The molecular basis of pH sensing by the human fungal pathogen Candida albicans TOK potassium channel

Two-pore domain, outwardly rectifying potassium (TOK) channels are exclusively expressed in fungi. Human fungal pathogen TOK channels are potential antifungal targets, but TOK channel modulation in general is poorly understood. Here, we discovered that Candida albicans TOK (CaTOK) is regulated by extracellular pH, in contrast to TOK channels from other fungal species tested. Low pH increased CaTOK channel outward currents (pKa = 6.0), hyperpolarized the voltage-dependence of TOK activation, and increased pore selectivity for K+ over Na+, shifting the reversal potential (EREV) toward EK. Mutating H144 in the S1-S2 extracellular linker partially diminished pH sensitivity, suggesting H144 forms part of the CaTOK pH sensor. Functional analysis of chimeras made with pH-insensitive Saccharomyces cerevisiae TOK and point mutants revealed that CaTOK V462 and S466 in the final transmembrane segment complete the pH-responsive elements. A tripartite network of residues thus endows CaTOK with the ability to respond functionally to changes in pH.