studied polymorphic amino acids contributing to luminal Ca²⁺ sensitivity. We found that the acidic residues E457, E605 and D606 of the Ca²⁺-sensitive Arabidopsis AtTPC1 channel were neutralized by either asparagine or alanine in Vicia faba and many other Fabaceae as well. When expressed in the Arabidopsis loss-of-AtTPC1 function background, the wild type VfTPC1 was hypersensitive to vacuole depolarization and insensitive to blocking luminal Ca²⁺. When AtTPC1 was mutated for the three VfTPC1-homologous polymorphic site residues, the Arabidopsis At-VfTPC1 channel mutant gained VfTPC1-like voltage and luminal Ca²⁺ insensitivity that together made vacuoles hyperexcitable. These findings indicate that natural TPC1 channel variants in plant families exist which differ in vacuole excitability and very likely respond to changes in environmental settings of their ecological niche.