In most healthy individuals the yeast Candida albicans is found within the oral cavity as part of the normal microflora. Though under immunocompromising conditions, this benign microbe can become an opportunistic pathogen causing oral candidiasis (i.e. thrush). Although antifungal drugs are typically efficacious, the paucity of drugs and their increasing usage has led to rising drug resistance. Thus, researchers have begun to look at alternative therapeutics, such as the candidacidal salivary peptide histatin-5 (Hst-5). To date, little is known about the initial binding and subsequent internalization that facilitates Hst-5's killing activity. Thus, our study aims to determine how Hst-5 is internalized into C. albicans. It was thought that Hst-5 is transported into the cytoplasm without disruption of the plasma membrane. However, our studies have determined that Hst-5, under physiological concentrations, disrupts the plasma membrane and is rapidly translocated into the cytoplasm, leading to cell death. Interestingly, the internalization of Hst-5 is initiated from a single spatially restricted site on the plasma membrane rather than multiple breaches on the cell surface. We propose that Hst-5, unlike classical pore forming antimicrobial peptides, enters C. albicans in a membrane dependent manner. This mechanism of cell membrane disruption may provide species-specific killing with minimal damage to commensal flora and the host. We will investigate the functional domain of Hst-5 to determine which characteristic of the peptides (i.e. hydrophobicity, amphipathicit, etc.) allow for the nove cytoplasmic translocation of Hst-5 into C. albicans. We will also investigate using planar bilayer membrane whether Hst-5 disruption of the cellular membrane is voltage dependent. We will also determine whether cell surface proteins, which are associated with the osmotic stress response of C. albicans, play a role in resistance/susceptibility at low to moderate physiological concentrations of Hst-5. Public Health Relevance: This study should provide new insights into how Hst-5 and potentially other antimicrobial peptides may be acting on C. albicans and may provide urgently needed alternative therapeutics to antifungal drugs
