Killing of Candida albicans by human salivary histatin 5 is modulated, but not determined, by the potassium channel TOK1

Salivary histatin 5 (Hst 5), a potent toxin for the human fungal pathogen Candida albicans, induces noncytolytic efflux of cellular ATP, potassium, and magnesium in the absence of cytolysis, implicating these ion movements in the toxin's fungicidal activity. Hst 5 action on Candida resembles, in many respects, the action of the K1 killer toxin on Saccharomyces cerevisiae, and in that system the yeast plasma membrane potassium channel, Tok1p, has recently been reported to be a primary target of toxin action. The question of whether the Candida homologue of Saccharomyces Tok1p might be a primary target of Hst 5 action has now been investigated by disruption of the C. albicans TOK1 gene. The resultant strains (TOK1/tok1) and (tok1/tok1) were compared with wild-type Candida (TOK1/TOK1) for relative ATP leakage and killing in response to Hst 5. Patch-clamp measurements on Candida protoplasts were used to verify the functional deletion of Tok1p and to provide its first description in Candida. Tok1p is an outwardly rectifying, noisily gated, 40-pS channel, very similar to that described in Saccharomyces. Knockout of CaTOK1 (tok1/tok1) completely abolishes the currents and gating events characteristic of Tok1p. Also, knockout (tok1/tok1) increases residual viability of Candida after Hst 5 treatment to 27%, from 7% in the wild type, while the single allele deletion (TOK1/tok1) increases viability to 18%. Comparable results were obtained for Hst-induced ATP efflux, but quantitative features of ATP loss suggest that wild-type TOKI genes function cooperatively. Overall, very substantial killing and ATP efflux are produced by Hst 5 treatment after complete knockout of wild-type TOKI, making clear that Tok1p channels are not the primary site of Hst 5 action, even though they do play a modulating role.