Structural basis of haem-iron acquisition by fungal pathogens.
Pathogenic microorganisms must cope with extremely low free-iron concentrations in the host's tissues. Some fungal pathogens rely on secreted haemophores that belong to the Common in Fungal Extracellular Membrane (CFEM) protein family, to extract haem from haemoglobin and to transfer it to the cell's interior, where it can serve as a source of iron. Here we report the first three-dimensional structure of a CFEM protein, the haemophore Csa2 secreted by Candida albicans. The CFEM domain adopts a novel helical-basket fold that consists of six α-helices, and is uniquely stabilized by four disulfide bonds formed by its eight signature cysteines. The planar haem molecule is bound between a flat hydrophobic platform located on top of the helical basket and a peripheral N-terminal 'handle' extension. Exceptionally, an aspartic residue serves as the CFEM axial ligand, and so confers coordination of Fe3+haem, but not of Fe2+haem. Histidine substitution mutants of this conserved Asp acquired Fe2+haem binding and retained the capacity to extract haem from haemoglobin. However, His-substituted CFEM proteins were not functional in vivo and showed disturbed haem exchange in vitro, which suggests a role for the oxidation-state-specific Asp coordination in haem acquisition by CFEM proteins.
B. Rappaport Faculty of Medicine, Technion - Israel Institute of Technology, and the Rappaport Institute for Research in the Medical Sciences, Haifa 31096, Israel.
Lena
L
Nasser
B. Rappaport Faculty of Medicine, Technion - Israel Institute of Technology, and the Rappaport Institute for Research in the Medical Sciences, Haifa 31096, Israel.
Ziva
Z
Weissman
B. Rappaport Faculty of Medicine, Technion - Israel Institute of Technology, and the Rappaport Institute for Research in the Medical Sciences, Haifa 31096, Israel.
Mariel
M
Pinsky
Wolfson Centre for Applied Structural Biology, Hebrew University of Jerusalem, Jerusalem, Israel.
Hadar
H
Amartely
Technion Center for Structural Biology, Technion - Israel Institute of Technology, Haifa 32000, Israel.
Hay
H
Dvir
B. Rappaport Faculty of Medicine, Technion - Israel Institute of Technology, and the Rappaport Institute for Research in the Medical Sciences, Haifa 31096, Israel.
Daniel
D
Kornitzer
D001224:Aspartic Acid / Q000737:chemistry / Q000378:metabolism; D001692:Biological Transport; D002176:Candida albicans / Q000737:chemistry / Q000378:metabolism* / Q000472:pathogenicity; D018360:Crystallography, X-Ray; D003545:Cysteine / Q000737:chemistry / Q000378:metabolism; D005656:Fungal Proteins / Q000737:chemistry* / Q000378:metabolism; D006418:Heme / Q000378:metabolism*; D006420:Hemeproteins / Q000737:chemistry / Q000378:metabolism; D006639:Histidine / Q000737:chemistry / Q000378:metabolism; D007501:Iron / Q000378:metabolism*; D008565:Membrane Proteins / Q000737:chemistry*; D008968:Molecular Conformation
D005656:Fungal Proteins; D006420:Hemeproteins; D008565:Membrane Proteins; D001224:Aspartic Acid; D006418:Heme; D006639:Histidine; D007501:Iron; D003545:Cysteine