Novel molecular mechanisms of iron sensing and homeostasis in filamentous fungi

Iron is essential for numerous cellular processes but toxic in excess. Moreover, adaptation to iron starvation is a crucial virulence determinant of pathogens including fungi. Due to the fundamental differences in the mechanism employed by host and pathogens to maintain iron homeostasis, iron metabolism is an attractive new target for improvement of antifungal therapy and diagnosis of fungal infections. However, a better understanding of the molecular basis of fungal iron homeostasis is required. In various fungi including the most common airborne fungal pathogen Aspergillus fumigatus, the key iron regulator HapX has been shown to be required for adaptation to iron starvation via ist interaction with the CCAAT-binding complex (CBC). Recently, we found that HapX is in addition essential for iron resistance, which underlines that HapX senses the cellular iron status to coordinate the transcriptional response. The planned collaborative studies by geneticists, biochemists and structural biologists aim to explore in molecular depth the functions of the HapX/CBC complex as iron sensor, transcriptional repressor and activator, as well as promoter organizer. In vivo and in vitro analyses are combined with structural examinations of the involved protein/protein and protein/DNA complexes to elucidate these novel regulatory mechanisms in eukaryotes.

Iron is essential for numerous cellular processes but toxic in excess. Moreover, adaptation to iron starvation is a crucial virulence determinant of pathogens including fungi. Due to the fundamental differences in the mechanism employed by host and pathogens to maintain iron homeostasis, iron metabolism is an attractive new target for improvement of antifungal therapy and diagnosis of fungal infections. However, a better understanding of the molecular basis of fungal iron homeostasis is required. In various fungi including the most common airborne fungal pathogen Aspergillus fumigatus, the key iron regulator HapX has been shown to be required for adaptation to iron starvation via its interaction with the CCAAT-binding complex (CBC). In this collaborative study combining genetics, biochemistry and structural biology including in vitro and in vivo analyses, we found that the HapX-CBC complex additionally regulates iron resistance and elucidated molecular mechanisms of HapX-mediated iron-sensing and iron-regulation, e.g. (i) we identified HapX protein domains that are important for adaptation to iron starvation and iron resistance, respectively, (ii) elucidated the mode of DNA-binding by the HapX-CBC complex, (iii) identified cellular components required for iron sensing (monothiol glutaredoxin and glutathione), and (iv) demonstrate that iron sensing requires a signal derived from mitochondrial iron-sulfur-cluster biosynthesis. Moreover, we functionally and structurally characterized several cellular components involved in iron homeostasis maintenance. Furthermore, ourstudies identified and analyzed links between iron regulation with secondary metabolism, ergosterol biosynthesis, azole resistance, leucine biosynthesis, microbial interaction, and mammalian virulence. Taken together, this project revealed similarities but also significant differences in iron sensing and regulation between the ascomycete role model A. fumigatus and other model fungal species, which might be exploited in antifungal therapy as well as in biotechnology. The results of this study, reflected by the publication output, emphasize the successful collaboration within the DACH consortium as well as with other national and international partners.