Human Salivary Histatin 5 Causes Disordered Volume Regulation and Cell Cycle Arrest in Candida albicans

Abstract

ABSTRACT Human salivary histatin 5 (Hst 5) is a nonimmune salivary protein with antifungal activity against an important human pathogen, Candida albicans. The candidacidal activity of histatins appears to be a distinctive multistep mechanism involving depletion of the C. albicans intracellular ATP content as a result of nonlytic ATP efflux. Hst 5 caused a loss of cell viability concomitant with a decrease in cellular volume as determined both by a classical candidacidal assay with exogenous Hst 5 and by using a genetically engineered C. albicans strain expressing Hst 5. Preincubation of C. albicans cells with pharmacological inhibitors of anion transport provided complete or substantial protection from Hst 5-induced killing and volume reduction of cells. Moreover, intracellular expression of Hst 5 resulted in a reduction in the population mean cell volume that was accompanied by an increase in the percentage of unbudded cells and C. albicans cells in the G 1 phase. Following expression of Hst 5, the smallest cells sorted by fluorescence-activated cell sorting from the total population did not replicate and were exclusively in the G 1 phase. Cells with intracellularly expressed Hst 5 had greatly reduced G 1 cyclin transcript levels, indicating that they arrested in the G 1 phase before the onset of Start. Our data demonstrate that a key determinant in the mechanism of Hst 5 toxicity in C. albicans cell s is the disruption of regulatory circuits for cell volume homeostasis that is closely coupled with loss of intracellular ATP. This novel process of fungicidal activity by a human salivary protein has highlighted potential interactions of Hst 5 with volume regulatory mechanisms and the process of yeast cell cycle control.