Csa2 is a member of both the Candida albicans Rbt5 protein family and the Common in Fungal Extracellular Membranes (CFEM) protein superfamily. CFEM proteins are characterized by an internal domain containing eight equally spaced cysteine residues. Csa2 is involved in iron uptake from hemoglobin and heme proteins; however, its precise role is unclear. Here, we provide quantitative evidence of the involvement of Csa2 in the utilization of iron from human hemoglobin during C. albicans hyphal growth. The ability of the hyphal form of the wild-type (wt), a homozygote csa2Δ mutant, and a complemented strain of C. albicans to utilize hemoglobin as an iron source under iron-restricted conditions was examined through growth studies and a crystal violet-staining assay. Hemoglobin-binding activity was assessed indirectly using a hemoglobin-sensitized tube method. Although hyphal growth of the wt and csa2Δ/Δ::CSA2 strains was completely recovered when a high concentration of human hemoglobin was added to the iron-restricted culture medium, the recovery of the csa2Δ/Δ mutant was significantly diminished. Furthermore, hemoglobin binding was impaired in the csa2Δ/Δ mutant compared with the wt and csa2Δ/Δ::CSA2 strains, revealing that Csa2 is involved in the utilization of hemoglobin as an iron source by the hyphal form of C. albicans.
Keywords: Candida albicans; Rbt5 protein family; hemoglobin; hyphal growth; iron utilization.
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