Abstract
Candida albicans is the predominant species of yeast isolated from patients with oral candidiasis, which is frequently a symptom of human immunodeficiency virus infection and is a criterion for staging and progression of AIDS. Salivary histatins (Hsts) are potent in vitro antifungal agents and have great promise as therapeutic agents in humans with oral candidiasis. The molecular mechanisms by which Hsts kill yeast cells are not known. We report here, that unlike other antimicrobial proteins, Hsts do not display lytic activities to lipid membranes, measured by release and dequenching of the fluorescent dye calcein. Analysis of the magnitude and time course of Hst-induced calcein release from C. albicans cells further showed that loss of cell integrity was a secondary effect following cell death, rather than the result of primary disruption of the yeast cell membrane. 125I-Hst 5 binding studies indicated that C. albicans expressed a class of saturable binding sites (KD = 1 microM), numbering 8.6 x 10(5) sites/cell. Both Hst 3 and Hst 4 competed for these binding sites with similar affinities, which is consistent with the micromolar concentration of Hsts required for candidacidal activity. Specific 125I-Hst 5 binding was not detected to C. albicans spheroplasts, which were 14-fold less susceptible to Hst 5 killing, compared with intact cells in candidacidal assays. In overlay experiments, 125I-Hst 5 bound to a 67-kDa protein detected in C. albicans whole cell lysates and crude membrane fractions, but not in the yeast cell wall fraction. Consistent with the overlay data, cross-linking of 125I-Hst 5 to C. albicans resulted in the appearance of a specific 73-kDa 125I-Hst 5-containing complex that was not detected in the cell wall. 125I-Hst 5-binding protein of similar size was also observed in susceptible S. cerevisiae strain TI#20. This is the first description of Hst 5 binding sites on C. albicans which mediate cell killing and identification of a 67-kDa yeast Hst 5-binding protein. The binding characteristics of Hst 5 are in agreement with the observed potency of its biological effect and provide crucial information to the use of Hst 5 as a therapeutic agent. The presence of a specific C. albicans Hst 5-binding protein provides further insight into the potential mechanism of yeast killing and suggests a basis for differential activity between yeast killing and the nontoxic nature of Hsts to humans.
Highlights
Candida albicans is the predominant species of yeast isolated from patients with oral candidiasis, which is frequently a symptom of human immunodeficiency virus infection and is a criterion for staging and progression of AIDS
Consistent with the overlay data, cross-linking of 125I-histatin 5 (Hst 5) to C. albicans resulted in the appearance of a specific 73-kDa 125I-Hst 5-containing complex that was not detected in the cell wall. 125I-Hst 5-binding protein of similar size was observed in susceptible S. cerevisiae strain TI#20
Hst C, which represents the eight-amino acid C terminus of histatin 3 (Hst 3), did not produce loss of yeast cell viability even at high concentrations (500 M). These data demonstrate that the middle and N-terminal regions of full-length Hsts are required for optimal candidacidal activity and that the C-terminal region may not be essential for yeast killing and may even reduce Hsts’ fungicidal activity
Summary
Materials—C. albicans strain DS1 was isolated from the palate of a denture stomatitis patient [39] and Saccharomyces cerevisiae strains TI#20 and BJ2169 were obtained from Dr J. Whole cells were loaded with calcein-AM and subsequently converted to spheroplasts as described above and measurements were performed in 10 mM phosphate buffer containing 1 M sorbitol. Suspension assays were performed at cell densities of approximately 106 cells (25 l) for each point in a final volume of 50 l binding buffer containing 125I-Hst 5 for 30 min at room temperature. Cross-linking Studies—Washed C. albicans cells were resuspended in binding buffer at 1.8 ϫ 109 cells/ml, and 150 l (3 ϫ 108 cells) were incubated for 20 min at room temperature with 100 nM 125I-Hst 5 in a final incubation volume of 200 l. Cross-linked C. albicans cells were resuspended in 200 l of spheroplasting buffer (10 mM Tris, pH 7.4, 1 mM EDTA, 1 M sorbitol, 480 g of Zymolyase-20T, 30 mM 2-mercaptoethanol) and incubated for 1 h at 30 °C with gentle shaking. Autoradiography was performed by exposing dried gels at Ϫ80 °C for 5–15 days
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