Candida albicans Flu1-Mediated Efflux of Salivary Histatin 5 Reduces Its Cytosolic Concentration and Fungicidal Activity

Abstract

ABSTRACTHistatin 5 (Hst 5) is a salivary human antimicrobial peptide that is toxic to the opportunistic yeastCandida albicans. Fungicidal activity of Hst 5 requires intracellular translocation and accumulation to a threshold concentration for it to disrupt cellular processes. Previously, we observed that total cytosolic levels of Hst 5 were gradually reduced from intact cells, suggesting thatC. albicanspossesses a transport mechanism for efflux of Hst 5. Since we identifiedC. albicanspolyamine transporters responsible for Hst 5 uptake, we hypothesized that one or more polyamine efflux transporters may be involved in the efflux of Hst 5.C. albicans FLU1andTPO2were found to be the closest homologs ofSaccharomyces cerevisiae TPO1, which encodes a major spermidine efflux transporter, indicating that the products of these two genes may be involved in efflux of Hst 5. We found thatflu1Δ/Δ cells, but nottpo2Δ/Δ cells, had significant reductions in their rates of Hst 5 efflux and had significantly higher cytoplasmic Hst 5 and Hst 5 susceptibilities than did the wild type. We also found thatflu1Δ/Δ cells had reduced biofilm formation compared to wild-type cells in the presence of Hst 5. Transcriptional levels ofFLU1were not altered over the course of treatment with Hst 5; therefore, Hst 5 is not likely to induceFLU1gene overexpression as a potential mechanism of resistance. Thus, Flu1, but not Tpo2, mediates efflux of Hst 5 and is responsible for reduction of its toxicity inC. albicans.