Heme-iron acquisition pathway of C. albicans

Candida albicans is a commensal organism that can cause life threating systemic candidiasis in immunocompromised patients. Like all organisms, C. albicans requires iron as a critical cofactor of essential proteins. Although iron is abundant on the earth’s crust, inside the human host, free iron is scarce. Iron-protoporphyrin IX, or heme, constitutes the largest iron pool in the human host, found mainly as cofactor of hemoglobin. Like many pathogens, C. albicans has evolved a mechanism to utilize hemoglobin heme. Three extracellular CFEM-type hemophores, Pga7 (membrane-anchored), Rbt5 (cell wall-anchored), and Csa2 (soluble), can extract heme from hemoglobin and albumin, and transfer it to the cell surface. There, two transmembrane proteins, Frp1 and Frp2, are predicted to capture the heme from the CFEM hemophores and internalize it. My work focuses on finding the specific amino acids within the Frp1/2 protein structure that interacts with heme. We took advantage of the AlphaFold 2 protein structure predictions and looked for potential heme-iron coordinating residues (His, Tyr, Cys, Met) properly positioned to function as part of a heme-binding site. We performed site-directed mutagenesis at these predicted sites in Frp1 and analyzed their localization, their ability to utilize hemoglobin-heme and their sensitivity to toxic heme analogues. We found that Frp1 is localized in the membrane, ER and vacuole of C. albicans and we identified several residues that are important for Frp1’s function in heme acquisition.