1.A.1.7.1
Tok1 twin (dual) barrel outward rectifying K⁺ channel with exterme assymmetry which includes an extra 4 N-terminal TMSs for a total of 16 TMSs.  (Transports K⁺ and Cs⁺) (Bertl et al., 2003; Roller et al. 2008).  TOKs are outwardly rectifying K⁺ channels in fungi with two pore-loops and eight transmembrane spans. Lewis et al. 2020 described the TOKs from four fungal pathogens. These TOKs pass large currents only in the outward direction like this ScTOK. ScTOK, AfTOK1 (Aspergillus fumigatus), and H99TOK (Cryptococcus neoformans grubii) are K⁺-selective and pass current above the K⁺ reversal potential. CaTOK (Candida albicans) and CnTOK (Cryptococcus neoformans neoformans) pass both K⁺ and Na⁺ and conduct above a reversal potential, reflecting the mixed permeability of their selectivity filter. Mutations in CaTOK and ScTOK at sites homologous to those that open the internal gates in classical K⁺ channels are shown to produce inward TOK currents. Possibly the reversal potential determines ion occupancy, and thus, conductivity, of the selectivity filter gate that is coupled to an imperfectly restrictive internal gate, permitting the filter to sample ion concentrations on both sides of the membrane (Lewis et al. 2020). TOK (tandem-pore outward-rectifying K⁺) channels consist of eight TMSs and two pore domains per subunit, organized in dimers. They play a role in cellular K⁺ homeostasis and possibly also in plant-fungus symbioses (Houdinet et al. 2022).